Heritability
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Heritability is the proportion of phenotypic variation in a population that is inherited among individuals. Phenotypic variation among individuals may be due to genetic and/or environmental factors. Heritability analyses estimate the relative contributions of differences in genetic and non-genetic factors to the total phenotypic variance in a population. Often measured empirically, heritability is an important notion in quantitative genetics, particularly in selective breeding, but less so in the general theory of population genetics.
Pay close attention to the variation part of "phenotypic variation": if a trait has a heritability of 0.5, it means that the phenotypic variation is 50% due to genetic variation. It does not imply that the trait is 50% caused by genetics.
Heritability is specific to a particular population in a particular environment, but the extent of the dependence on environment is also a function of the genes involved. Individuals with the same genotype can exhibit different phenotypes through a mechanism called phenotypic plasticity, which makes their heritability difficult to measure in some cases. Recent insights in molecular biology have identified changes in transcriptional activity of individual genes associated with environmental changes. However, there are a large number of genes whose transcription is not affected by the environment.
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