The Principles of Clinical Cytogenetics - Extra Materials - Springer

138 Jin-Chen Wang
Fig. 3. Schematic representation of meiosis II nondisjunction: (a) prophase I, (b) metaphase I, (c) anaphase
I, (d) telophase I, (e) products of meiosis I, (f) metaphase II, (g) anaphase II, with both sister chromatids
segregating together, (h) meiotic products—two gametes with a normal chromosome complement, one gamete
lacking one chromosome, and one gamete containing two copies of one chromosome.
study, together with one on trisomy 16 (21), suggests that, at least for trisomies 16 and 21, distal
chiasmata are less efficient in preventing nondisjunction in MI. In contrast, in maternal MII nondisjunction,
the number of recombination events appeared to be increased, especially in proximal 21q.
These proximal recombinations could cause an “entanglement” effect. Entanglement of the two homologs
can cause the bivalent to move to the same pole at MI, and then at MII, the two homologs
finally separate, resulting in two disomic gametes, each having two chromatids with identical centromeres.
Alternatively, the entanglement may disrupt sister chromatid cohesion resulting in premature
separation of the sister chromatids at MI. If the two separated sister chromatids travel to the same
pole at MI and again at MII, an apparent MII nondisjunction would be observed. Thus, these data
suggest that all nondisjunction events could be initiated during MI. Lamb et al. showed that the
alteration in recombination pattern was not maternal age dependent. They proposed a “two-hit” model
(45,46) and hypothesized that certain recombination configurations are less likely to be processed
properly in older women. This could result from, for example, an age-dependent loss of spindle-