Question

Why can't a man transfer a sex-linked disease directly to his son?

Answer 1

Sex-linked diseases may be either X-linked or Y-linked. Y-linked diseases can be passed on to the son, but the Y chromosome is very small with only a few genes on it. Therefore, diseases caused by the genes on the Y chromosome are very few. Also, the chances of occurrence of these diseases in a population are quite rare.
X-linked diseases are not passed on directly from the father to the son as the X chromosome comes from the mother.