Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. The patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein. They rarely live past their 20s. How likely is it for a woman to have this condition?
A) Women can never have this condition.
B) One-fourth of the daughters of an affected man would have this condition.
C) One-half of the daughters of an affected father and a carrier mother could have this condition.
D) Only if a woman is XXX could she have this condition.

Question

Duchenne muscular dystrophy is a serious condition caused by a recessive allele of a gene on the human X chromosome. The patients have muscles that weaken over time because they have absent or decreased dystrophin, a muscle protein. They rarely live past their 20s. How likely is it for a woman to have this condition?
A) Women can never have this condition.
B) One-fourth of the daughters of an affected man would have this condition.
C) One-half of the daughters of an affected father and a carrier mother could have this condition.
D) Only if a woman is XXX could she have this condition.

Quizplus · Accepted Answer

Women have two X chromosomes, so if one chromosome has the recessive allele for Duchenne muscular dystrophy and the other X chromosome does not, the woman is a carrier but does not express the disease because she has a normal copy of the gene on the other X chromosome. However, if the woman has a son, there is a 50% chance that she passes the recessive allele to him, which would cause him to have Duchenne muscular dystrophy. If a woman is a carrier and has a daughter, there is a 50% chance that she passes the recessive allele to her daughter, but the daughter would need to inherit the recessive allele from the father's X chromosome to express the disease. So, the probability that a carrier mother and an affected father would have a daughter with Duchenne muscular dystrophy is 1/2 x 1/2 = 1/4.

Duchenne Muscular Dystrophy Is a Serious Condition Caused by a Recessive