Red green colour blindness is far commoner in men than in women because
Correct answer: A. a man has only one X chromosome, so a single recessive allele is expressed
- 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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About X-linked Recessive Inheritance
X-linked recessive inheritance involves genes on the X chromosome, so males express a recessive allele with no corresponding allele on their single X chromosome, while females may be carriers. Pedigrees cover haemophilia and red-green colour blindness, including carrier mothers, affected sons and the absence of father-to-son transmission.
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