The genetic analysis of a chromosome-specific meiotic mutant that permits a premature separation of sister chromatids in Drosophila melanogaster.

mei- G87 is a recessive meiotic mutant that increases second chromosome nondisjunction in both males and females. A significant proportion of the diplo -2 exceptions are equational . In females, diplo -2 reductional exceptions are usually noncrossovers , but, in equational exceptions, crossover frequency and distribution are the same as that found in the haplo-2 controls. The frequencies of nondisjunction are relatively low: 0.6% in females and 1.3% in males. Nondisjunction frequency is affected by environmental conditions (possibly humidity). The defect in mei- G87 , as in other "second division" mutants, appears to be a failure to maintain sister-chromatid cohesion. mei- G87 increases nondisjunction of only the second chromosome. This may indicate either a weak mutant with only the second chromosome being sensitive enough to misbehave or it may indicate that chromosome-specific regions responsible for sister-chromatid cohesion exist.