Chromosome 21 behavior during fetal oogenesis in Down's syndrome.

: Oogenesis of four cases of 47,XX,+21 at gestational ages of 19 and 20 weeks was studied using pachytene cytogenetic methods. We found a variable pattern of pairing behavior of the 21 chromosomes among the cases, which included partially synapsed trivalents, a bivalent plus a univalent, and three univalents. The bivalent/univalent conformation of 21 chromosomes predominated. Pachytene chromomere maps were normal for all autosomal bivalents in 84 oocytes analyzed, except for minor variations in some 21 chromosomes. This complex system of pairing behavior of the 21 chromosomes theoretically affects subsequent disjunctional behavior and therefore may account for the observations of normal and trisomic progeny observed for Down's syndrome mothers. Further study of meiotic behavior during all stages of oogenesis in such patients would provide essential data for predicting chromosomal outcome of pregnancy in this population.