Write out the phenotypes. A person with widow's peak always has at least one parent who also has the trait, whereas people with a straight frontal hairline may occur in families in which one or even both parents have widow's peak. When both parents have a straight frontal hairline, all children also have the same trait. Using the letter W express the most likely genotypes for people with and without widow's peak.
Some individuals have one whorl of hair on the back of the head, while others have two.
Albinism, the total lack of pigment, is due to a recessive allele. A man and woman plan to marry and wish to know the probability of their having any albino children. These homozygous people have a much higher chloride content in their sweat than normal people. Heterozygotes, who live a normal lifespan, have an intermediate chloride content. A couple planning marriage decide to have a sweat test because a brother of the man died in infancy from the disorder.