Question

1. certain genetic conditions, like down syndrome, result from an individual inheriting an extra copy of a chromosome which is termed trisomy. in the case of down syndrome (also known as trisomy 21), the individual would have three copies of chromosome 21 instead of the typical two copies of chromosome 21. other conditions, like turner syndrome, involve an individual inheriting a single copy of a chromosome, which is known as monosomy. turner syndrome occurs in females who only inherit one copy of chromosome 23 (specifically the x - chromosome), instead of receiving two copies of chromosome 23. these genetic conditions result from an error that occurs during meiosis. describe an event in meiosis i and/or meiosis ii that could end up producing a daughter cell with zero or two chromosomes for a set.

Explanation:

To determine the meiotic event causing abnormal chromosome numbers, we analyze nondisjunction. In meiosis I, homologous chromosomes may fail to separate (nondisjunction), leading to one daughter cell with two copies of a chromosome set and the other with zero. In meiosis II, sister chromatids may not separate, resulting in a daughter cell with two copies (if the chromatids stay together) or zero (if the other cell gets both, leaving this with none). For example, in meiosis I nondisjunction of a homologous pair means one cell has \( n + 1 \) and the other \( n - 1 \) chromosomes for that set. In meiosis II, if sister chromatids of a chromosome don't separate, a cell gets two copies (the chromatids) and another gets zero for that chromosome's set.

Answer:

An event in meiosis I is nondisjunction of homologous chromosomes: during anaphase I, a pair of homologous chromosomes fails to separate. This results in one daughter cell receiving both chromosomes of the pair (two copies for the set) and the other daughter cell receiving none (zero copies for the set).

An event in meiosis II is nondisjunction of sister chromatids: during anaphase II, the sister chromatids of a chromosome fail to separate. This causes one daughter cell (from that secondary oocyte or spermatocyte) to receive both sister chromatids (two copies for the chromosome’s set) and the other daughter cell to receive none (zero copies for that chromosome’s set).