mei-S51¹ disrupts a number of aspects of chromosome pairing and alignment prior to metaphase and therefore both reduces exchanges and prevents proper partner choice within the distributive system.

Non-disjunction of the X, 2nd, 3rd and 4th chromosomes is increased compared to wild-type in females. There is a positive correlation in the probability of nondisjunction of nonhomologous chromosomes, nondisjoining nonhomologs tend to separate from each other and nondisjunction occurs at the first meiotic division. Crossing over on the X chromosome is uniformly reduced (with the possible exception of the most proximal region which shows a lesser effect) in homozygous females to about 1/2 the control value. Segregation of the sex and 4th chromosomes is normal in mutant males.

Females homozygous for mei-S51¹ exhibit reduced exchange and high frequencies of nonhomologous disjunction, particularly with respect to the X and fourth chromosomes (XX <-> 44 segregations are frequent). In addition mei-S51¹ decreases the frequency of secondary nondisjunction in structurally normal XXY females and increases the frequency of nondisjunction in X inversion heterozygotes.