Question

While crossing pure lines of white eyed female and red eyed males of Drosophila, a young breeder was much puzzled to find a rare specimen of white-eyed female along with a large population of red-eyed females and white-eyed males. What possible explanations could you suggest for the presence of this exceptional female in his experiment?

Answer 1

The presence of a rare white-eyed female along with many red-eyed females and white-eyed males in a cross of pure white-eyed females with red-eyed males of Drosophila can be explained by an event called non-disjunction of the X chromosomes.

Normally, eye color in Drosophila is a sex-linked trait carried on the X chromosome, with red eyes being dominant and white eyes recessive. Females have two X chromosomes (XX) and males have one X and one Y (XY). White-eye mutation is recessive, so a female must have two copies of the white-eye allele to express white eyes, while a male needs only one copy on his single X chromosome.

During normal meiosis, X chromosomes segregate so that gametes receive only one X chromosome. However, non-disjunction is a failure of the sex chromosomes to separate properly during meiosis. In the case of non-disjunction, an egg could receive two X chromosomes or no X chromosome.

• If a rare egg contains two X chromosomes both carrying the white-eye allele, when fertilized by a sperm carrying an X chromosome with white-eye allele, the resulting female offspring would be white-eyed because she is homozygous for the white-eye allele.
• This could explain the rare white-eyed female appearing among a majority of red-eyed females (which have one red-eye allele making them red-eyed) and white-eyed males (which have one X with the white-eye allele).

Therefore, the exceptional white-eyed female is likely due to non-disjunction of X chromosomes leading to an abnormal XX composition with white-eye alleles on both X chromosomes.