In my recent note on R. A. Fisher and epistasis, I mentioned that Fisher's theory of the evolution of dominance relied on the epistatic effect of 'modifier' genes. On looking again at the chapter in The Genetical Theory of Natural Selection dealing with the evolution of dominance, I see that there is a more general statement of the principle that the effect of a gene depends in part on the genetic background against which it occurs:

The fashion of speaking of a given factor, or gene substitution, as causing a given somatic change, which was prevalent among the earlier geneticists, has largely given way to a realization that the change, although genetically determined, may be influenced or governed either by the environment in which the substitution is examined, or by the other elements in the genetic composition. Cases were fairly early noticed in which a factor, B, produced an effect when a second factor, A, was represented by its recessive gene, but not when the dominant gene was present. Factor A was then said to be epistatic to factor B, or more recently B would be said to be a specific modifier of A. .... These are evidently only particular examples of the more general fact that the visible effect of a gene substitution depends both on the gene substitution itself and on the genetic complex, or organism, in which this gene substitution is made.
- The Genetical Theory of Natural Selection, page 54, variorum edition, 1999, from the first edition text of 1930. There is a slight change of wording in the second (1958) edition.

Labels: Burbridge, Population genetics