Nonadditive genetic effects

So far we have simplistically assumed that phenotypic variation is influenced in an additive fashion. However, the contribution of genes and environment is likely to be more complex than this. We have already referred to genetic dominance effects where there is non-additive interaction of alleles within a locus. Another potential source of influence is the non-additive interaction between alleles at different loci which is known as gene-gene interaction or epistasis.

Similarly, it is possible that genes and environment also interact non-additively. For example, it has been shown that although the performance of mice on certain tasks improves when they are reared in an enriched environment, the degree of response to this environment depends on the genotype of the mouse. However, so far it has been difficult to show these types of gene-environment effects for human behaviour and psychopathology.

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