Mutants have a strong meiotic phenotype. Homozygous, Mcm5^A7/Df(3R)BSC38, Mcm5^A7/Df(3R)Exel6159 and Mcm5^A7/Df(3R)Exel7305 females and females carrying homozygous germ-line clones show approximately 30% X chromosome nondisjunction.

Mutant females show a 10-fold decrease in the absolute frequency of recombination along the entire length of the X chromosome and a more than 10-fold increase in the fraction of oocytes in which the X chromosomes fail to undergo exchange. The distribution of exchange is also altered in the mutant females compared to wild type, with the reduction in exchange being most severe in the two regions (cv-v and v0-f) that show the highest levels of exchange in control females).

Mutant third instar larvae have a normal central nervous system and normal imaginal discs. The polytene chromosomes of the salivary glands appear normal.

Mutant oocytes do not show defects in prometaphase figures, but there is a failure to arrest at metaphase I in 42% of the mutant oocytes.

There is no significant difference in the relative rates of viability of homozygotes compared to heterozygous siblings after treatment with methyl methanesulfonate or X rays, indicating that the mutants are proficient in somatic DNA repair.

Assays for meiotic crossing over in Mcm5^A7 homozygotes reveals a 90% decrease in crossover frequency.