To investigate the origin of nondisjunction, 200 human oocytes that had failed to be ferti...

To investigate the origin of nondisjunction, 200 human oocytes that had failed to be fertilized during in vitro fertilization procedures were subsequently examined (Angel, R. 1997. Am. J. Hum. Genet. 61: 23-32). These oocytes had completed meiosis I and were arrested in metaphase II (MII). The majority (67 percent) had a normal MII-metaphase complement, showing 23 chromosomes, each consisting of two sister chromatids joined at a common centromere. The remaining oocytes all had abnormal chromosome compositions. Surprisingly, when trisomy was considered, none of the abnormal oocytes had 24 chromosomes.

(a) Interpret these results in regard to the origin of trisomy, as it relates to nondisjunction, and when it occurs. Why are the results surprising?

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(b) A large number of the abnormal oocytes contained 22 1 / 2 chromosomes; that is, 22 chromosomes plus a single chromatid representing the 1 / 2 chromosome. What chromosome compositions will result in the zygote if such oocytes proceed through meiosis and are fertilized by normal sperm?

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(c) How could the complement of 22 1 / 2 chromosomes arise? Provide a drawing that includes several pairs of MII chromosomes.

(d) Do your answers support or dispute the generally accepted theory regarding nondisjunction and trisomy, as outlined in Figure?

Figure Nondisjunction during the first and second meiotic divisions. In both cases, some of the gametes that are formed either contain two members of a specific chromosome or lack that chromosome. After fertilization by a gamete with a normal haploid content, monosomic, disomic (normal), or trisomic zygotes are produced.