If a woman is heterozygous for widow's peak (widow's peak is dominant over a smooth hairline) has children with a man with a smooth hairline, what
are the possible phenotypes of their children?
It is impossible to determine the possible phenotypes.
O All of their children will have a widow's peak.
O None of their children will have a widow's peak.
It is possible for their children to either have widow's peak or a smooth hairline.

It is possible for their children to either have widow's peak or a smooth hairline.

Explanation:

Let's say that the widow's peak allele is W (dominant) and the smooth hairline is w (recessive).

A woman heterozygous for widow's peak is Ww. A man with a smooth hairline must have two copies of the smooth allele since it is a recessive trait, so he must be ww.

The cross is Ww x ww

                    W               w

    w            Ww            ww

    w           Ww           ww

There is a 50% chance that the children will have the genotype Ww, and therefore have a widow's peak. There is a 50% chance the children will be ww, and will therefore have a smooth hairline.

So, It is possible for their children to either have widow's peak or a smooth hairline.