As you can see from the pedigree presented below, Joseph and Alice both have Widow's peak, but one of their children possesses the trait and the other doesn't. If this were a trait that was determined by a recessive allele, this would be impossible, because the two parents would possess two recessive alleles and their offspring could only get the recessive allele from them.
But because this is a trait determined by a dominant allele, an individual can have the trait, but also have offspring that do not possess the trait.
