Triple chromosome synapsis in oocytes from a human foetus with trisomy 21.

Oocytes from a human foetus with trisomy 21 were spread using detergent and examined by light and electron microscopy. The three chromosomes 21 occurred as bivalent and univalent or trivalent configurations. In the trivalents the lateral elements of the synaptonemal complex were associated in threes, either completely along the length of the trivalent, or partially, forming a variety of forked structures. This triple association demonstrates that, contrary to the classical view of chromosome pairing, three homologous chromosomes can be held in register at the same site.