Question

When red eyed males are crossed with white eyed females, one expects white eyed males and red eyed females in F₁ generation. However, Bridges found that sometimes, in such a cross, red eyed males and white eyed females could be observed. What was the reason for this observation?

• A. Non-disjunction of X chromosomes
• B. Epistasis
• C. Codominance
• D. Homologous recombination

Answer 1

The correct option is A Non-disjunction of X chromosomes

Eye colour is a sex linked trait in Drosophila with red eye colour being dominant over white eye colour. When red eyed males are crossed with white eyed females, one expects white eyed males and red eyed females in F₁ generation, using the knowledge of Morgan's experiments. However, Bridges found that there could arise a situation wherein red eyed males and white eyed females are observed. He observed that such females had a chromosomal pattern XXY instead of the usual XX. Bridges explained this instance on the basis of non-disjunction of X chromosomes in female individuals. Non-disjunction of X chromosomes means an absence of separation of the two homologous X chromosomes during anaphase I of meiosis. Instead, both X chromosomes move together to the same pole. The other pole obviously would receive no X chromosomes. When an egg with two X chromosomes - both containing only the alleles for white eye colour, is fertilised by a sperm carrying Y chromosome (without any allele for eye colour, since this gene is present only on X chromosome), the resultant offspring turns out to be a white eyed female (XXY).