View source for Genotype–phenotype distinction

{{short description|Distinction made in genetics}}
[[Image:PAX6 Phenotypes Washington etal PLoSBiol e1000247.png|right|400px|thumb|Similar genotypic changes may result in similar phenotypic alterations, even across a wide range of species.<ref>{{Cite journal|last1=Washington|first1=Nicole L.|last2=Haendel|first2=Melissa A.|last3=Mungall|first3=Christopher J.|last4=Ashburner|first4=Michael|last5=Westerfield|first5=Monte|last6=Lewis|first6=Suzanna E.|date=2009-11-24|editor-last=Buetow|editor-first=Kenneth H.|title=Linking Human Diseases to Animal Models Using Ontology-Based Phenotype Annotation|journal=PLOS Biology|language=en|volume=7|issue=11|pages=e1000247|doi=10.1371/journal.pbio.1000247|issn=1545-7885|pmc=2774506|pmid=19956802 |doi-access=free }}</ref>]]

The '''genotype–phenotype distinction''' is drawn in [[genetics]]. The "[[genotype]]" is an organism's full [[hereditary]] information. The "[[phenotype]]" is an organism's actual observed properties, such as [[morphology (biology)|morphology]], [[Developmental biology|development]], or [[behavior]]. This distinction is fundamental in the study of [[biological inheritance|inheritance]] of [[trait (biology)|trait]]s and their [[evolution]].

==Overview==
The terms "genotype" and "phenotype" were created by [[Wilhelm Johannsen]] in 1911,<ref>{{Cite journal |doi=10.1086/279202 |title=The Genotype Conception of Heredity |journal=[[The American Naturalist]] |volume=45 |issue=531 |pages=129–159 |year=1911 |last1=Johannsen |first1=W. |jstor=2455747|pmid=24691957 |pmc=4258772 }}</ref> although the meaning of the terms and the significance of the distinction have evolved since they were introduced.<ref>{{cite web |url=https://plato.stanford.edu/entries/genotype-phenotype/ |title=The Genotype-Phenotype Distinction |last1=Taylor |first1=Peter  |last2=Lewontin |first2=Richard |access-date=21 June 2017}}</ref>

It is the organism's physical properties that directly determine its chances of survival and reproductive output, but the inheritance of physical properties is dependent on the inheritance of genes. Therefore, understanding the theory of evolution via [[natural selection]] requires understanding the genotype–phenotype distinction. The genes contribute to a trait, and the phenotype is the observable manifestation of the genes (and therefore the genotype that affects the trait). If a white mouse had recessive genes that caused the genes responsible for color to be inactive, its genotype would be responsible for its phenotype (the white color).{{cn|date=March 2023}}

The [[Function (mathematics)|mapping]] of a set of genotypes to a set of phenotypes is sometimes referred to as the [[genotype–phenotype map]].<ref>{{cite journal |title=Genotype–phenotype mapping and the end of the 'genes as blueprint' metaphor |last1=Pigliucci |first1=Massimo |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |year=2010 |volume=365 |issue=1540 |pages=557–566 |doi=10.1098/rstb.2009.0241 |pmid=20083632 |pmc=2817137 }}</ref>

[[File:De Albina y Español, Torna atrás (Juan Patricio Morlete).jpg|thumb|''De [[Quadroon|Albina]] y Español, [[Torna atrás]]''. Attributed to [[Juan Patricio Morlete Ruiz]] (1701-1770) In [[casta]] paintings, "torna atrás" described mixed-race individuals who expressed phenotypes dissimilar to their parents. In this painting, the daughter and mother are both of partial [[Sub-Saharan]] and European ancestry, yet have noticeably differing phenotypes.]]
An organism's genotype is a major (the largest by far for [[morphology (biology)|morphology]]) influencing factor in the development of its phenotype, but it is not the only one. Even two organisms with identical genotypes may differ in their phenotypes, due to [[phenotypic plasticity]]. To what extent a particular genotype influences a phenotype depends on the relative [[Dominance (genetics)|dominance]], [[penetrance]], and [[Expressivity (genetics)|expresivity]] of the alleles in question.{{cn|date=February 2025}}

One experiences this in everyday life with [[identical twins|monozygous (i.e. identical) twins]]. Identical twins share the same genotype, since their genomes are identical; but they never have the same phenotype, although their phenotypes may be very similar. This is apparent in the fact that close relations can always tell them apart, even though others might not be able to see the subtle differences. Further, identical twins can be distinguished by their [[fingerprint]]s, which are never completely identical.{{cn|date=March 2023}}

==Phenotypic plasticity==
The concept of [[phenotypic plasticity]] defines the degree to which an organism's phenotype is determined by its genotype. A high level of plasticity means that [[environmental factor]]s have a strong influence on the particular phenotype that develops. If there is little plasticity, the phenotype of an organism can be reliably predicted from knowledge of the genotype, regardless of [[environment (biophysical)|environment]]al peculiarities during development. An example of high plasticity can be observed in [[larva]]l [[newt]]s<sup>1</sup>: when these larvae sense the presence of [[predator]]s such as [[dragonfly|dragonflies]], they develop larger heads and tails relative to their body size and display darker [[pigmentation]]. Larvae with these traits have a higher chance of [[Survival skills|survival]] when exposed to the predators, but grow more slowly than other phenotypes.{{cn|date=March 2023}}

==Genetic canalization==
In contrast to phenotypic plasticity, the concept of [[canalisation (genetics)|genetic canalization]] addresses the extent to which an organism's phenotype allows conclusions about its genotype. A phenotype is said to be canalized if [[mutation]]s (changes in the genome) do not noticeably affect the physical properties of the organism. This means that a canalized phenotype may form from a large variety of different genotypes, in which case it is not possible to exactly predict the genotype from knowledge of the phenotype (i.e. the genotype–phenotype map is not invertible). If canalization is not present, small changes in the genome have an immediate effect on the phenotype that develops.{{cn|date=February 2025}}

==Importance to evolutionary biology==

According to [[Richard Lewontin|Lewontin]],<ref>{{cite book|last1=Lewontin|first1=Richard C.|title=The genetic basis of evolutionary change|url=https://archive.org/details/geneticbasisofev00lewo|url-access=registration|date=1974|publisher=Columbia University Press|location=New York|isbn=978-0231083188|edition=[4th printing.]}}</ref> the theoretical task for population genetics is a process in two spaces: a "genotypic space" and a "phenotypic space". The challenge of a ''complete'' theory of population genetics is to provide a set of laws that predictably map a population of [[genotype]]s (''G''<sub>1</sub>) to a [[phenotype]] space (''P''<sub>1</sub>), where [[natural selection|selection]] takes place, and another set of laws that map the resulting population (''P''<sub>2</sub>) back to genotype space (''G''<sub>2</sub>) where [[Mendelism|Mendelian]] genetics can predict the next generation of genotypes, thus completing the cycle. Even if non-Mendelian aspects of [[molecular genetics]] are ignored, this is a gargantuan task. Visualizing the transformation schematically:

:<math>G_1 \; \stackrel{T_1}{\rightarrow} \; P_1 \; \stackrel{T_2}{\rightarrow} \; P_2 \; \stackrel{T_3}{\rightarrow} \; G_2 \;
\stackrel{T_4}{\rightarrow} \; G_1' \; \rightarrow \cdots</math>

(adapted from Lewontin 1974, p.&nbsp;12). ''T''<sub>1</sub> represents the genetic and [[epigenetic]] laws, the aspects of functional biology, or [[developmental biology|development]], that transform a genotype into phenotype. This is the "[[genotype–phenotype map]]". ''T''<sub>2</sub> is the transformation due to natural selection, ''T''<sub>3</sub> are epigenetic relations that predict genotypes based on the selected phenotypes and finally ''T''<sub>4</sub> the rules of Mendelian genetics.

In practice, there are two bodies of evolutionary theory that exist in parallel, traditional population genetics operating in the genotype space and the [[biometry|biometric]] theory used in [[plant breeding|plant]] and [[animal breeding]], operating in phenotype space. The missing part is the mapping between the genotype and phenotype space. This leads to a "sleight of hand" (as Lewontin terms it) whereby variables in the equations of one domain, are considered parameters or ''constants'', where, in a full-treatment, they would be transformed themselves by the evolutionary process and are ''[[function (mathematics)|function]]s'' of the state variables in the other domain. The "sleight of hand" is assuming that the mapping is known. Proceeding as if it is understood is enough to analyze many cases of interest. For example, if the phenotype is almost one-to-one with genotype ([[sickle-cell disease]]) or the time-scale is sufficiently short, the "constants" can be treated as such; however, there are also many situations where that assumption does not hold.{{cn|date=February 2025}}

==References==
{{reflist}}

==External links==
* [http://plato.stanford.edu/entries/genotype-phenotype/ Stanford Encyclopedia of Philosophy entry]
* [http://embryo.asu.edu/handle/10776/4206/ "Wilhelm Johannsen's Genotype-Phenotype Distinction" at the Embryo Project Encyclopedia] {{Webarchive|url=https://web.archive.org/web/20191118022318/https://embryo.asu.edu/handle/10776/4206/ |date=2019-11-18 }}

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[[Category:Genetics]]