As you learned in an earlier lesson, the reason you inherit traits from both parents is because of the process of meiosis. Because of meiosis, only one allele (one gene variation) will end up in each of your parents' egg or sperm cells. Which gene ends up in the egg or sperm that created you is very important, though. That's because an allele can be dominant, indicated by a capital letter (A), or recessive, indicated as a lowercase letter (a).

Because allele A is dominant, its presence in a newly formed zygote causes that trait to be expressed. For instance, if allele A expresses a widow's peak, and it ends up in a fertilized egg, the resulting child will have a widow's peak. If, instead, allele a ends up in the zygote, the child may have a straight hairline, not a widow's peak.

The diagram above shows the combination of allele A and allele a from the father and mother, also called a genotype. If the combination is AA, the alleles are identical, and the pair is called a homozygous dominant pair of chromosomes. When the combination is Aa, the alleles are non-identical, and the pair is called a heterozygous dominant pair of chromosomes. If the combination is aa, the alleles are identical but recessive, so the pair is called a homozygous recessive pair of chromosomes.