Liam's father has blue eyes, but Liam has brown eyes, like his mother. As an adult, Liam marries a woman with blue eyes, and now they are expecting their first baby. Is it possible for the child to be born with blue eyes?

A) Yes. Liam may carry the recessive gene for blue eyes.

B) Yes. When one parent has blue eyes and the other has brown, the child always has a 50% chance of getting the blue eyes.

C) No. As long as one parent has brown eyes the child will always have brown eyes.

D) No. Since Liam has brown eyes, he only carries genes for brown eyes.

Okay, so:

If we write the blue (recessive) gene as b and the brown (dominant) gene as B, then we can deduce from Liam's ancestry that he must have Bb genes (since otherwise his father couldn't have had blue eyes).

Therefore, Liam's genes of Bb and the woman's genes of bb can combine in the following ways:

Bb, Bb, bb, bb (repeats shown)

This means that half the time (the half with the B) the child will have brown eyes. The other half, the child will have blue eyes.

However, the answers to this are a bit confusing, and I'm not sure which one you should pick. B is correct in that it says the part about 50%, but it is not true that a child will always have this chance, as one parent may not carry a recessive gene. A is correct where it says yes, but incorrect in that it says Liam may carry the blue-eyed gene - he actually must have it.

Either way, you now know how the genetics works, and perhaps writing an explanation will get you the mark instead. ¯/_ (ツ) _/¯

Good luck!