Phenotype

In

genetics

, the phenotype () is the set of observable characteristics or traits of an

organism

. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological properties, its behavior, and the products of behavior. An organism's phenotype results from two basic factors: the

expression

of an organism's genetic code, or its genotype, and the influence of environmental factors. Both factors may interact, further affecting phenotype. When two or more clearly different phenotypes exist in the same population of a species, the species is called polymorphic. A well-documented example of polymorphism is Labrador Retriever coloring; while the coat color depends on many genes, it is clearly seen in the environment as yellow, black, and brown.

Richard Dawkins

in 1978 and then again in his 1982 book ''The Extended Phenotype'' suggested that one can regard bird nests and other built structures such as caddis-fly larva cases and

beaver dam

s as "extended phenotypes".

Wilhelm Johannsen proposed the genotype–phenotype distinction in 1911 to make clear the difference between an organism's

heredity

and what that heredity produces. The distinction resembles that proposed by

August Weismann

(1834–1914), who distinguished between germ plasm (heredity) and somatic cells (the body).

The genotype–phenotype distinction should not be confused with

Francis Crick

's

central dogma of molecular biology

, a statement about the directionality of molecular sequential information flowing from

DNA

to protein, and not the reverse.

Difficulties in definition

Despite its seemingly straightforward definition, the concept of the phenotype has hidden subtleties. It may seem that anything dependent on the genotype is a phenotype, including

molecule

s such as RNA and proteins. Most molecules and structures coded by the genetic material are not visible in the appearance of an organism, yet they are observable (for example by Western blotting) and are thus part of the phenotype; human blood groups are an example. It may seem that this goes beyond the original intentions of the concept with its focus on the (living) organism in itself. Either way, the term phenotype includes inherent traits or characteristics that are observable or traits that can be made visible by some technical procedure. A notable extension to this idea is the presence of "organic molecules" or metabolites that are generated by organisms from chemical reactions of enzymes.

The term "phenotype" has sometimes been incorrectly used as a shorthand for phenotypic difference from wild type, yielding the statement that a
"mutation has no phenotype".

Another extension adds behavior to the phenotype, since behaviors are observable characteristics. Behavioral phenotypes include cognitive, personality, and behavioral patterns. Some behavioral phenotypes may characterize psychiatric disorders or syndromes.

Phenotypic variation

Phenotypic variation (due to underlying heritable genetic variation) is a fundamental prerequisite for

evolution

by natural selection. It is the living organism as a whole that contributes (or not) to the next generation, so natural selection affects the genetic structure of a population indirectly via the contribution of phenotypes. Without phenotypic variation, there would be no evolution by natural selection.

The interaction between genotype and phenotype has often been conceptualized by the following relationship:

:genotype (G) + environment (E) → phenotype (P)

A more nuanced version of the relationship is:

:genotype (G) + environment (E) + genotype & environment interactions (GE) → phenotype (P)

Genotypes often have much flexibility in the modification and expression of phenotypes; in many organisms these phenotypes are very different under varying environmental conditions (see ecophenotypic variation). The plant '' Hieracium umbellatum'' is found growing in two different

habitat

s in Sweden. One habitat is rocky, sea-side

cliff

s, where the plants are bushy with broad leaves and expanded inflorescences; the other is among

sand dunes

where the plants grow prostrate with narrow leaves and compact inflorescences. These habitats alternate along the coast of Sweden and the habitat that the seeds of ''Hieracium umbellatum'' land in, determine the phenotype that grows.

An example of random variation in ''

Drosophila

'' flies is the number of

ommatidia

, which may vary (randomly) between left and right eyes in a single individual as much as they do between different genotypes overall, or between

clones

raised in different environments.

The concept of phenotype can be extended to variations below the level of the gene that affect an organism's fitness. For example, silent mutations that do not change the corresponding amino acid sequence of a gene may change the frequency of

guanine

-

cytosine

base pairs (GC content). These base pairs have a higher thermal stability (''melting point'') than

adenine

-

thymine

, a property that might convey, among organisms living in high-temperature environments, a selective advantage on variants enriched in GC content.

The extended phenotype

Richard Dawkins

described a phenotype that included all effects that a gene has on its surroundings, including other organisms, as an extended phenotype, arguing that "An animal's behavior tends to maximize the survival of the genes 'for' that behavior, whether or not those genes happen to be in the body of the particular animal performing it." For instance, an organism such as a

beaver

modifies its environment by building a

beaver dam

; this can be considered an expression of its

gene

s, just as its incisor teeth are—which it uses to modify its environment. Similarly, when a bird feeds a brood parasite such as a

cuckoo

, it is unwittingly extending its phenotype; and when genes in an

orchid

affect orchid bee behavior to increase pollination, or when genes in a

peacock

affect the copulatory decisions of peahens, again, the phenotype is being extended. Genes are, in Dawkins's view, selected by their phenotypic effects.

Other biologists broadly agree that the extended phenotype concept is relevant, but consider that its role is largely explanatory, rather than assisting in the design of experimental tests.

Phenome and phenomics

Although a phenotype is the ensemble of observable characteristics displayed by an organism, the word '' phenome'' is sometimes used to refer to a collection of traits, while the simultaneous study of such a collection is referred to as '' phenomics''. Phenomics is an important field of study because it can be used to figure out which genomic variants affect phenotypes which then can be used to explain things like health, disease, and evolutionary fitness. Phenomics forms a large part of the Human Genome Project

Phenomics has applications in agriculture. For instance, genomic variations such as drought and heat resistance can be identified through phenomics to create more durable GMOs.

Phenomics may be a stepping stone towards personalized medicine, particularly drug therapy. Once the phenomic database has acquired more data, a person's phenomic information can be used to select specific drugs tailored to an individual.

Large-scale phenotyping and genetic screens

Large-scale genetic screens can identify the genes or mutations that affect the phenotype of an organism. Analyzing the phenotypes of mutant genes can also aid in determining gene function. For example, a large-scale phenotypic screen has been used to study lesser understood phenotypes such as behavior. In this screen, the role of mutations in mice were studied in areas such as learning and

memory

,

circadian rhythmicity

, vision, responses to stress and response to psychostimulants (see table for details).

This experiment involved the progeny of mice treated with

ENU

, or N-ethyl-N-nitrosourea, which is a potent mutagen that causes

point mutations

. The mice were phenotypically screened for alterations in the different behavioral domains in order to find the number of putative mutants (see table for details). Putative mutants are then tested for heritability in order to help determine the inheritance pattern as well as map out the mutations. Once they have been mapped out, cloned, and identified, it can be determined whether a mutation represents a new gene or not.

These experiments showed that mutations in the

rhodopsin

gene affected vision and can even cause retinal degeneration in mice. The same

amino acid

change causes human familial blindness, showing how phenotyping in animals can inform medical diagnostics and possibly therapy.

Evolutionary origin of phenotype

The RNA world is the hypothesized pre-cellular stage in the evolutionary history of life on earth, in which self-replicating

RNA

molecules proliferated prior to the evolution of

DNA

and

protein

s.Michod R (1983) Population biology of the first replicators: On the origin of the genotype, phenotype and organism. Am Zool 23:5–14 The folded three-dimensional physical structure of the first RNA molecule that possessed ribozyme activity promoting replication while avoiding destruction would have been the first phenotype, and the nucleotide sequence of the first self-replicating RNA molecule would have been the original genotype.

See also

* Ecotype
* Endophenotype
* Genotype
* Genotype-phenotype distinction
* Molecular phenotyping
*Race and genetics

References

External links

Mouse Phenome Database

Human Phenotype Ontology

Europhenome: Access to raw and annotated mouse phenotype data

"Wilhelm Johannsen's Genotype-Phenotype Distinction" by E. Peirson at the Embryo Project Encyclopedia

{{Authority control

Classical genetics
Polymorphism (biology)