Analyses of diplotene chiasma frequencies in mouse oocytes and spermatocytes in relation to ageing and sexual dimorphism.

Diplotene chiasma frequencies from "early" (day 16) fetal oocytes are significantly greater than the average from "late" (day 18) oocytes. A decrease in chiasma frequencies from both ages was found at diakinesis/MI in oocytes of young or old adults females, indicating a loss of chiasmata and/or terminalization. A greater number of type I univalents was found in "late" oocytes than "early" oocytes entering diplotene. No difference were found in chiasma frequencies of male diplotene spermatocytes studied at 23 days, 2 months, or 24 months of age. A greater chiasma frequency was generally found in the female than the male at all ages studied.