Their children have one of three genes

Let's look at Mendel's peas, for example.Let's write the dominant yellow allele as an uppercase "Y"and the recessive green allele as a lowercase "y."The uppercase Y always overpowers his lowercase friend,so the only time you get green babies is if you have lowercase Y's.In Mendel's first generation,the yellow homozygous pea mom will give each pea kid a yellow-dominant allele, and the green homozygous pea dad will give a green-recessive allele.So all the pea kids will be yellow heterozygous.Then, in the second generation, where the two heterozygous kids marry,their babies could have any of the three possible genotypes,showing the two possible phenotypes in a three-to-one proportion.