Below is a diagram of nondisjunction in meiosis: Nondisjunction in Meiosis XX XX Meiosis | XX Meiosis / Which of the following diagrams (left vs. right) depicts nondisjunction in meiosis I? Which depicts nondisjunction in meiosis II?
The Correct Answer and Explanation is:
Correct Answer:
- The left diagram depicts nondisjunction in meiosis I.
- The right diagram depicts nondisjunction in meiosis II.
Explanation:
Nondisjunction is a chromosomal error that occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis. This leads to gametes with an abnormal number of chromosomes, which can result in genetic disorders such as Down syndrome, Turner syndrome, or Klinefelter syndrome.
Meiosis consists of two consecutive cell divisions: meiosis I and meiosis II.
1. Nondisjunction in meiosis I:
In this phase, homologous chromosomes are supposed to separate and move into different daughter cells. If nondisjunction happens here, both homologous chromosomes go into the same daughter cell. As a result, two of the four gametes will have an extra chromosome (n+1), and two will have one less chromosome (n−1). None of the gametes will be normal. This is what the left diagram shows. The initial failure to separate happens in the first meiotic division.
2. Nondisjunction in meiosis II:
Here, the sister chromatids of a chromosome are supposed to separate. If they do not, one daughter cell ends up with both chromatids, and the other ends up with none. This error occurs in only one of the two cells from meiosis I. As a result, among the four gametes produced, two will be normal, one will have an extra chromosome (n+1), and one will have one less (n−1). This is shown in the right diagram, where the issue occurs in the second meiotic division.
In summary, nondisjunction in meiosis I affects all gametes, while in meiosis II, only half of the gametes are affected. Understanding the difference is crucial in diagnosing the origin of chromosomal disorders.
