Chromosome 21 is one of the 23 pairs of

chromosomes A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

s. Chromosome 21 is both the smallest human

autosome An autosome is any chromosome that is not a sex chromosome. The members of an autosome pair in a diploid cell have the same morphology, unlike those in allosomal (sex chromosome) pairs, which may have different structures. The DNA in autosomes ...

and

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

, with 46.7 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 DNA ...

s (the building material of

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

) representing about 1.5 percent of the total DNA in

cells Cell most often refers to: * Cell (biology), the functional basic unit of life * Cellphone, a phone connected to a cellular network * Clandestine cell, a penetration-resistant form of a secret or outlawed organization * Electrochemical cell, a d ...

. Most people have two copies of chromosome 21, while those with three copies of chromosome 21 (trisomy 21) have Down syndrome. Researchers working on 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, mapping and sequencing all of the genes of the human genome from both a ...

announced in May 2000 that they had determined the sequence of

s that make up this chromosome. Chromosome 21 was the second human chromosome to be fully sequenced, after

chromosome 22 Chromosome 22 is one of the 23 pairs of chromosomes in human cells. Humans normally have two copies of chromosome 22 in each cell. Chromosome 22 is the second smallest human chromosome, spanning about 51 million DNA base pairs and representing b ...

Genes

Number of genes

The following are some of the gene count estimates of human chromosome 21. Because researchers use different approaches to

genome annotation In molecular biology and genetics, DNA annotation or genome annotation is the process of describing the structure and function of the components of a genome, by analyzing and interpreting them in order to extract their biological significance and ...

, their predictions of the

number of genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: prote ...

on each chromosome varies (for technical details, see

gene prediction In computational biology, gene prediction or gene finding refers to the process of identifying the regions of genomic DNA that encode genes. This includes protein-coding genes as well as RNA genes, but may also include prediction of other functio ...

). Among various projects, the collaborative consensus coding sequence project ( CCDS) takes an extremely conservative strategy. Thus CCDS's gene number prediction represents a lower bound on the total number of human protein-coding genes.

Gene list

The following is a partial list of genes on human chromosome 21. For complete list, see the link in the infobox at the top of the article. In addition, the chromosome has many genes for keratin-associated protein, with symbols: KRTAP6-1, KRTAP6-2, KRTAP6-3, KRTAP7-1, KRTAP8-1, KRTAP10-1, KRTAP10-2, KRTAP10-3, KRTAP10-4, KRTAP10-5, KRTAP10-6, KRTAP10-7, KRTAP10-8, KRTAP10-9, KRTAP10-10, KRTAP10-11, KRTAP10-12, KRTAP11-1, KRTAP12-1, KRTAP12-1, KRTAP12-2, KRTAP12-3, KRTAP12-4, KRTAP13-1, KRTAP13-2, KRTAP13-3, KRTAP13-4, KRTAP15-1, KRTAP19-1, KRTAP19-2, KRTAP19-3, KRTAP19-4, KRTAP19-5, KRTAP19-6, KRTAP19-7, KRTAP19-8, KRTAP20-1, KRTAP20-2, KRTAP20-3, KRTAP20-4, KRTAP21-1, KRTAP21-2, KRTAP21-3, KRTAP22-1, KRTAP22-2, KRTAP23-1, KRTAP24-1, KRTAP25-1, KRTAP26-1, KRTAP27-1.

Diseases and disorders

The following diseases and disorders are some of those related to genes on chromosome 21: *

Acute myeloid leukemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with haematopoiesis, normal blood cell production. Sympt ...

Alzheimer's disease Alzheimer's disease (AD) is a neurodegenerative disease and the cause of 60–70% of cases of dementia. The most common early symptom is difficulty in remembering recent events. As the disease advances, symptoms can include problems wit ...

Amyotrophic lateral sclerosis Amyotrophic lateral sclerosis (ALS), also known as motor neuron disease (MND) or—in the United States—Lou Gehrig's disease (LGD), is a rare, Terminal illness, terminal neurodegenerative disease, neurodegenerative disorder that results i ...

Atrial fibrillation Atrial fibrillation (AF, AFib or A-fib) is an Heart arrhythmia, abnormal heart rhythm (arrhythmia) characterized by fibrillation, rapid and irregular beating of the Atrium (heart), atrial chambers of the heart. It often begins as short periods ...

, familial *

Autoimmune polyendocrine syndrome type 1 Autoimmune polyendocrine syndrome type 1 (APS-1), is a subtype of autoimmune polyendocrine syndrome (autoimmune polyglandular syndrome). It causes the dysfunction of multiple endocrine glands due to autoimmunity. It is a genetic disorder, inherit ...

* Basal ganglia calcification * Bartsocas–Papas syndrome *

Bethlem myopathy Bethlem myopathy is predominantly an autosomal dominant myopathy, classified as a congenital form of limb-girdle muscular dystrophy. There are two types of Bethlem myopathy, based on which type of collagen is affected. Bethlem myopathy 1 (BTHLM1) ...

Closed angle glaucoma Glaucoma is a group of eye diseases that can lead to damage of the optic nerve. The optic nerve transmits visual information from the eye to the brain. Glaucoma may cause vision loss if left untreated. It has been called the "silent thief of ...

Cataract A cataract is a cloudy area in the lens (anatomy), lens of the eye that leads to a visual impairment, decrease in vision of the eye. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colours, blurry or ...

CHAND syndrome Ankyloblepharon is a medical condition, defined as the adhesion of the edges of the upper eyelid with the lower eyelid. Ankyloblepharon must be differentiated from blepharophimosis, in which palpebral aperture is reduced and there is telecanthus, b ...

* Down syndrome *

Epilepsy Epilepsy is a group of Non-communicable disease, non-communicable Neurological disorder, neurological disorders characterized by a tendency for recurrent, unprovoked Seizure, seizures. A seizure is a sudden burst of abnormal electrical activit ...

Erondu–Cymet syndrome Erondu–Cymet syndrome is a syndrome caused by a translocation on the 21st chromosome. The genetic karyotype for people with this condition is 46, XY, inv(21)(q11.2q22.1). Findings in these patients include hypotension, hypoxemia, seizures, and ...

* Ewing sarcoma * Galloway Mowat syndrome * Glucocorticoid deficiency *

Hepatitis B Hepatitis B is an infectious disease caused by the '' hepatitis B virus'' (HBV) that affects the liver; it is a type of viral hepatitis. It can cause both acute and chronic infection. Many people have no symptoms during an initial infection. ...

(susceptibility to) *

Hereditary motor and sensory neuropathy Hereditary motor and sensory neuropathies (HMSN) is a name sometimes given to a group of different neuropathy, neuropathies which are all characterized by their impact upon both Afferent nerve fiber, afferent and Efferent nerve fiber, efferent neur ...

Holocarboxylase synthetase deficiency Holocarboxylase synthetase deficiency is an inherited metabolic disorder in which the body is unable to use the vitamin biotin effectively. This disorder is classified as a multiple carboxylase deficiency, a group of disorders characterized by ...

Homocystinuria Homocystinuria (HCU) is an inherited disorder of the metabolism of the amino acid methionine due to a deficiency of cystathionine beta synthase or methionine synthase. It is an inherited autosomal recessive trait, which means a child needs to ...

Hyperhomocysteinemia Hyperhomocysteinemia is a medical condition characterized by an abnormally high level of total homocysteine (that is, including homocystine and homocysteine-cysteine disulfide) in the blood, conventionally described as above 15 μmol/L. As ...

Hypotrichosis Hair loss, also known as alopecia or baldness, refers to a loss of hair from part of the head or body. Typically at least the head is involved. The severity of hair loss can vary from a small area to the entire body. Inflammation or scarring ...

Immunodeficiency Immunodeficiency, also known as immunocompromise, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that aff ...

Inflammatory bowel disease Inflammatory bowel disease (IBD) is a group of inflammatory conditions of the colon and small intestine, with Crohn's disease and ulcerative colitis (UC) being the principal types. Crohn's disease affects the small intestine and large intestine ...

Intellectual developmental disorder Intellectual disability (ID), also known as general learning disability (in the United Kingdom), and formerly mental retardation (in the United States), Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010).Archive is a generalized neurodevelopmental d ...

Jervell and Lange-Nielsen syndrome Jervell and Lange-Nielsen syndrome (JLNS) is a rare type of long QT syndrome associated with severe, bilateral sensorineural hearing loss. Those with JLNS are at risk of abnormal heart rhythms called arrhythmias, which can lead to fainting, seizu ...

Keppen–Lubinsky syndrome Keppen–Lubinsky syndrome is an extremely rare congenital disorder. The minimal clinical criteria for the Keppen–Lubinsky syndrome are as follows: normal growth parameters at birth, postnatal growth failure, peculiar face with an aged appearanc ...

Knobloch syndrome Knobloch syndrome is a rare genetic disorder presenting severe eyesight problems and often a defect in the skull. It was named after the ophthalmologist William Hunter Knobloch (1926–2005), who first described the syndrome in 1971. A usual occur ...

Leukocyte adhesion deficiency-1 Leukocyte adhesion deficiency-1 (LAD1) is a rare and often fatal genetic disorder in humans. Signs and symptoms The main sign of the disease is life-threatening, recurrent bacterial or fungal soft tissue infections. These infections are often ap ...

Majewski osteodysplastic primordial dwarfism type II Microcephalic osteodysplastic primordial dwarfism type II (MOPD II) is a form of primordial dwarfism associated with brain and skeletal abnormalities. It was characterized in 1982. MOPD II is listed as a rare disease by the Office of Rare Diseas ...

(MOPD II, or MOPD2) *

Non-small cell lung carcinoma Non-small-cell lung cancer (NSCLC), or non-small-cell lung carcinoma, is any type of epithelial lung cancer other than Small-cell carcinoma#Small-cell lung cancer, small-cell lung cancer (SCLC). NSCLC accounts for about 85% of all lung cancers. ...

Neurodevelopmental disorder Neurodevelopmental disorders are a group of mental conditions negatively affecting the development of the nervous system, which includes the brain and spinal cord. According to the American Psychiatric Association Diagnostic and Statistical Manu ...

Nonsyndromic deafness Nonsyndromic deafness is hearing loss that is not associated with other signs and symptoms. In contrast, syndromic deafness involves hearing loss that occurs with abnormalities in other parts of the body. Nonsyndromic deafness constitutes 75% of a ...

Parkinson disease Parkinson may refer to: *Parkinson (surname) * ''Parkinson'' (TV series), British chat show, presented by Sir Michael Parkinson *Parkinson, Queensland, suburb of Brisbane, Australia *The Parkinsons (fl. early 20th century), American father-and-son ...

Peripheral neuropathy Peripheral neuropathy, often shortened to neuropathy, refers to damage or disease affecting the nerves. Damage to nerves may impair sensation, movement, gland function, and/or organ function depending on which nerve fibers are affected. Neuropa ...

* Phosphofructokinase deficiency *

Primary ciliary dyskinesia Primary ciliary dyskinesia (PCD) is a rare, autosomal recessive genetic ciliopathy, that causes defects in the action of cilia lining the upper and lower respiratory tract, sinuses, Eustachian tube, middle ear, fallopian tube, and flagella of spe ...

Primary immunodeficiency Primary immunodeficiencies are disorders in which part of the body's immune system is missing or does not function normally. To be considered a ''primary'' immunodeficiency (PID), the immune deficiency must be inborn, not caused by secondary facto ...

Primitive neuroectodermal tumor Primitive neuroectodermal tumor is a malignant (cancerous) neural crest tumor. It is a rare tumor, usually occurring in children and young adults under 25 years of age. The overall 5 year survival rate is about 53%. It gets its name because the ...

Prostate cancer Prostate cancer is the neoplasm, uncontrolled growth of cells in the prostate, a gland in the male reproductive system below the bladder. Abnormal growth of the prostate tissue is usually detected through Screening (medicine), screening tests, ...

Romano–Ward syndrome Romano–Ward syndrome is the most common form of congenital long QT syndrome (LQTS), a genetic heart condition that affects the electrical properties of heart muscle cells. Those affected are at risk of abnormal heart rhythms which can lead to fa ...

Spastic quadriplegia Spastic quadriplegia, also known as spastic tetraplegia, is a subset of spastic cerebral palsy that affects all four limbs (both arms and legs). Compared to quadriplegia, spastic tetraplegia is defined by spasticity of the limbs as opposed to s ...

Ullrich congenital muscular dystrophy Ullrich congenital muscular dystrophy (UCMD) is a form of congenital muscular dystrophy. There are two forms: UCMD1 and UCMD2. UCMD1 is associated with variants of type VI collagen, while UCMD2 is associated with variants of type XII collagen. UC ...

Unverricht–Lundborg disease Unverricht–Lundborg disease (abbreviated ULD or EPM1) is the most common form of an uncommon group of Genetics, genetic epilepsy disorders called the progressive myoclonus epilepsies. It is caused due to a mutation in the cystatin B gene (CSTB).J ...

, a form of

progressive myoclonus epilepsy Progressive Myoclonic Epilepsies (PME) are a rare group of inherited neurodegenerative diseases characterized by myoclonus, resistance to treatment, and neurological deterioration. The cause of PME depends largely on the type of PME. Most PMEs are ...

ZTTK syndrome ZTTK syndrome (Zhu-Tokita-Takenouchi-Kim syndrome) is a rare multisystem disease caused in humans by a genetic mutation of the SON (gene), SON gene. Common symptoms include Developmental disability, developmental delay and often light to severe in ...

Chromosomal conditions

The following conditions are caused by changes in the structure or number of copies of chromosome 21: *

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. Po ...

s: Rearrangements (

translocations In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes "balanced" and "unbalanced" translocation, with three main types: "reciprocal", "nonreciprocal" and "Robertsonian" transloc ...

) of genetic material between chromosome 21 and other chromosomes have been associated with several types of cancer. For example,

acute lymphoblastic leukemia Acute lymphoblastic leukemia (ALL) is a cancer of the Lymphocyte, lymphoid line of blood cells characterized by the development of large numbers of lymphoblast, immature lymphocytes. Symptoms may include feeling tired, pale skin color, fever, ...

(a type of blood cancer most often diagnosed in childhood) has been associated with a translocation between chromosomes 12 and 21. Another form of leukemia,

acute myeloid leukemia Acute myeloid leukemia (AML) is a cancer of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with haematopoiesis, normal blood cell production. Sympt ...

, has been associated with a translocation between chromosomes 8 and 21. * In a small percentage of cases, Down syndrome is caused by a rearrangement of chromosomal material between chromosome 21 and another chromosome. As a result, a person has the usual two copies of chromosome 21, plus extra material from chromosome 21 attached to another chromosome. These cases are called translocation Down syndrome. Researchers believe that extra copies of genes on chromosome 21 disrupt the course of normal development, causing the characteristic features of Down syndrome and the increased risk of medical problems associated with this disorder. * Other changes in the number or structure of chromosome 21 can have a variety of effects, including

intellectual disability Intellectual disability (ID), also known as general learning disability (in the United Kingdom), and formerly mental retardation (in the United States), Rosa's Law, Pub. L. 111-256124 Stat. 2643(2010).Archive is a generalized neurodevelopmental ...

, delayed development, and characteristic facial features. In some cases, the signs and symptoms are similar to those of Down syndrome. Changes to chromosome 21 include a missing segment of the chromosome in each cell (partial monosomy 21) and a circular structure called ring chromosome 21. A ring chromosome occurs when both ends of a broken chromosome are reunited. * Duplication in

amyloid precursor protein Amyloid-beta precursor protein (APP) is an integral membrane protein expressed in many biological tissue, tissues and concentrated in the synapses of neurons. It functions as a cell surface receptor and has been implicated as a regulator of s ...

(APP) locus (duplicated segment varies in length but includes APP) on chromosome 21 was found to cause early onset familial

in a French family set and a Dutch family set. Compared to Alzheimer's caused by missense mutations in APP, the frequency of the Alzheimer's caused by APP duplications is significant. All patients that have an extra copy of APP gene due to the locus duplication show Alzheimer's with severe

cerebral amyloid angiopathy Cerebral amyloid angiopathy (CAA) is a form of angiopathy in which amyloid beta peptide deposits in the walls of small to medium blood vessels of the central nervous system and meninges. The term ''congophilic'' is sometimes used because the pres ...

Genes

Number of genes

Gene list

Diseases and disorders

Chromosomal conditions

Cytogenetic band

References

External links