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 twenties. how likely is it for a woman to have this condition?

Answer 1

Very very unlikely. Since the recessive allele is on the X chromosome, in order for a woman to have this condition, both copies of her X chromosome has to have the recessive allele. One copy of the allele would have to come from her mother, and the second copy from her father. The copy from her mother is a 50/50 chance if her mother was heterozygous with one copy of the recessive allele and one copy of a normal allele and therefore didn't exhibit the dystrophy and therefore didn't know she was a carrier. But the father only has 1 copy of the allele and therefore would exhibit the dystrophy and as such would be unlikely to have engaged in procreative sex that would pass on the defective allele to his offspring. This is especially true since duchenne muscular dystrophy will typically start to manifest itself in male children around starting around age 4.