POSITION EFFECTS AT THE HAIRY LOCUS IN DROSOPHILA MELANOGASTER

Radiation-induced chromosomal rearrangements of h+ have given rise to several Drosophila stocks that exhibit apparent position-effect inactivation; i.e., flies carrying the rearranged chromosomes heterozygously with h show varying degrees of hairiness. The numbers of hairy chaetae produce a quantifiable index of position effect. Six such "position-allele'' stocks are here discussed, both as to their basic expressions and in all possible pair-wise combinations with each other. Such crosses reveal complex interactions between the respective position alleles; little evidence is seen for clear-cut dominance or recessiveness. The stocks appear not to conform unequivocally to classical distinctions between variegated and stable types of position effects, nor to usual dicta relating the degree of inactivity to the proximity to heterochromatin. Indeed, these stocks appear to suggest additional dimensions to several of the principles to which position effects usually subscribe. The evidence additionally suggests that the hairy locus itself is associated with a tissue-specific suppressor effect on an otherwise polygenic system that produces the chaetae associated with the hairy phenotype.