Red green colour blindness is far commoner in men than in women because
- A. a man has only one X chromosome, so a single recessive allele is expressed
- B. the gene lies on the Y chromosome
- C. the allele is dominant in men and recessive in women
- D. women lose the allele during meiosis
Explanation
The gene is on the X chromosome and a man is hemizygous, so there is no second copy that could mask a defective allele. A woman needs the recessive allele on both of her X chromosomes to be colour blind, which is a far less likely combination, and with only one she is a carrier with normal vision. The Y chromosome carries no corresponding gene.
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