Question at position 23 Duchenne muscular dystrophy (DMD) is caused by rare X-linked recessive allele. It results in progressive muscular wasting and usually leads to death before age 20. An affected person is one with a severe form of DMD caused by hemizygosity or homozygosity for the disease allele. What is the probability that the son of a woman whose brother is affected will be affected? 1/41/21/803/41/187/8

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BlackTom AI · Accepted Answer

The scenario involves an X-linked recessive disorder (Duchenne muscular dystrophy). An affected male has genotype X^d Y. The sister of an affected male has a mother who must be a carrier (X^D X^d) for the affected son to exist in the family, because affected males come from mothers who pass the mutant X to their sons. For a daughter from a carrier mother, there is a 50% chance she will be a carrier herself (X^D X^d) and a 50% chance she will be non-carrier (X^D X^D).

If the woman in question is a carrier, each of her sons has a 50% chance of being affected (X^d from carrier mother times Y from father). Therefore the probability that the son of this woman will be affected is the product of the probability that she is a carrier (50%) and the probability that a carrier mother passes the mutant allele to a son (50%), giving 0.5 × 0.5 = 0.25, i.e., 1/4.

Now, evaluating each option:
- Option 1: 1/4 — This matches the calculated probability (carrier status × affected-son probability). It is correct based on Mendelian inheritance for X-linked recessive traits.
- Option 2: 1/2 — This would be correct if we knew the woman is definitely a carrier and were calculating only the chance a son is affected given carrier status, but that ignores the initial 50% chance she is a carrier. It overestimates by assuming carrier status with certainty.
- Option 3: 1/8 — This would correspond to 25% × 1/2 or 50% × 1/4 in some miscomputed logic; it wrongly combines probabilities and does not reflect the two-stage process (carrier probability and then affected-son probability).
- Option 4: 0 — Suggests impossibility; this is incorrect since there is a real risk for the son to be affected if the mother is a carrier.
- Option 5: 3/4 — This would imply a very high probability, inconsistent with X-linked recessive inheritance and the two-gene (carrier) considerations involved.
- Option 6: 1/18 — A nonsensical fraction in this genetic context, not arising from standard Mendelian probabilities for this scenario.
- Option 7: 7/8 — An even larger fraction that does not fit the two-step inheritance calculation for an X-linked recessive condition in this family context.

In summary, the correct reasoning leads to a final probability of 1/4 for the son to be affected, while all other listed options misstate the risk due to overlooking the necessary carrier probability of the mother or applying incorrect inheritance logic.

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