Phenotype
The
phenotype of an individual
organism is either its total physical appearance and constitution or a specific manifestation of a
trait, such as size, eye color, or
behavior that varies between individuals. Phenotype is determined to some extent by
genotype, or by the identity of the
alleles that an individual carries at one or more positions on the
chromosomes. Many phenotypes are determined by multiple
genes and influenced by
environmental factors. Thus, the identity of one or a few known alleles does not always enable prediction of the phenotype.
Nevertheless, because phenotypes are much easier to observe than genotypes (it doesn't take chemistry or
sequencing to determine a person's eye color),
classical genetics uses phenotypes to deduce the functions of genes. Breeding experiments can then check these inferences. In this way, early
geneticists were able to trace inheritance patterns without any knowledge of
molecular biology.
The interaction between genotype and phenotype has often been described using a simple equation:
genotype + environment â†' phenotypeA slightly more nuanced version of the equation is:
genotype + environment + random-variation â†' phenotypeAn 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 do the distinctions that define any particular trait at any particular state of the organism´s life cycle.
The idea of the phenotype has been also generalized by
Richard Dawkins to include effects on other organisms or the environment in
The Extended Phenotype.
