Question

If the offspring in question come from a father who does not exhibit the trait and a mother who does, what would you expect for the phenotype of the two sons and one daughter?
1) The daughter will exhibit the trait, but the sons will not.

2) The sons will exhibit the trait, but the sons will not.

3) None of the offspring will exhibit the trait.

4) All of the offspring will exhibit the trait.

Answer 1

"Offspring in question..." What's the context here? Judging by the phrasing and given answer choices, I would guess you're working on something involving sex-linked traits (particularly X-linked ones).

Recall that a zygote obtains one X chromosome each from the mother and father. Suppose you have some condition that shows up in homozygous recessive females (let's denote this scenario \(X^nX^n\)). If a male does not exhibit the trait, then he must have the dominant allele on the X chromosome, such that his genotype is \(X^NY\). If the offspring receives one X chromosome from each parent, then there is only one possible X it can get from the father (\(X^N\)) and one possible X from the mother (\(X^n\)), which means the genotype of the female offspring would be \(X^NX^n\). Depending on the inheritance pattern, the phenotype will differ, but if we assume normal dominance, this means each female would not exhibit the recessive trait.

Male offspring, however, have a 100% chance of obtaining the recessive trait. This is because they are guaranteed to receive the Y chromosome from the father and one of the affected X chromosomes from the mother, so that his genotype would be \(X^nY\).

By the way... option 2? Whoever wrote that simply isn't trying.

Answer 2

Also, for future reference, let me remind you this is the math section.