The Pseudoallelism of White and Apricot in Drosophila Melanogaster

The classical example of multiple allelism is the series of eye-color mutants at the white (w) locus in Drosophila melanogaster. The altemative interpretation of this series, namely, that it is made up of "pseudoalleles," or closely linked genes with similar effects, has usually been considered ruled out by two kinds of evidence. In the first place early attempts to resolve the series by crossing over failed in spite of numerous tests involving most of the mutants available at the time.(1-4) Secondly, a heterozygote for two different mutant genes of the series does not have the phenotype expected for non-allelic genes, namely, wild-type (or red) eye color, but instead has a mutant eye color which is usually intermediate between the colors of the two respective homozygotes. In recent years, however, several cases have been found in which non-allelic genes give a positive phenotypic test for allelism by virtue of a position effect.(5-7) In such cases, which have been termed "position pseudoalleles,"(7) mutant genes at the different loci (say, a and b) give a mutant phenotype in the a +/+ b heterozygote, but a wild-type, or more nearly wild-type, phenotype in the a b/+ + heterozygote.