Produces 27% XO male self progeny; nondisjunction is correlated with a high frequency of achiasmate X chromosomes in oocyte nuclei, and a reduced frequency of X chromosome crossovers. meDf4 disrupts the function of the cis-acting X chromosome meiotic pairing center. meDf4/+ heterozygotes produce 4-6% XO progeny, so the presence of meDf4 can be followed in heterozygotes by this weak Him phenotype.