The Principles of Clinical Cytogenetics - Extra Materials - Springer

136 Jin-Chen Wang
Fig. 1. Schematic representation of meiosis I nondisjunction: (a) prophase I, (b) metaphase I, (c) anaphase I,
(d) telophase I, with both homologs of one chromosome pair segregating together, (e) products of meiosis I,
(f) metaphase II, (g) anaphase II, (h) meiotic products—two gametes lack one chromosome and two gametes
contain two copies of one chromosome.
(1.3–5.5%). Further studies are needed to substantiate this observation. Another example is the “limited
oöcyte pool” model (38). At the antral stage of each menstrual cycle, multiple follicles at various
stages of development are present. When stimulated with high levels of plasma follicle-stimulating
hormone (FSH), only one follicle, presumably the one at the most optimal stage, will complete MI
and eventually achieve ovulation. The number of follicles in the antral stage decreases with increasing
maternal age. When the number of these follicles is low, it is more likely that an oöcyte that is not
at the optimal stage will be selected for ovulation. If such “less optimal” oöcytes are more likely to
undergo MI nondisjunction, then the ovulated oöcytes of older women will have higher rates of
aneuploidy. Recent data, however, do not appear to support this hypothesis (42).
One probable factor that predisposes gametes to nondisjunction is aberrant recombination (see
Chapter 2, reviewed in ref. 43). Data on recombination patterns are available for trisomies 15, 16, 18,
and 21. Studies of chromosome 15 nondisjunction in uniparental disomy (see Chapter 19) revealed
that there was a mild reduction in recombination in association with maternal nondisjunction, with an