Question

Eye colour in fruitfly is a sex-linked trait and the cross of a white-eyed female with red-eyed male gives red-eyed females and white-eyed male. Rarely this cross may give all white-eyed females and red eyed males. This was found to be due to nondisjunction of
A) Two Y chromosomes in female fly
B) Two X chromosomes in female fly
C) Four Y chromosomes in the male fly
D) Four X chromosomes in the male fly

Answer 1

These are recessive disorders associated with one of the allosomes. They are recessive and dominant diseases. X -linked recessive diseases are more common in males than in females.

Complete answer:
Sex-linked diseases are mostly observed in humans. These are traits that are found on either one among the chromosomes that determine the sex, or the sex chromosomes. And in humans, this is often linked to X or Y chromosomes. Some familiar sex-linked traits are haemophilia, red-green colour blindness, congenital night blindness, some high blood pressure genes, Duchenne dystrophy and the fragile X syndrome. The cross of a white-eyed female with red eyed male gives red eyed female and white-eyed males. Rarely, this cross may give all white-eyed females and red eyed males because this results from non-separation of two X chromosomes during anaphase I of meiosis. Both X chromosomes go together to the same pole resulting in the formation of one gamete with XX and others with no X chromosome. Fertilization of XX gamete with Y sperm forms XXY and fertilization of cabinets with no X chromosome to X-sperm forms red-eyed males. This phenomenon is called non-disjunction of two X chromosomes.
Thus, the correct answer is option B.

Note: Sex-linked diseases can be dominant and recessive. Most of the males are observed to get affected by diseases of X and Y sex-linked chromosomes. Sex-linked diseases are controlled by the genes present on the sex chromosomes.