The Principles of Clinical Cytogenetics - Extra Materials - Springer

Autosomal Aneuploidy 137
Fig. 2. Schematic representation of meiosis I error resulting from premature sister chromatid separation:
(a) prophase I, (b) metaphase I, (c) anaphase I, with premature separation of centromere of one chromosome,
(d) telophase I, with one prematurely separated chromatid segregating with its homologous chromosome, (e)
products of meiosis I, (f) metaphase II, (g) anaphase II, (h) meiotic products—two gametes with a normal
chromosome complement, one gamete lacking one chromosome, and one gamete containing two copies of one
chromosome.
excess of cases that have zero or one crossover and a deficiency of cases that have multiple crossovers
(16). In contrast, in a study of trisomy 18, approximately one-third (5/16) of maternal MI
nondisjunctions were associated with a complete absence of recombination, whereas the remaining
MI and all MII nondisjunctions appeared to have normal rates of recombination (22). Studies of
trisomy 16 and trisomy 21 reported similar findings between the two. In trisomy 16, it was shown that
recombination was reduced, but not absent, and that distribution of recombination was altered, with
rare crossovers in the proximal regions of the chromosome (21). In trisomy 21, there was an overall
reduction in recombination with an increase in both zero and one crossover in maternal MI nondisjunction
(44). Lamb et al. (45) showed that in maternal MI nondisjunction for chromosome 21, the
average number of recombination events was decreased, with approximately 35–45% of cases having
no crossovers. When at least one crossover was present, it occurred largely at distal 21q. This