How genetic background masks single-gene heterosis in Drosophila (polymorphism-selection-isoenzymes-inbreeding-fly).

Conditional heterosis associated with two isoallelic forms of octanol dehydrogenase in Drosophila pseudoobscura has been detected in flies that have been (a) inbred for several generations to make the background genotype as homozygous as practicable, and (b) grown on a medium containing a small amount of octanol. The heterosis was first found in inbred males. The possibility that the effect in females was masked by X-linked polymorphic genes affecting the same fitness components as the locus for octanol dehydrogenase was shown to be correct when, on further inbreeding, heterosis was found in the females of one of the lines. The implications of this and other findings for the current understanding of the genetic structure of natural populations is discussed.