AbstractThe meiotic segregants of male mice heterozygous for Rb(6.16)24Lub and Rb(16.17)7Bnr were viewed, for the first time, at first cleavage metaphase. Chromosomes were analyzed after G‐banding, C‐banding, and karyotyping. To study sperm aging effects, chromosomes of 202 one‐cell zygotes derived from males mating at intervals of approximately 3,14, and 21 days were examined. At least 89.6% of sperm‐derived complements were products of 2:2 segregation; at most, a possible 6.4% were 3:1 segregants. The six expected types of 2:2 segregants, both balanced and unbalanced, were equifrequent in the total zygote population derived from sperm of all ages. When the data were analyzed according to mating frequency, the 3‐day sperm population considered most likely to be fresh showed a deficiency of the segregant nullisomic for chromosome 6 and disomic for chromosome 17, when compared with the reciprocal segregant (P<0.025) as well as to all other 2:2 segregants (P<0.05). However, these sperm fertilized in greater numbers (P<0.01) than their reciprocal segregant (disomic for 6 and nullisomic for 17) in the 14‐day sperm population. While sperm with chromosomal abnormalities are capable of fertilization, the competence of segregants nullisomic for 6 and disomic for 17 apparently depends on the prior storage period in the male. Further, the results suggest that the effect of aneuploidy on sperm function is dependent on the specific chromosome(