Abstract
The effects of a female-specific meiotic mutation, altered disjunction (ald: 3-61), are described. Although ald females show normal levels of meiotic exchange, sex- and 4th-chromosome nondisjunction occurs at an elevated level. A large proportion of the nondisjunction events is the result of nonhomologous disjunction of the sex and 4th chromosomes. These nonhomologous disjunction events, and probably all nondisjunction events occurring in ald females are the result of two anomalies in chromosome behavior: (1) X chromosomes derived from exchange tetrads undergo nonhomologous disjunction and (2) the 4th chromosomes nonhomologously disjoin from larger chromosomes. There is at best a marginal effect of ald on the meiotic behavior of chromosomes 2 or 3. The results suggest that the ald+ gene product acts to prevent the participation of exchange X chromosomes and all 4th chromosomes in nonhomologous disjunction events. The possible role of ald+ in current models of the disjunction process is considered.
