Non-disjunction of chromosome 18.

A sample of 100 trisomy 18 conceptuses analysed separately and together with a published sample of 61 conceptuses confirms that an error in maternal meiosis II (MII) is the most frequent cause of non-disjunction for chromosome 18. This is unlike all other human trisomies that have been studied, which show a higher frequency in maternal meiosis I (MI). Maternal MI trisomy 18 shows a low frequency of recombination in proximal p and medial q, but not the reduction in proximal q observed in chromosome 21 MI non-disjunction. Maternal MII non-disjunction does not fit the entanglement model that predicts increased recombination, especially near the centromere. Whereas recent data on MII trisomy 21 show the predicted increase in recombination proximally, maternal MII trisomy 18 has non-significantly reduced recombination. Therefore, chromosome-specific factors must complicate the simple model of susceptible chiasma distributions interacting with age-dependent deterioration of the meiotic mechanism. For chromosome 18, 30% of tetrads are nullichiasmate in maternal MI non-disjunction, but nullichiasmates are not observed in maternal MII non-disjunction. Chiasma distributions from normal chromosome 18 meioses provide no evidence for normal disjunction from nullichiasmate tetrads. We extend this study to examine the remaining autosomes and find no evidence for normal disjunction from nullichiasmate tetrads generally.