Question detail
A couple has two children. The mother is a carrier of a recessive X‑linked disorder and the father has a normal X chromosome. What proportion of the children are expected to be affected?
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At a glance
Question
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exam_style
Style
Topic
Reproduction
Question
A couple has two children. The mother is a carrier of a recessive X‑linked disorder and the father has a normal X chromosome. What proportion of the children are expected to be affected?
Answer
Since the mother is a carrier (X^dY) and the father is normal (X^NY), the children’s genotypes are X^dX^N (female) or X^NY (male). Only the female can be affected, so 1 out of 2 children, or 50 %, are expected to be affected.
Explanation
The answer shows understanding of X‑linked inheritance, correct calculation of genotype ratios and correct application of direct proportion to a simple cross.
Common mistake
Misinterpreting Ratios in Sex‑Determination Crosses
Students often treat the 1:1 ratio of male to female offspring as a fixed rule, ignoring that the ratio depends on the sex of the parent and the specific cross (e.g., XX × XY vs. XY × XY).
Explain that the 1:1 ratio applies only when one parent contributes a single sex chromosome (XX or XY) and the other contributes a single sex chromosome (XY). Use a simple Punnett square to show how the ratio changes when both parents are XY (giving 1/4 XX, 1/2 XY, 1/4 YY) or when both are XX (all XX). Emphasise that the ratio is derived from the proportion of gametes, not a universal rule.
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