Question

Which statement most accurately describes the pattern of inheritance for a human disease caused by a dominant allele on the X chromosome (assuming the disease allele is rare)?
a. Affected mothers have all affected sons, but no affected daughters.
b. Only females inherit the disease.
c. Affected fathers have all affected daughters, but no affected sons.
d. Only males inherit the disease.

Answer 1

Answer:

c. Affected fathers have all affected daughters, but no affected sons.

Explanation:

Given that the disease is caused by a dominant allele on X chromosome, an affected father would have a disease allele in the X chromosome that passes along to the male gamete, while the gametes containing a Y chromosome would not have it (since it is X-linked).

During sexual reproduction, the father passes an X chromosome to its daughters (the second X chromosome being contributed by the mother) but an Y chromosome to its sons (the single X chromosome being contributed by the mother).

Indeed, within an affected father's offspring, all his daughters will receive the affected X chromosome from him, while his sons will receive an unaffected Y chromosome (since the disease is X-linked). These males will not inherit the disease, for they will receive an unaffected X chromosome from her mother (which is expected to be healthy since the disease is rare).