A woman who is a carrier for colour blindness marries a man with normal colour vision. The probability that any son of theirs is colour blind is
Correct answer: C. one in two
- A. 0
- B. one in four
- C. one in two
- D. certain
Explanation
A son takes his single X from his mother, and half of her X chromosomes carry the defective allele, so half the sons are colour blind. The answer of one in four applies to the different question of the chance that a child of either sex is colour blind, which is the usual mistake here. None of the daughters is colour blind, since each receives a normal X from the father, but half are carriers.
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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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