A dominant allele masks the expression of the recessive gene’s trait. The alleles are homozygous if they code for the same trait and heterozygous if they code for different traits. A homozygous pair might have two dominant or two recessive alleles. Dominant genes are indicated by capital letters, recessives by small letters. For example, P stands for round peas and w stands for the wrinkled variety. A heterozygous pea plant has the Pw allele pair, while a dominant homozygous plant has two dominant genes, PP. Both have round peas. Codominance. Animal scientists have identified the genes that give a horse a roan-colored coat as being codominant.