Epistasis facts for kids
When Gregor Mendel did his experiments on the sweet pea, he chose seven traits, such as round vs wrinkled peas, and tall vs short plants. He found the plants had pairs of 'factors' (genes) which controlled their appearance (phenotype). However, one pair of factors never affected the other pairs. But with epistasis, the action of one gene does affect another gene, even one on a different chromosome.
Use of the term has varied somewhat. Its original meaning was quite narrow: for instance, a gene for no wings in Drosophila will obviously mask genes for any other aspect of the wings. In general, however, epistasis denotes any departure from independence of the effects of different genetic loci.
Images for kids
Quantitative trait values after two mutations either alone (Ab and aB) or in combination (AB). Bars contained in the grey box indicate the combined trait value under different circumstances of epistasis. Upper panel indicates epistasis between beneficial mutations (blue). Lower panel indicates epistasis between deleterious mutations (red).
The top row indicates interactions between two genes that are either additive (a), show positive epistasis (b) or reciprocal sign epistasis (c). Below are fitness landscapes which display greater and greater levels of global epistasis between large numbers of genes. Purely additive interactions lead to a single smooth peak (d), as increasing numbers of genes exhibit epistasis, the landscape becomes more rugged (e) and when all genes interact epistatically the landscape becomes so rugged that mutations have seemingly random effects (f).
Epistasis Facts for Kids. Kiddle Encyclopedia.