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Genetics is the study of
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanin ...
s,
genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, bu ...
, and
heredity Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...
in
organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...
s.Hartl D, Jones E (2005) It is an important branch in
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
because
heredity Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...
is vital to organisms'
evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...
.
Gregor Mendel Gregor Johann Mendel, Augustinians, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinians, Augustinian friar and abbot of St Thomas's Abbey, Brno, St. Thomas' Abbey in Br ...
, a
Moravia Moravia ( , also , ; cs, Morava ; german: link=yes, Mähren ; pl, Morawy ; szl, Morawa; la, Moravia) is a historical region in the east of the Czech Republic and one of three historical Czech lands, with Bohemia and Czech Silesia. The me ...
n Augustinian friar working in the 19th century in
Brno Brno ( , ; german: Brünn ) is a city A city is a human settlement of notable size.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd ...
, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene. Trait inheritance and
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded to study the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the
cell Cell most often refers to: * Cell (biology) The cell is the basic structural and functional unit of life forms. Every cell consists of a cytoplasm enclosed within a Cell membrane, membrane, and contains many biomolecules such as proteins, D ...
, the organism (e.g. dominance), and within the context of a population. Genetics has given rise to a number of subfields, including
molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
,
epigenetics In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' imp ...
and
population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...
. Organisms studied within the broad field span the domains of life (
archaea Archaea ( ; singular archaeon ) is a Domain (biology), domain of Unicellular organism, single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially Taxonomy (biology), classified as bacter ...
,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, and
eukarya Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
). Genetic processes work in combination with an organism's environment and experiences to influence development and
behavior Behavior (American English) or behaviour (British English) is the range of Action (philosophy), actions and mannerisms made by individuals, organisms, systems or Artificial Intelligence, artificial entities in some environment. These systems c ...
, often referred to as
nature versus nurture Nature versus nurture is a long-standing debate in biology and society about the balance between two competing factors which determine fate: genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones ...
. The
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
or
extracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
environment of a living cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate (lacking sufficient waterfall or rain). While the average height of the two corn stalks may be genetically determined to be equal, the one in the
arid climate The desert climate or arid climate (in the Köppen climate classification ''BWh'' and ''BWk''), is a dry climate sub-type in which there is a severe excess of evaporation over precipitation. The typically bald, rocky, or sandy surfaces in desert ...
only grows to half the height of the one in the temperate climate due to lack of water and nutrients in its environment. We need to know a lot more about the biology of viruses because of genetic analysis. The virus's genome, which is made up of 11 double-stranded RNA segments, serves as its defining characteristic. The primary characteristic of viral genetics is the genome's segmented structure's ability to reassign genome segments during mixed infections


Etymology

The word ''genetics'' stems from the
ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek Dark ...
' meaning "genitive"/"generative", which in turn derives from ' meaning "origin".


History

The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through
selective breeding Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to Selection (biology), selectively develop particular phenotypic traits (characteristics) by choosing which typically ...
. The modern science of genetics, seeking to understand this process, began with the work of the Augustinian friar
Gregor Mendel Gregor Johann Mendel, Augustinians, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinians, Augustinian friar and abbot of St Thomas's Abbey, Brno, St. Thomas' Abbey in Br ...
in the mid-19th century. Prior to Mendel,
Imre Festetics Count Imre Festetics de Tolna (1764 – 1847) was a noble landowner and geneticist. Scientific works Many of the central principles the discipline of genetics were formulated by Imre Festetics through the study of sheep. Festetics formulated a nu ...
, a Hungarian noble, who lived in Kőszeg before Mendel, was the first who used the word "genetic" in hereditarian context. He described several rules of biological inheritance in his works ''The genetic laws of the Nature'' (Die genetischen Gesetze der Natur, 1819). His second law is the same as what Mendel published. In his third law, he developed the basic principles of mutation (he can be considered a forerunner of
Hugo de Vries Hugo Marie de Vries () (16 February 1848 – 21 May 1935) was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while apparently unaware of ...
). Festetics argued that changes observed in the generation of farm animals, plants, and humans are the result of scientific laws. Festetics empirically deduced that organisms inherit their characteristics, not acquire them. He recognized recessive traits and inherent variation by postulating that traits of past generations could reappear later, and organisms could produce progeny with different attributes. These observations represent an important prelude to Mendel’s theory of particulate inheritance insofar as it features a transition of heredity from its status as myth to that of a scientific discipline, by providing a fundamental theoretical basis for genetics in the twentieth century. Text was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
Other theories of inheritance preceded Mendel's work. A popular theory during the 19th century, and implied by
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
's 1859 ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'', was
blending inheritance Blending may refer to: * The process of mixing (process engineering), mixing in process engineering * Paint mixing, Mixing paints to achieve a greater range of colors * Blending (alcohol production), a technique to produce alcoholic beverages by mi ...
: the idea that individuals inherit a smooth blend of traits from their parents. Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the
inheritance of acquired characteristics Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also calle ...
: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with
Jean-Baptiste Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist, biologist, academic, and soldier. He was an early proponent of the idea that biologi ...
) is now known to be wrong—the experiences of individuals do not affect the genes they pass to their children. Other theories included Darwin's
pangenesis Pangenesis was Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that a ...
(which had both acquired and inherited aspects) and
Francis Galton Sir Francis Galton, Fellow of the Royal Society, FRS Royal Anthropological Institute of Great Britain and Ireland, FRAI (; 16 February 1822 – 17 January 1911), was an English Victorian era polymath: a statistician, sociologist, psycholo ...
's reformulation of pangenesis as both particulate and inherited.


Mendelian genetics

Modern genetics started with Mendel's studies of the nature of inheritance in plants. In his paper "''Versuche über Pflanzenhybriden''" ("
Experiments on Plant Hybridization "Experiments on Plant Hybridization" (German language, German: "Versuche über Pflanzen-Hybriden") is a seminal paper written in 1865 and published in 1866 by Gregor Mendel, an Augustinian friar considered to be the founder of modern genetics. Th ...
"), presented in 1865 to the ''Naturforschender Verein'' (Society for Research in Nature) in Brünn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically. Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios. The importance of Mendel's work did not gain wide understanding until 1900, after his death, when
Hugo de Vries Hugo Marie de Vries () (16 February 1848 – 21 May 1935) was a Dutch botanist and one of the first geneticists. He is known chiefly for suggesting the concept of genes, rediscovering the laws of heredity in the 1890s while apparently unaware of ...
and other scientists rediscovered his research.
William Bateson William Bateson (8 August 1861 – 8 February 1926) was an English biologist A biologist is a scientist who conducts research in biology. Biologists are interested in studying life on Earth, whether it is an individual Cell (biology), cell, ...
, a proponent of Mendel's work, coined the word ''genetics'' in 1905. (The adjective ''genetic'', derived from the Greek word ''genesis''—γένεσις, "origin", predates the noun and was first used in a biological sense in 1860.) Bateson both acted as a mentor and was aided significantly by the work of other scientists from Newnham College at Cambridge, specifically the work of Becky Saunders, Nora Darwin Barlow, and
Muriel Wheldale Onslow Muriel Wheldale Onslow (31 March 1880 – 19 May 1932) was a British biochemist, born in Birmingham, England. She studied the inheritance of flower colour in the common snapdragon Antirrhinum and the biochemistry of anthocyanin Anthocyani ...
. Bateson popularized the usage of the word ''genetics'' to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in
London London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
in 1906. :Initially titled the "International Conference on Hybridisation and Plant Breeding", the title was changed as a result of Bateson's speech. See: After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1900, Nettie Stevens began studying the mealworm. Over the next 11 years, she discovered that females only had the X chromosome and males had both X and Y chromosomes. She was able to conclude that sex is a chromosomal factor and is determined by the male. In 1911,
Thomas Hunt Morgan Thomas Hunt Morgan (September 25, 1866 – December 4, 1945) was an American evolutionary biologist, geneticist, Embryology, embryologist, and science author who won the Nobel Prize in Physiology or Medicine in 1933 for discoveries elucidating t ...
argued that genes are on
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s, based on observations of a sex-linked white eye mutation in
fruit flies Fruit fly may refer to: Organisms * Drosophilidae, a family of small flies, including: ** ''Drosophila'', the genus of small fruit flies and vinegar flies ** ''Drosophila melanogaster'' or common fruit fly ** ''Drosophila suzukii'' or Asian fruit ...
. In 1913, his student Alfred Sturtevant used the phenomenon of
genetic linkage Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separ ...
to show that genes are arranged linearly on the chromosome.


Molecular genetics

Although genes were known to exist on chromosomes, chromosomes are composed of both
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metabo ...
and DNA, and scientists did not know which of the two is responsible for inheritance. In 1928, Frederick Griffith discovered the phenomenon of transformation: dead bacteria could transfer
genetic material Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the monomers made of three components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. ...
to "transform" other still-living bacteria. Sixteen years later, in 1944, the
Avery–MacLeod–McCarty experiment The Avery–MacLeod–McCarty experiment was an experimental demonstration, reported in 1944 by Oswald Avery, Colin Munro MacLeod, Colin MacLeod, and Maclyn McCarty, that DNA is the substance that causes bacterial transformation, in an era when it ...
identified DNA as the molecule responsible for transformation. Reprint: The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hämmerling in 1943 in his work on the single celled alga '' Acetabularia''. The
Hershey–Chase experiment The Hershey–Chase experiments were a series of experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight ...
in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.
James Watson James Dewey Watson (born April 6, 1928) is an American molecular biology, molecular biologist, geneticist, and zoologist. In 1953, he co-authored with Francis Crick the academic paper proposing the Nucleic acid double helix, double helix struc ...
and
Francis Crick Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was an English molecular biology, molecular biologist, biophysics, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in de ...
determined the structure of DNA in 1953, using the
X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract into many specific directions. By measurin ...
work of
Rosalind Franklin Rosalind Elsie Franklin (25 July 192016 April 1958) was a British chemist and X-ray crystallographer whose work was central to the understanding of the molecular structures of DNA (deoxyribonucleic acid), RNA (ribonucleic acid), viruses, coa ...
and
Maurice Wilkins Maurice Hugh Frederick Wilkins (15 December 1916 – 5 October 2004) was a New Zealand-born British people, British Biophysics, biophysicist and Nobel Laureate, Nobel laureate whose research spanned multiple areas of physics and biophysics, co ...
that indicated DNA has a helical structure (i.e., shaped like a corkscrew). Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what look like rungs on a twisted ladder. The a-helix is a secondary structure and the twisting in the a-helix is caused by hydrogen bonds between the carboxyl (C=O) and the amine H (N-H) constituents of the polypeptide backbone. This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand. Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of
protein production Protein production is the biotechnology, biotechnological process of generating a specific protein. It is typically achieved by the manipulation of gene expression in an organism such that it heterologous expression, expresses large amounts of a ...
. It was discovered that the cell uses DNA as a template to create matching
messenger RNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
, molecules with
nucleotide Nucleotides are Organic compound, organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential ...
s very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the
genetic code The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into protein Proteins are large biomolecules and macromolecules ...
. With the newfound molecular understanding of inheritance came an explosion of research. A notable theory arose from Tomoko Ohta in 1973 with her amendment to the
neutral theory of molecular evolution The neutral theory of molecular evolution holds that most evolutionary changes occur at the molecular level, and most of the variation within and between species are due to random genetic drift of mutation, mutant alleles that are selectively neut ...
through publishing the nearly neutral theory of molecular evolution. In this theory, Ohta stressed the importance of natural selection and the environment to the rate at which genetic
evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...
occurs. One important development was chain-termination
DNA sequencing DNA sequencing is the process of determining the nucleic acid sequence – the order of nucleotides in DNA. It includes any method or technology that is used to determine the order of the four bases: adenine, guanine, cytosine, and thymine. The ...
in 1977 by
Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was an English biochemist who received the Nobel Prize in Chemistry twice. He won the 1958 Chemistry Prize for determining the amino acid sequence of insulin and numerous other pr ...
. This technology allows scientists to read the nucleotide sequence of a DNA molecule. In 1983, Kary Banks Mullis developed the
polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...
, providing a quick way to isolate and amplify a specific section of DNA from a mixture. The efforts of the
Human Genome Project The Human Genome Project (HGP) was an international scientific research project with the goal of determining the base pairs that make up human DNA, and of identifying, gene mapping, mapping and DNA sequencing, sequencing all of the genes of the ...
, Department of Energy, NIH, and parallel private efforts by
Celera Genomics Celera is a subsidiary of Quest Diagnostics which focuses on genetic sequencing and related technologies. It was founded in 1998 as a business unit of Applera, spun off into an independent company in 2008, and finally acquired by Quest Diagnostic ...
led to the sequencing of the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual Mitochondrial DNA, mitochondria. These are usually treated s ...
in 2003.


Features of inheritance


Discrete inheritance and Mendel's laws

At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanin ...
s, from parents to offspring. This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in
pea The pea is most commonly the small spherical seed or the seed-pod of the flowering plant species ''Pisum sativum''. Each pod contains several peas, which can be green or yellow. Botanically, pea pods are fruit, since they contain seeds and d ...
plants, showing for example that flowers on a single plant were either purple or white—but never an intermediate between the two colors. The discrete versions of the same gene controlling the inherited appearance (phenotypes) are called
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the Locus (genetics), same place on a long DNA molecule, as described in leading textbooks on genetics and evoluti ...
s. In the case of the pea, which is a
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
species, each individual plant has two copies of each gene, one copy inherited from each parent. Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a given gene are called
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
at that gene locus, while organisms with two different alleles of a given gene are called
heterozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
. The set of alleles for a given organism is called its
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleot ...
, while the observable traits of the organism are called its
phenotype In genetics, the phenotype () is the set of observable characteristics or phenotypic trait, traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, dev ...
. When organisms are heterozygous at a gene, often one allele is called dominant as its qualities dominate the phenotype of the organism, while the other allele is called
recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The first ...
as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have
incomplete dominance In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The first ...
by expressing an intermediate phenotype, or
codominance In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The first ...
by expressing both alleles at once. When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation. However, the probability of getting one gene over the other can change due to dominant, recessive, homozygous, or heterozygous genes. For example, Mendel found that if you cross homozygous dominate trait and homozygous recessive trait your odds of getting the dominant trait is 3:1. Real geneticist study and calculate probabilities by using theoretical probabilities, empirical probabilities, the product rule, the sum rule, and more.


Notation and diagrams

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual, non-mutant allele for a gene. In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the
Punnett square The Punnett square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is named after Reginald Punnett, Reginald C. Punnett, who devised the approach in 1905. The diagram is used by biologi ...
. When studying human genetic diseases, geneticists often use
pedigree chart A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses. Definition The word pedigree ...
s to represent the inheritance of traits. These charts map the inheritance of a trait in a family tree.


Multiple gene interactions

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as " Mendel's second law" or the "law of independent assortment," means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. Different genes often interact to influence the same trait. In the Blue-eyed Mary (''Omphalodes verna''), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are white—regardless of whether the first gene has blue or magenta alleles. This interaction between genes is called
epistasis Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is dep ...
, with the second gene epistatic to the first. Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and
skin color Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in pigment A pigment is a colored material that is completely or nearly Solubility, insoluble in water ...
). These
complex traits Complex traits, also known as quantitative traits, are traits that do not behave according to simple Mendelian inheritance laws. More specifically, their inheritance cannot be explained by the genetic segregation of a single gene. Such traits show ...
are products of many genes. The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called
heritability Heritability is a statistic used in the fields of Animal husbandry, breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. T ...
. Measurement of the heritability of a trait is relative—in a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and
health care Health care or healthcare is the improvement of health via the prevention, diagnosis, treatment, amelioration or cure A cure is a substance or procedure that ends a medical condition, such as a medication, a surgery, surgical operation, ...
, height has a heritability of only 62%.


Molecular basis for inheritance


DNA and chromosomes

The
molecular A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
basis for genes is deoxyribonucleic acid (DNA). DNA is composed of
deoxyribose Deoxyribose, or more precisely 2-deoxyribose, is a monosaccharide with idealized formula H−(C=O)−(CH2)−(CHOH)3−H. Its name indicates that it is a deoxy sugar, meaning that it is derived from the sugar ribose by loss of a hydroxy group. Di ...
(sugar molecule), a phosphate group, and a base (amine group). There are four types of bases:
adenine Adenine () (nucleoside#List of nucleosides and corresponding nucleobases, symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. Th ...
(A),
cytosine Cytosine () (nucleoside#List of nucleosides and corresponding nucleobases, symbol C or Cyt) is one of the four Nucleobase, nucleobases found in DNA and RNA, along with adenine, guanine, and thymine (uracil in RNA). It is a pyrimidine derivative, ...
(C),
guanine Guanine () (symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between othe ...
(G), and
thymine Thymine () (nucleoside#List of nucleosides and corresponding nucleobases, symbol T or Thy) is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine ...
(T). The phosphates make hydrogen bonds with the sugars to make long phosphate-sugar backbones. Bases specifically pair together (T&A, C&G) between two backbones and make like rungs on a ladder. The bases, phosphates, and sugars together make a
nucleotide Nucleotides are Organic compound, organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential ...
that connects to make long chains of DNA. Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain. These chains coil into a double a-helix structure and wrap around proteins called
Histones In biology, histones are highly Base (chemistry), basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosom ...
which provide the structural support. DNA wrapped around these histones are called chromosomes.
Virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es sometimes use the similar molecule
RNA Ribonucleic acid (RNA) is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical substance, substance or material consisting of very large molecules called macromolecules, composed of many ...
instead of DNA as their genetic material. DNA normally exists as a double-stranded molecule, coiled into the shape of a
double helix A double is a look-alike or doppelgänger; one person or being that resembles another. Double, The Double or Dubble may also refer to: Film and television * Double (filmmaking), someone who substitutes for the credited actor of a character * Th ...
. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand. Genes are arranged linearly along long chains of DNA base-pair sequences. In
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
, each cell usually contains a single circular
genophore The nucleoid (meaning ''cell nucleus, nucleus-like'') is an irregularly shaped region within the prokaryotic, prokaryotic cell that contains all or most of the genome, genetic material. The chromosome of a prokaryote is circular chromosome, circula ...
, while
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million
base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both ...
s in length. The DNA of a chromosome is associated with structural proteins that organize, compact, and control access to the DNA, forming a material called
chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important ...
; in eukaryotes, chromatin is usually composed of
nucleosome A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone, histone proteins and resembles thread wrapped around a spool. The nucleosome is the f ...
s, segments of DNA wound around cores of
histone In biology, histones are highly Base (chemistry), basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei. They act as spools around which DNA winds to create structural units called nucleosomes. Nucleosom ...
proteins. The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding gene ...
. DNA is most often found in the nucleus of cells, but Ruth Sager helped in the discovery of nonchromosomal genes found outside of the nucleus. In plants, these are often found in the chloroplasts and in other organisms, in the mitochondria. These nonchromosomal genes can still be passed on by either partner in sexual reproduction and they control a variety of hereditary characteristics that replicate and remain active throughout generations. While
haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
organisms have only one copy of each chromosome, most animals and many plants are
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
, containing two of each chromosome and thus two copies of every gene. The two alleles for a gene are located on identical loci of the two
homologous chromosomes A couple of homologous chromosomes, or homologs, are a set of one maternal and one paternal chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DN ...
, each allele inherited from a different parent. Many species have so-called
sex chromosome A sex chromosome (also referred to as an allosome, heterotypical chromosome, gonosome, heterochromosome, or idiochromosome) is a chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In m ...
s that determine the sex of each organism. In humans and many other animals, the
Y chromosome The Y chromosome is one of two sex chromosomes ( allosomes) in therian mammals, including humans, and many other animals. The other is the X chromosome. Y is normally the sex-determining chromosome in many species In biology, a species ...
contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the
X chromosome The X chromosome is one of the two sex-determining chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging protei ...
is similar to the other chromosomes and contains many genes. This being said, Mary Frances Lyon discovered that there is X-chromosome inactivation during reproduction to avoid passing on twice as many genes to the offspring. Lyon's discovery led to the discovery of X-linked diseases.


Reproduction

When cells divide, their full genome is copied and each
daughter cell Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...
inherits one copy. This process, called
mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...
, is the simplest form of reproduction and is the basis for asexual reproduction. Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called
clones Clone or Clones or Cloning or Cloned or The Clone may refer to: Places * Clones, County Fermanagh * Clones, County Monaghan, a town in Ireland Biology * Clone (B-cell), a lymphocyte clone, the massive presence of which may indicate a pathologi ...
.
Eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome (
haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
) and double copies (
diploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Sets of chromosomes refer to the number of mat ...
). Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell
gamete A gamete (; , ultimately ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also r ...
s such as
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...
or
eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
. Although they do not use the haploid/diploid method of sexual reproduction,
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
have many methods of acquiring new genetic information. Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium. Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation. These processes result in
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
, transmitting fragments of genetic information between organisms that would be otherwise unrelated. Natural bacterial transformation occurs in many
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometre The micrometre (Amer ...
l species, and can be regarded as a sexual process for transferring DNA from one cell to another cell (usually of the same species). Transformation requires the action of numerous bacterial
gene product A gene product is the biochemical material, either RNA or protein, resulting from Gene expression, expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product c ...
s, and its primary adaptive function appears to be
repair The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery A machine is a physical system using Power (physics), power to apply Force, forces and control ...
of DNA damages in the recipient cell.


Recombination and genetic linkage

The diploid nature of chromosomes allows for genes on different chromosomes to assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do, via the cellular process of
chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid repro ...
. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes. This process of chromosomal crossover generally occurs during
meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
, a series of cell divisions that creates haploid cells. Meiotic recombination, particularly in microbial
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s, appears to serve the adaptive function of repair of DNA damages. The first cytological demonstration of crossing over was performed by Harriet Creighton and
Barbara McClintock Barbara McClintock (June 16, 1902 – September 2, 1992) was an American scientist and cytogenetics, cytogeneticist who was awarded the 1983 Nobel Prize in Physiology or Medicine. McClintock received her PhD in botany from Cornell University in ...
in 1931. Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other. The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated. For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.


Gene expression


Genetic code

Genes generally
express Express or EXPRESS may refer to: Arts, entertainment, and media Films * ''Express: Aisle to Glory'', a 1998 comedy short film featuring Kal Penn * ''The Express: The Ernie Davis Story'', a 2008 film starring Dennis Quaid Music * Express (album) ...
their functional effect through the production of proteins, molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each composed of a sequence of
amino acid Amino acids are organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon chemical bond, bonds. Due to carbon's ability to Catenation, catenate (form chains with ot ...
s. The DNA sequence of a gene is used to produce a specific
amino acid sequence Protein primary structure is the Biomolecular structure#Primary structure, linear sequence of amino acids in a peptide or protein. By convention, the Protein structure#Primary structure, primary structure of a protein is reported starting from the ...
. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called
transcription Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including: Genetics * Transcription (biology), the copying of DNA into RNA, the fir ...
. This
messenger RNA In molecular biology, messenger ribonucleic acid (mRNA) is a single-stranded molecule of RNA that corresponds to the genetic sequence of a gene, and is read by a ribosome in the process of Protein biosynthesis, synthesizing a protein. mRNA is ...
molecule then serves to produce a corresponding amino acid sequence through a process called
translation Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
. Each group of three nucleotides in the sequence, called a
codon The genetic code is the set of rules used by living cell (biology), cells to Translation (biology), translate information encoded within genetic material (DNA or RNA sequences of nucleotide triplets, or codons) into proteins. Translation is accom ...
, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the
genetic code The genetic code is the set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets, or codons) into protein Proteins are large biomolecules and macromolecules ...
. The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNA—a phenomenon
Francis Crick Francis Harry Compton Crick (8 June 1916 – 28 July 2004) was an English molecular biology, molecular biologist, biophysics, biophysicist, and neuroscientist. He, James Watson, Rosalind Franklin, and Maurice Wilkins played crucial roles in de ...
called the
central dogma of molecular biology The central dogma of molecular biology is an explanation of the flow of genetic information within a biological system. It is often stated as "DNA makes RNA, and RNA makes protein", although this is not its original meaning. It was first stated by ...
. The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions. Some are simple structural molecules, like the fibers formed by the protein
collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...
. Proteins can bind to other proteins and simple molecules, sometimes acting as
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
s by facilitating
chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood. A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example,
sickle-cell anemia Sickle cell disease (SCD) is a group of blood disorder Hematologic diseases are disorders which primarily affect the blood & Blood formation, blood-forming organs. Hematologic diseases include rare genetic disorders, anemia, HIV, sickle cell ...
is a human
genetic disease A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
that results from a single base difference within the
coding region The coding region of a gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can ...
for the β-globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties. Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...
s carrying the protein. These sickle-shaped cells no longer flow smoothly through
blood vessel The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide aw ...
s, having a tendency to clog or degrade, causing the medical problems associated with this disease. Some DNA sequences are transcribed into RNA but are not translated into protein products—such RNA molecules are called
non-coding RNA A non-coding RNA (ncRNA) is a functional RNA molecule that is not translated into a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins ...
. In some cases, these products fold into structures which are involved in critical cell functions (e.g.
ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...
and
transfer RNA Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the Messenger RNA, mRNA a ...
). RNA can also have regulatory effects through hybridization interactions with other RNA molecules (such as
microRNA MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. ...
). The genetic code is a dictionary that matches together the amino acid and nucleotide sequences called add-ons. 64 genetic codons are there in which every codon has 3 bases. In 64 codons,20 amino acids are coded by 61 codons which are found in proteins and 3 codons don’t code for any amino acid. There are different types of codons. The codons that code for amino acids are called Sense codons and the codons that code for protein synthesis are called Signal codons. Signal codons are of two types which are Terminating codons and Initiating codons. UAA UAG UGA is termed as termination codons or also called nonsense codons. AUG is called an initiation codon used to code for the first amino acids in every protein. During the translation process the t-RNA base sequence pairs with the codon of m RNA which is known as an Anticodon. The difference between codon and anticodon is that codon is present not only in DNA but in RNA, whereas anticodon is present only in RNA but not in DNA. Codons will be directed from 5’ end to 3’ end in the same way anticodons are directed in the opposite way i.e., 3’ end to 5’ end. In some t RNA molecules, anticodons must pair with more than one codon. The arrangement of codons is sequence manner based while an arrangement of anticodons is discretely present in cells with amino acids.


Nature and nurture

Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. The phrase "
nature and nurture Nature versus nurture is a long-standing debate in biology and society about the balance between two competing factors which determine fate: genetics (nature) and environment (nurture). The alliterative expression "nature and nurture" in English h ...
" refers to this complementary relationship. The phenotype of an organism depends on the interaction of genes and the environment. An interesting example is the coat coloration of the
Siamese cat The Siamese cat ( th, แมวไทย, Maeo Thai; แมวสยาม, Maeo Seeaam) is one of the first distinctly recognized Cat breed, breeds of Asian cat. Derived from the Wichianmat landrace, one of several varieties of cat native to ...
. In this case, the body temperature of the cat plays the role of the environment. The cat's genes code for dark hair, thus the hair-producing cells in the cat make cellular proteins resulting in dark hair. But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. The protein remains functional in areas of skin that are colder—such as its legs, ears, tail, and faceso the cat has dark hair at its extremities. Environment plays a major role in effects of the human genetic disease
phenylketonuria Phenylketonuria (PKU) is an inborn error of metabolism that results in decreased metabolism of the amino acid phenylalanine. Untreated PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. It may also resu ...
. The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid
phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of al ...
, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive intellectual disability and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy. A common method for determining how genes and environment ("nature and nurture") contribute to a phenotype involves studying identical and fraternal twins, or other siblings of
multiple birth A multiple birth is the culmination of one multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such bir ...
s. Identical siblings are genetically the same since they come from the same zygote. Meanwhile, fraternal twins are as genetically different from one another as normal siblings. By comparing how often a certain disorder occurs in a pair of identical twins to how often it occurs in a pair of fraternal twins, scientists can determine whether that disorder is caused by genetic or postnatal environmental factors. One famous example involved the study of the Genain quadruplets, who were identical quadruplets all diagnosed with
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...
.


Gene regulation

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell.
Transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding to ...
s are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene. Within the genome of ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaer ...
'' bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid
tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an Alpha_and_beta_carbon , α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with ...
. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genes—tryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating
negative feedback Negative feedback (or balancing feedback) occurs when some function (Mathematics), function of the output of a system, process, or mechanism is feedback, fed back in a manner that tends to reduce the fluctuations in the output, whether caused by ...
regulation of the tryptophan synthesis process. Differences in gene expression are especially clear within
multicellular organism A multicellular organism is an organism that consists of more than one cell (biology), cell, in contrast to unicellular organism. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few ...
s, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells. Within
eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
s, there exist structural features of
chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important ...
that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells. These features are called "
epigenetic In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' imp ...
" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types grown within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.


Genetic change


Mutations

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can affect the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low—1 error in every 10–100 million bases—due to the "proofreading" ability of
DNA polymerase A DNA polymerase is a member of a family of enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the e ...
s. Processes that increase the rate of changes in DNA are called
mutagenic In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material Nucleic acids are biopolymers, macromolecules, essential to all Organism, known forms of life. They are composed of nucleotides, which are the mono ...
: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while
UV radiation Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
induces mutations by causing damage to the DNA structure. Chemical damage to DNA occurs naturally as well and cells use
DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolism, metabolic activities and environmental factors such as r ...
mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence. A particularly important source of DNA damages appears to be
reactive oxygen species In chemistry, reactive oxygen species (ROS) are highly Reactivity (chemistry), reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen. The reduction of ...
produced by cellular aerobic respiration, and these can lead to mutations. In organisms that use
chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, life cycle in which a gamete (haploid repro ...
to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations. Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence— duplications, inversions, deletions of entire regions—or the accidental exchange of whole parts of sequences between different chromosomes,
chromosomal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...
. GENETIC MUTATION A highly pathogenic, more or less permanent alteration to the genetic code (genome) of a virus or a cell in a living creature that can be handed down to the progeny of the original cell or virus. A somatic mutation is a change in a multicellular organism's DNA that can progress to progeny by DNA replication. Alterations can happen as a result of subjection to electromagnetic spectrum with high intensity (such as X-rays, ultraviolet light), mishaps that can place during the normal chemical transactions of DNA, most frequently during replication. The bulk of variations are expected to be harmful because they are random changes, however, certain mutations might be helpful in certain circumstances. Types of mutations Mutations and perhaps other gene changes can be inherited or acquired. An inherited gene mutation is one that, as its name suggests, is passed down from one parent to the next. As a result, it can be found in the very first cell that develops into a human after the egg cell and sperm cell have mated. Because it began in the first cell, which gave rise to all the other cells in the body, this modification is present in every cell in the body and can be passed on to the next generation. Because the cells that give rise to eggs and sperm are known as germ cells, also known as a hereditary alteration. A gene mutation that is acquired does not come from the parent. As opposed to that, it emerges at some point in a person's life. Acquired mutations start in one cell and spread to any subsequent cells that develop from that cell. Because this mutation does not affect sperm or egg cells, it cannot be passed down to a person's offspring. Somatic mutation or spontaneous mutation are other names for this kind of mutation.


Natural selection and evolution

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive fitness. Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial. Studies in the fly ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
'' suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations are harmful with the remainder being either neutral or weakly beneficial.
Population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...
studies the distribution of genetic differences within populations and how these distributions change over time. Changes in the frequency of an allele in a population are mainly influenced by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable Phenotypic trait, traits characteristic of a populati ...
, where a given allele provides a selective or reproductive advantage to the organism, as well as other factors such as
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
,
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear co ...
,
genetic hitchhiking Genetic may refer to: *Genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Mora ...
,
artificial selection Selective breeding (also called artificial selection) is the process by which humans use animal breeding and plant breeding to Selection (biology), selectively develop particular phenotypic traits (characteristics) by choosing which typically ...
and
migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
. Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a stat ...
, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.
Earlier related ideas were acknowledged in
New species are formed through the process of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
, often caused by geographical separations that prevent populations from exchanging genes with each other. By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form
evolutionary tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree (graph theory), tree showing the evolutionary relationships among va ...
s; these trees represent the
common descent Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...
and most common in bacteria).


Model organisms

Although geneticists originally studied inheritance in a wide variety of organisms, the range of species studied has narrowed. One reason is that when significant research already exists for a given organism, new researchers are more likely to choose it for further study, and so eventually a few
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biology, biological phenomena, with the expectation that discoveries made in the model organism will provide insight into th ...
s became the basis for most genetics research. Common research topics in model organism genetics include the study of
gene regulation Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are wide ...
and the involvement of genes in development and
cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
. Organisms were chosen, in part, for convenience—short generation times and easy
genetic manipulation Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the ...
made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaer ...
'', the plant ''
Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter an ...
'', baker's yeast (''
Saccharomyces cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...
''), the nematode ''
Caenorhabditis elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a Hybrid word, blend of the Greek ''caeno-'' (recent), ''rh ...
'', the common fruit fly (''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (the taxonomic order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly" or "pomace fly". Starting with Ch ...
''), the zebrafish (''
Danio rerio The zebrafish (''Danio rerio'') is a freshwater fish Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 1.05%. These environments differ from marine conditio ...
''), and the common house mouse (''
Mus musculus Mus or MUS may refer to: Abbreviations * MUS, the NATO country code for Mauritius Mauritius ( ; french: Maurice, link=no ; mfe, label=Mauritian Creole, Moris ), officially the Republic of Mauritius, is an island nation in the Indian Oce ...
'').


Medicine

Medical genetics Medical genetics is the branch tics in that human genetics is a field of scientific research that may or may not apply to medicine, while medical genetics refers to the application of genetics to medical care. For example, research on the caus ...
seeks to understand how genetic variation relates to human health and disease. When searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic
pedigree chart A pedigree chart is a diagram that shows the occurrence and appearance of phenotypes of a particular gene or organism and its ancestors from one generation to the next, most commonly humans, show dogs, and race horses. Definition The word pedigree ...
s to find the location on the genome associated with the disease. At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a method especially useful for multigenic traits not clearly defined by a single gene. Once a candidate gene is found, further research is often done on the corresponding (or homologous) genes of model organisms. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of
pharmacogenetics Pharmacogenomics is the study of the role of the genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nucl ...
: the study of how genotype can affect drug responses. Individuals differ in their inherited tendency to develop
cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
, and cancer is a genetic disease. The process of cancer development in the body is a combination of events. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger
cell death Cell death is the event of a biological cell ceasing to carry out its functions. This may be the result of the natural process of old cells dying and being replaced by new ones, as in programmed cell death, or may result from factors such as dis ...
, but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable Phenotypic trait, traits characteristic of a populati ...
occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous
tumor A neoplasm () is a type of abnormal and excessive growth of tissue (biology), tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tiss ...
that grows and invades various tissues of the body. Normally, a cell divides only in response to signals called
growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it is a secreted protein or a steroid hormone. Growth factors are important for regul ...
s and stops growing once in contact with surrounding cells and in response to growth-inhibitory signals. It usually then divides a limited number of times and dies, staying within the
epithelium Epithelium or epithelial tissue is one of the four basic types of animal Tissue (biology), tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed Cell (biology), ...
where it is unable to migrate to other organs. To become a cancer cell, a cell has to accumulate mutations in a number of genes (three to seven). A cancer cell can divide without growth factor and ignores inhibitory signals. Also, it is immortal and can grow indefinitely, even after it makes contact with neighboring cells. It may escape from the epithelium and ultimately from the
primary tumor A primary tumor is a tumor growing at the anatomical site where tumor progression began and proceeded to yield a cancerous mass. Most cancers develop at their primary site but then go on to metastasize or spread to other parts of the body. These ...
. Then, the escaped cell can cross the endothelium of a blood vessel and get transported by the bloodstream to colonize a new organ, forming deadly
metastasis Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...
. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny (
somatic mutation A somatic mutation is a change in the Nucleic acid sequence, DNA sequence of a somatic cell of a multicellular organism with dedicated Germline, reproductive cells; that is, any mutation that occurs in a cell other than a gamete, germ cell, or ga ...
s). The most frequent mutations are a loss of function of p53 protein, a
tumor suppressor A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or ...
, or in the p53 pathway, and gain of function mutations in the Ras proteins, or in other
oncogene An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or Gene expression, expressed at high levels.
s.


Research methods

DNA can be manipulated in the laboratory.
Restriction enzymes A restriction enzyme, restriction endonuclease, REase, ENase or'' restrictase '' is an enzyme that cleaves DNA into fragments at or near specific recognition sites within molecules known as restriction sites. Restriction enzymes are one class of ...
are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA. DNA fragments can be visualized through use of
gel electrophoresis Gel electrophoresis is a method for separation and analysis of biomolecule, biomacromolecules (DNA, RNA, proteins, etc.) and their fragments, based on their size and charge. It is used in clinical chemistry to separate proteins by charge or ...
, which separates fragments according to their length. The use of ligation enzymes allows DNA fragments to be connected. By binding ("ligating") fragments of DNA together from different sources, researchers can create
recombinant DNA Recombinant DNA (rDNA) molecules are DNA molecules formed by laboratory methods of genetic recombination (such as molecular cloning) that bring together genetic material from multiple sources, creating DNA sequence, sequences that would not othe ...
, the DNA often associated with
genetically modified organism A genetically modified organism (GMO) is any organism whose genetic material has been altered using genetic engineering techniques. The exact definition of a genetically modified organism and what constitutes genetic engineering varies, with ...
s. Recombinant DNA is commonly used in the context of
plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from gDNA, chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; ...
s: short circular DNA molecules with a few genes on them. In the process known as
molecular cloning Molecular cloning is a set of experimental methods in molecular biology that are used to assemble recombinant DNA molecules and to direct their DNA replication, replication within Host (biology), host organisms. The use of the word ''cloning'' re ...
, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate clones of bacteria cells). "Cloning" can also refer to the various means of creating cloned ("clonal") organisms. DNA can also be amplified using a procedure called the
polymerase chain reaction The polymerase chain reaction (PCR) is a method widely used to rapidly make millions to billions of copies (complete or partial) of a specific DNA sample, allowing scientists to take a very small sample of DNA and amplify it (or a part of it) t ...
(PCR). By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.


DNA sequencing and genomics

DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of chain-termination sequencing, developed in 1977 by a team led by
Frederick Sanger Frederick Sanger (; 13 August 1918 – 19 November 2013) was an English biochemist who received the Nobel Prize in Chemistry twice. He won the 1958 Chemistry Prize for determining the amino acid sequence of insulin and numerous other pr ...
, is still routinely used to sequence DNA fragments. Using this technology, researchers have been able to study the molecular sequences associated with many human diseases. As sequencing has become less expensive, researchers have sequenced the genomes of many organisms using a process called
genome assembly In bioinformatics, sequence assembly refers to sequence alignment, aligning and merging fragments from a longer DNA sequence in order to reconstruct the original sequence. This is needed as DNA sequencing technology might not be able to 'read' who ...
, which uses computational tools to stitch together sequences from many different fragments. These technologies were used to sequence the human genome in the Human Genome Project completed in 2003. New high-throughput sequencing technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars.
Next-generation sequencing Massive parallel sequencing or massively parallel sequencing is any of several high-throughput approaches to DNA sequencing using the concept of massively parallel processing; it is also called next-generation sequencing (NGS) or second-generation s ...
(or high-throughput sequencing) came about due to the ever-increasing demand for low-cost sequencing. These sequencing technologies allow the production of potentially millions of sequences concurrently. The large amount of sequence data available has created the subfield of
genomics Genomics is an interdisciplinary field of biology focusing on the structure, function, evolution, mapping, and editing of genomes. A genome is an organism's complete set of DNA, including all of its genes as well as its hierarchical, three-dime ...
, research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of
bioinformatics Bioinformatics () is an interdisciplinary field that develops methods and software tools for understanding biological data, in particular when the data sets are large and complex. As an interdisciplinary field of science, bioinformatics combi ...
, which uses computational approaches to analyze large sets of
biological data Biological data refers to a compound or information derived from living organisms and their products. A medicinal compound made from living organisms, such as a serum or a vaccine, could be characterized as biological data. Biological data is highly ...
. A common problem to these fields of research is how to manage and share data that deals with human subject and
personally identifiable information Personal data, also known as personal information or personally identifiable information (PII), is any information related to an identifiable person. The abbreviation PII is widely accepted in the United States The United States of Amer ...
.


Society and culture

On 19 March 2015, a group of leading biologists urged a worldwide ban on clinical use of methods, particularly the use of
CRISPR CRISPR () (an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryote, prokaryotic organisms such as bacteria and archaea. These sequences are derived from DNA fragmen ...
and
zinc finger A zinc finger is a small protein structural motif that is characterized by the Coordination complex, coordination of one or more zinc ions (Zn2+) in order to stabilize the fold. It was originally coined to describe the finger-like appearance of ...
, to edit the human genome in a way that can be inherited. In April 2015, Chinese researchers reported results of
basic research Basic research, also called pure research or fundamental research, is a type of scientific research with the aim of improving scientific theory, theories for better understanding and prediction of natural or other phenomena. In contrast, applied ...
to edit the DNA of non-viable human embryos using CRISPR.


See also

* Bacterial genome size *
Cryoconservation of animal genetic resources Cryoconservation of animal genetic resources is a strategy wherein samples of animal genetic materials are preserved cryogenically."Cryoconservation of Animal Genetic Resources", Rep. Rome: Food and Agriculture Organization of the United Nations ...
*
Eugenics Eugenics ( ; ) is a Fringe science, fringe set of beliefs and practices that aim to improve the genetics, genetic quality of a human population. Historically, eugenicists have attempted to alter human gene pools by excluding people and groups ...
*
Embryology Embryology (from Ancient Greek, Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logy, -logia'') is the branch of animal biology that studies the Prenatal development (biology), prenatal development of gametes (sex ...
*
Genetic disorder A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
*
Genetic diversity Genetic diversity is the total number of Genetics, genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species and can be attributed to the span of survival for a species. ...
*
Genetic engineering Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of Genetic engineering techniques, technologies used to change the gene ...
* Genetic enhancement * Glossary of genetics (M−Z) * Index of genetics articles *
Medical genetics Medical genetics is the branch tics in that human genetics is a field of scientific research that may or may not apply to medicine, while medical genetics refers to the application of genetics to medical care. For example, research on the caus ...
* Molecular tools for gene study * Neuroepigenetics * Outline of genetics * Timeline of the history of genetics * Plant genetic resources


References


Further reading

* * * * *


External links

* * * * * {{Authority control
Genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinians, Augustinian fr ...