a. The following are the parents' most likely genotypes:
The mother (XNXn): She has one normal X chromosome (XN) and one X chromosome with the Duchenne muscular dystrophy recessive allele (Xn).
The father (XY) has one X chromosome and one Y chromosome. He must have a normal X chromosome (XN) because he does not have the disease.
b. The allele for Duchenne muscular dystrophy (DMD) originated from the mother. Because the mother is a carrier (XNXn), the boy inherited the X chromosome with the DMD allele from her. The father (XY) provided his Y chromosome, which lacks the DMD allele.
c. It is unlikely that the young man's sister will also have DMD. She will receive one X chromosome from her mother (Xn), which may carry the DMD allele, and one from her father (XN). As a result, her genotype will be XNXn, and she will be a carrier like her mother. However, she is unlikely to get the condition unless she inherits the X chromosome containing the DMD variant from both parents.
d. Male children are more likely than female children to exhibit sex-linked features since they only inherit one X chromosome from their mother, whereas females acquire two X chromosomes. Males have no equivalent X chromosome with the normal allele to hide the effects of the recessive allele if the X chromosome carries a recessive gene for a sex-linked trait. Females, on the other hand, have two X chromosomes. Thus, if one bears a recessive allele, the other X chromosome may have the normal allele, offering protection. This is especially true for sex-linked disorders because males have just one X chromosome and are more likely to express the trait if they receive the recessive allele, but females often require the inheritance of two recessive alleles to express the phenotype.