Non-disjunction of chromosome 18

A sample of 100 trisomy 18 conceptuses analysed separately and togetherwith a published sample of 61 conceptuses confirms that an error inmaternal meiosis II (MII) is the most frequent cause of non-disjunction forchromosome 18. This is unlike all other human trisomies that have beenstudied, which show a higher frequency in maternal meiosis I (MI). MaternalMI trisomy 18 shows a low frequency of recombination in proximal p andmedial q, but not the reduction in proximal q observed in chromosome 21 MInon-disjunction. Maternal MII non-disjunction does not fit the entanglementmodel that predicts increased recombination, especially near thecentromere. Whereas recent data on MII trisomy 21 show the predictedincrease in recombination proximally, maternal MII trisomy 18 hasnon-significantly reduced recombination. Therefore, chromosome-specificfactors must complicate the simple model of susceptible chiasmadistributions interacting with age-dependent deterioration of the meioticmechanism. For chromosome 18, 30% of tetrads are nullichiasmate in maternalMI non-disjunction, but nullichiasmates are not observed in maternal MIInon-disjunction. Chiasma distributions from normal chromosome 18 meiosesprovide no evidence for normal disjunction from nullichiasmate tetrads. Weextend this study to examine the remaining autosomes and find no evidencefor normal disjunction from nullichiasmate tetrads generally.