Phenotype

In genetics, the phenotype (from Ancient Greek φαίνω (phaínō) 'to appear, show, shine', and τύπος (túpos) 'mark, type') is the set of observable characteristics or traits of an organism.[1][2] 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[3] 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 dams as "extended phenotypes".

For a non-technical introduction to the topic, see Introduction to genetics.
The shells of individuals within the bivalve mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes.
Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent genes for color, and the pictures show the resultant phenotypes. This shows how multiple genotypes (BB and Bb) may yield the same phenotype (purple petals).

Wilhelm Johannsen proposed the genotype–phenotype distinction in 1911 to make clear the difference between an organism's hereditary material and what that hereditary material produces.[4][5] The distinction resembles that proposed by August Weismann (1834–1914), who distinguished between germ plasm (heredity) and somatic cells (the body). More recently, in the Selfish Gene (1976), Richard Dawkins distinguished these concepts as replicators and vehicles.

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 molecules 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.

ABO blood groups determined through a Punnett square and displaying phenotypes and genotypes

The term "phenotype" has sometimes been incorrectly used as a shorthand for the phenotypic difference between a mutant and its wild type, which (if not significant) leads to the statement that a "mutation has no phenotype".[6]

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[7] or syndromes.[8][9]

Biston betularia morpha typica, the standard light-colored peppered moth
B.betularia morpha carbonaria, the melanic form, illustrating discontinuous variation

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.[10]

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 habitats in Sweden. One habitat is rocky, sea-side cliffs, 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.[11]

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."[3] For instance, an organism such as a beaver modifies its environment by building a beaver dam; this can be considered an expression of its genes, 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.[12]

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.[13]

Phenome and phenomics
Not to be confused with Phoneme or Phonology.

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.[14][15] 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.[16] Phenomics forms a large part of the Human Genome Project[17]

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.[18][19]

Phenomics may be a stepping stone towards personalized medicine, particularly drug therapy.[20] Once the phenomic database has acquired more data, a person's phenomic information can be used to select specific drugs tailored to an individual.[20]

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.[21] 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).

Large-scale mutagenesis and phenotypic screens for the nervous system and behavior in mice.
Phenotypic Domain Assay Notes Software Package
Circadian Rhythm Wheel running behavior ClockLab
Learning and Memory Fear conditioning Video-image-based scoring of freezing FreezeFrame
Preliminary Assessment Open field activity and elevated plus maze Video-image-based scoring of exploration LimeLight
Psychostimulant response Hyperlocomotion behavior Video-image-based tracking of locomotion BigBrother
Vision Electroretinogram and Fundus photography L. Pinto and colleagues

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.

Phenotypic domain ENU Progeny screened Putative mutants Putative mutant lines with progeny Confirmed mutants
General assessment 29860 80 38 14
Learning and memory 23123 165 106 19
Psychostimulant response 20997 168 86 9
Neuroendocrine response to stress 13118 126 54 2
Vision 15582 108 60 6

These experiments showed that mutations in the rhodopsin gene affected vision and can even cause retinal degeneration in mice.[22] 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 proteins.[23] 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.[23]

See also
  • Ecotype
  • Endophenotype
  • Genotype
  • Genotype-phenotype distinction
  • Molecular phenotyping
  • Race and genetics

References
  1. "phenotype adjective – Definition, pictures, pronunciation and usage notes". Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com. Retrieved 2020-04-29. the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.
  2. "Genotype versus phenotype". Understanding Evolution. Retrieved 2020-04-29. An organism's genotype is the set of genes that it carries. An organism's phenotype is all of its observable characteristics — which are influenced both by its genotype and by the environment.
  3. Dawkins R (May 1978). "Replicator selection and the extended phenotype". Zeitschrift für Tierpsychologie. 47 (1): 61–76. doi:10.1111/j.1439-0310.1978.tb01823.x. PMID 696023.
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  9. O'Brien, Gregory, ed. (2002). Behavioural Phenotypes in Clinical Practice. London: Mac Keith Press. ISBN 978-1-898683-27-8. Retrieved 27 September 2010.
  10. Lewontin RC (November 1970). "The Units of Selection" (PDF). Annual Review of Ecology and Systematics. 1: 1–18. doi:10.1146/annurev.es.01.110170.000245. JSTOR 2096764.
  11. "Botany online: Evolution: The Modern Synthesis - Phenotypic and Genetic Variation; Ecotypes". Archived from the original on 2009-06-18. Retrieved 2009-12-29.
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  13. Hunter P (March 2009). "Extended phenotype redux. How far can the reach of genes extend in manipulating the environment of an organism?". EMBO Reports. 10 (3): 212–215. doi:10.1038/embor.2009.18. PMC 2658563. PMID 19255576.
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  16. Houle D, Govindaraju DR, Omholt S (December 2010). "Phenomics: the next challenge". Nature Reviews. Genetics. 11 (12): 855–866. doi:10.1038/nrg2897. PMID 21085204. S2CID 14752610.
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