Question

An affected father and a phenotypically normal woman have a son who is affected with the same trait as the father. Which of the following modes of inheritance can be excluded for this trait?

a)Autosomal recessive
b)X-linked recessive
c)Autosomal dominant
d)X-linked dominant
e)Y-linked

Answer 1

X-linked recessive inheritance can be excluded because a son with an affected phenotype inherited a Y chromosome from his father, not the disease-carrying X. Y-linked inheritance is also excluded as the son is affected, but this mode of inheritance would mean every male offspring from the father would be affected. The scenario is consistent with autosomal dominant inheritance.

Step-by-step explanation:

In the scenario provided, X-linked recessive inheritance can be ruled out as a possible mode of inheritance for the trait in question because for an X-linked recessive trait, females would have to carry two copies of the mutation to exhibit the phenotype. Since the mother is phenotypically normal and the son is affected, it suggests that he inherited a Y chromosome from his father, not the X chromosome that would carry the trait if it were X-linked recessive. Therefore, one allele from the mother would be normal.

Furthermore, Y-linked inheritance can also be excluded since the only way for a son to inherit a Y-linked trait is directly from his father, which typically results in all male offspring being affected. That leaves us with autosomal dominant inheritance, which is consistent with the affected father passing the trait to his son, as it requires only one copy of the dominant allele to express the trait.