A woman with thick lips marries a man with thin lips whose father had thick lips.

A woman with thick lips marries a man with thin lips whose father had thick lips. What is the likelihood of having children with thin and thick lips if the gene for thin lips is recessive to the gene for thick lips?

A recessive gene appears in the phenotype only when it is in a homozygous state. If thin lips are a recessive trait, then, regardless of which lips the father had, the man is homozygous for this trait. But a woman with thick lips can be either heterozygous or homozygous.

Let A be thick lips and a thin

1.RR f AA (T) x m aa (t)

m / f a A Aa (T)
By phenotype: 100% thick lips.

2.RR f Aa (T) x m aa (t)

m / f a A Aa (T) a aa (t)
By phenotype:

50% thick lips, 50% thin lips.