Origin of sex chromosome aneuploidy

Probabilities applicable to RFLPs are derived for the genotypes of XXY, XXX, and XO children, conditional on their mode of origin and the parental mating type, and the theory is applied to family data on the XG locus. The proportion of XXY males arising in spermatogenesis is shown to be 0.41 +/- 0.09. The large distance of XG from the centromere makes distinction between maternal meiosis I and II unreliable, but if man is like Drosophila, most of the maternal non-disjunctions arise in meiosis I, among tetrads that have undergone one or more exchanges. Data from XG show that 0.78 +/- 0.05 of XO females arise from an error involving the paternal sex chromosomes. The XG locus is virtually uninformative about the origin of XXX. Application of the theory to selected RFLPs will be much more incisive because of their large number, lack of dominance, greater heterozygosity, and distribution along the chromosome. Study of RFLPs will facilitate diagnosis of the parental origin of sex chromosome abnormalities and the comparison of recombination rates in regular and trisomic progeny of maternal mei I and mei II origin. A pseudocentromeric model that estimates map distances from exceptional progeny is applied to non-disjunction of the X chromosomes in D. melanogaster, giving good recovery of the expected map and thereby validating this approach for RFLPs in man.