AP BIOLOGY

Detailed explanation-1: -Nondisjunction in meiosis I occurs when the tetrads fail to separate during anaphase I.

Detailed explanation-2: -Nondisjunction in Meiosis 1 vs 2 Failure of homologous chromosomes to separate towards the poles during anaphase 1 is known as nondisjunction in meiosis 1. Failure of sister chromatids to separate towards the poles during anaphase 2 in meiosis is known as nondisjunction in meiosis 2.

Detailed explanation-3: -When nondisjunction occurs in meiosis II, cells divide normally during anaphase I (homologous chromosomes separate correctly), but sister chromatids fail to separate during anaphase II (as seen in the diagram above, on the left). This again results in four possible haploid gametes.

Detailed explanation-4: -Among the 188 maternal cases, nondisjunction occurred in meiosis I in 128 cases and in meiosis II in 38 cases; in 22 cases the DNA markers used were uninformative. Therefore meiosis I was responsible for 77.1% and meiosis II for 22.9% of maternal nondisjunction.

Detailed explanation-5: -Homologous chromosomes do not separate in nondisjunction errors in meiosis I, but sister chromatids separate properly in meiosis II. Thus, each of the germ cells with an extra chromosome will have a maternally derived chromosome and a paternally derived chromosome.