The use of translocation-derived "marker-bivalents" for studying the origin of meiotic instability in female mice.

Female mice of two age groups, 3--4 and 11--14 months old, homozygous for the T(1;13)70H reciprocal mouse translocation were used for cytological observations of bivalents (in primary oocytes) and metaphase II chromosomes (in secondary oocytes). Special attention was given to the behavior of the long (131) and short (113) marker chromosomes. In primary oocytes, univalents were considered "true" or "opposite". The aged females showed an eight-folded increase in "true" univalent frequency for chromosomes 113 over the young ones. A nine-fold rise for nondisjunction with regard to this chromosome was observed. For the other chromosomes, these factors were 2 and 1.7, respectively. The absolute levels of nondisjunction remained low at old age (1.42% for chromosome 113, 1.22% for all other chromosomes). The long marker bivalent 131 was used for chiasma counts. No change in chiasma number with age was observed. It is argued that poorer physiological conditions within the maturing oocytes of older females are the major cause for both the increasing frequencies of "true" and "opposite" univalents and the increased incidence for nondisjunction.