Mrs. and mr. williams both have widow's peaks (dominant). their first child also has a widow's peak, but their second child doesn't. mr. williams accuses mrs. williams of being unfaithful to him. is he necessarily justified?

Mr. William has no legitimate reason. It is unclear the genetics of the widow'speak  and how or if it is inherited. It's possible that if you have the widow's peak, somebody in your family also has one and it's not necessary to know if your siblings have the widow's peak doesn't mean that you have different genes from them.

Explanation:

Evidence is inadequate to conclude that a widow’s peak is the result of a single dominant gene. It may very well be that multiple genes are involved that can also result from the other child not to have widow's peak.