The origin of aneuploidy: bivalent instability and the maternal age effect in trisomy 21 Down syndrome.

The mechanism behind the high rate of maternal first meiotic non-disjunction and the maternal age effect in trisomy 21 is unknown. Much attention has been paid to the causal role of univalence 21 by lack of chiasma formation. Here I suggest that not only achiasmatic bivalents may be susceptible to nondisjunction, but the chiasmatic bivalent shape per se may play an important role. Considering the topology of the chiasmata and the orientation of the bivalent at first meta- and anaphase, some bivalent shapes may have a greater segregational potential than others. For any one chromosome there may at first meta- and anaphase be an optimal balance between chromosome coiling/condensation on the one hand and chiasma frequency distribution on the other. Chromosome coiling/condensation may play an interesting dual role in both determining bivalent flexibility and dictating chiasma formation. I propose that the higher rate of double- and triple-chiasma bivalents 21 in oocytes contributes to its higher rate of spontaneous first meiotic nondisjunction; and the maternal age effect is associated with chromosome laxity, secondary to an impairment of appropriate chromosome coiling/condensation in oocytes of older women. In spermatocytes of older men some compensatory increase in chiasma frequency with a shift towards double- and triple-chiasma bivalents 21 might take place, i.e., if a general age-related chromosome decondensation is counteracted by an auto-regulatory "length effect" remaining intact in pachytene spermatocytes.