Widow's peak, a pointed hairline on the forehead, is a genetic trait caused by a dominant allele. it can be traced back through a family's history using pedigree analysis. the pedigree shown here is of three generations of a family. notice that some individuals (shown in gray) have a widow's peak (w = dominant allele and w = recessive allele). this pedigree supports the fact that widow's peak is due to a dominant allele, because if it were due to a recessive allele and both parents show the recessive phenotype, then

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.

They must use the scientific method