Components of phenotypic variation

We will now consider the different influences that contribute to phenotypic variation in a population. The total observed variation in an observed trait (phenotype vp) at the simplest level (ignoring non-additive effects) can be partitioned into a proportion due to genetic influences ( vg), a component explained by shared environmental factors (vc) and a remainder accounted by non-shared environmental factors which includes error ( ve):

Shared or common environmental influences are aspects of the environment, such as poverty, that are shared by family members. Non-shared environmental factors refer to environmental influences that are specific to individuals (e.g. a head injury) and not common to family members.

Although we have so far only considered one type of genetic contribution, the genetic variance vg can be further subdivided into variance due to additive genetic influences (va) and dominance effects (vd).

The relative influence of genetic factors is expressed as heritability and when defined as the proportion of the total phenotypic variance attributable to additive genetic variance, is known as narrow-sense heritability:

Heritability is also sometimes used to describe the proportion of variance explained by the total genetic variance (additive and non-additive genetic variance) and it is then known as broad-sense heritability:

Similarly we can estimate the proportion of the total phenotypic variance explained by shared environment where c2 = vc/vp and the remaining proportion attributable to non-shared environmental factors and error ( e2).

It is important to remember that the estimate of heritability and the contribution of shared environment and non-shared environment are proportions of total variation within a given population, i.e. these parameters tell us about sources of difference between individuals in a population and have no meaning at an individual level. For example, if an individual was selected from a population where IQ had been shown to have a heritability of 50 per cent, it could not be said that 50 per cent of that individual's IQ was determined by genes. Another important point is that these estimates are specific to the population studied and may differ for other populations.

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