A small tandem duplication is responsible for the unstable white-ivory mutation in Drosophila.

The white-ivory (wi) mutation, an unstable allele of the white locus in Drosophila, reverts to wild-type at frequencies of 5 X 10(-5) in homozygous females, and 5 X 10(-6) in males and deletion heterozygous females. We show by molecular cloning and Southern blot analysis of DNA from wi flies that a 2.9 kilobase tandem duplication within the white locus is responsible for the mutation. Phenotypic reversion appears, in most cases, to be due to an exact excision of the extra copy of the sequence. Two derivative alleles of wi, one phenotypically wild-type, the other a partial revertant, carry insertions of moderately repetitive DNA from outside the locus, in addition to suffering deletions of some white locus DNA. Earlier genetic data preclude unequal crossing-over between homologs as an explanation for the precise reversions. Rather, an intrachromosomal meiotic event seems to be responsible. Our results suggest that intrachromosomal recombination may be responsible in other systems for a larger number of rearrangements than has been suspected, and that interallelic recombination frequencies in Drosophila do not always correlate in a simple way with DNA length or extent of homology.