Question

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement

(normal autosomal complement) and to have white eyes as contrasted with the normal red eye
color of wild type. The female's mother and father had red eyes. The mother, however, was
heterozygous for the gene for white eyes. Knowing that white eyes are X-linked and recessive,
present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed
individual.
A) nondisjunction in meiosis II of the mother
B) nondisjunction in meiosis I or II in father
C) nondisjunction in meiosis I of the father
D) nondisjunction in meiosis I of the mother
E) nondisjunction in meiosis I of both the mother and father

Answer 1

The XXY, white-eyed Drosophila female results from nondisjunction during meiosis I in the heterozygous mother, leading to the daughter receiving two X chromosomes from her mother (one with the recessive white eye allele) and one Y chromosome from her father.

Step-by-step explanation:

The genetic and chromosomal constitution of the XXY, white-eyed Drosophila individual can be explained by nondisjunction during meiosis in one of the parents. Nondisjunction is the failure of chromosome pairs to separate properly during cell division. Given that the XXY female has white eyes and the white eye color trait is X-linked and recessive, and considering that her mother is heterozygous for the white eye trait (XwXW), the most plausible explanation for the XXY female's genotype is nondisjunction occurring during meiosis I in the mother (option D).

This nondisjunction would result in one of the mother's eggs receiving two X chromosomes, one with the white eye allele (Xw) and one with the wild-type red eye allele (XW), which then gets fertilized by a sperm with a Y chromosome. Since the father has red eyes, he most likely possesses a normal XY genotype. Therefore, the nondisjunction event in the mother during meiosis I that leads to the XXY, white-eyed daughter would cause her to inherit two X chromosomes (one carrying the white eye allele) from her mother and one Y chromosome from her father.