The Regulation of White Locus Expression: A Dominant Mutant Allele at the White Locus of DROSOPHILA MELANOGASTER.

A new mutant allele (w(DZL)) at the white locus of Drosophila melanogaster is dominant to the wild-type allele, but apparently only when the two alleles are synapsed. When chromosomal rearrangements prevent somatic pairing between the two white alleles, w(DZL) is rendered recessive to wild type. This observation suggests that the dominance of w(DZL) is sensitive to a synapsis (transvection) effect. On the basis of this and other properties, it is proposed that w(DZL) causes the repression of transcription of a synapsed w(+) allele, but not of a w(+) allele elsewhere in the same nucleus. One model to account for this supposes that w(DZL) produces a repressor of white-locus transcription. This repressor is presumed to be so unstable that other white genes, removed from w(DZL) but in the same nucleus, are not detectably repressed. These properties may be simply understood if it is assumed that the repressor produced by the w(DZL) allele is an RNA molecule.