Final answer:
In a cross where three nostrils is a Y-linked trait, all male offspring will inherit the trait from their father and have three nostrils, while all female offspring will have two nostrils, as they cannot inherit a Y chromosome from their mother.
Step-by-step explanation:
When considering the cross where a trait is Y-linked, the Y chromosome is only passed from father to son, so all sons will exhibit the Y-linked traits of their fathers. A woman with two nostrils (not having the Y-linked allele for three nostrils) mating with a man with three nostrils will have all male offspring with three nostrils and all female offspring with two nostrils because the daughters cannot inherit a Y chromosome from their mother. So the expected phenotypic ratio of their offspring would be that all sons will have three nostrils (inherited from their father), and all daughters will have two nostrils (having inherited their mother's X chromosome and not inheriting a Y from the mother).