Fig. 1.

Stages at which non-disjunction can occur. In this cartoon, we consider the tetrad for one chromosome and do not represent the other 22. (a1) MI non-disjunction results in two daughter cells with two chromatids and two with zero chromatids. Alleles near the centromere are not identical by descent (NIBD), so may be heterozygous. In this case, a recombinant chromatid and a non-recombinant chromatid are transmitted, so the crossovers can be detected from proband and parental data. (a2) In contrast, arecombinant crossover cannot be detected if both recombinant (or non-recombinant) chromatids are transmitted. (b) MII non-disjunction results in one daughter cell with two chromatids, one with zero chromatid and two normal cells with one chromatid each. Alleles near the centromere are identical by descent (IBD), so are homozygous. The pattern near the centromere is therefore informative about the stage of meiosis at which non-disjunction events occurred