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The 'phenotype' (from Ancient Greek: pheno- "appearing, seeming" and -type "sort, kind") of an individual organism describes one of its traits or characteristics that is measurable and that is expressed in only a subset of the individuals within that population ^[1]. Examples include
"blue eyes", or "aggressive behavior". Some phenotypes are controlled entirely by the individual's genes. Others are controlled by genes but are significantly affected by extragenetic or environmental factors. Still other phenotypes are entirely extragenetic, for example, a person's language or physical traits that were altered by surgery.
Many phenotypes are determined by multiple genes and can also be influenced by environmental factors. Thus, the identity of one or a few alleles of an organism does not always enable prediction of its phenotype.
Geneticists use easily observable phenotypes to deduce an organism's genotype, and analyse complex phenotypes to help hypothesize about how individual genes function.

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Phenotypic variation

References

Phenotypic variation

Phenotypic variation (due to underlying heritable genetic variation) is a fundamental prerequisite for evolution by natural selection. The fitness of an organism is a high-level phenotype determined by the contributions of thousands of more specific phenotypes. Without phenotypic variation, individual organisms would all have the same fitness, and changes in phenotypic frequency would proceed without any selection (randomly).
The interaction between genotype and phenotype has often been conceptualized by the following relationship:
:'genotype + environment → phenotype'
A slightly more nuanced version of the relationships is:
:'genotype + environment + random-variation → phenotype'
An example of the importance of random variation in phenotypic expression is Drosophila flies in which number of eyes may vary (randomly) between left and right sides in a single individual as much as they do between different genotypes overall, or between clones raised in different environments.
A phenotype is any detectable characteristic of an organism (i.e., structural, biochemical, physiological and behavioral) determined by an interaction between its genotype and environment (see genotype-phenotype distinction and phenotypic plasticity for a further elaboration of this distinction).
According to the autopoietic notion of living systems by Humberto Maturana, the phenotype is epigenetically being constructed throughout ontogeny, and we as observers make the distinctions that define any particular trait at any particular state of the organism's life cycle.
The idea of the phenotype has been generalized by Richard Dawkins in ''The Extended Phenotype'' to mean all the effects a gene has on the outside world which may influence its chances of being replicated. These can be effects on the organism in which the gene resides, the environment or other organisms. For instance, a beaver dam can be considered a phenotype of beaver genes, the same way beaver's powerful incisor teeth are regarded as phenotype expressions of their genes.
The concept of phenotype can be extended to variations below the level of the gene that effect 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", see also DNA-DNA hybridization) than adenine-thymine, a property that might convey, among organisms living in high temperature environments, a selective advantage on variants enriched in GC content.

References

1. Encyclopedia of Genetics, Ed. Sydney Brenner and Jeffrey H. Miller. San Diego: Academic Press, 2002.