genotypes, for each characteristic: the homozygous dominant combination has two dominant alleles, the homozygous recessive combination has two recessive alleles, and heterozygous combination has one dominant allele and one recessive allele. Yet, there are generally only two possible physical appearances, called phenotypes, of each trait, dominant and recessive, since both the homozygous dominant and heterozygous genotypes will express the form of the character coded by the dominant allele.
In Mendel's experiment that crossed plants bearing green pods with those bearing yellow pods, the resulting genotypic and phenotypic combinations were as follows. In the P generation, the parent with the green pods was genotypically homozygous dominant having two alleles for green pods while the parent with yellow pods was genotypically homozygous recessive having two alleles for yellow pods. In the F1 generation, all the individuals were genotypically heterozygous, having one allele for green pods and one allele for yellow pods, and phenotypically dominant. In the F2 generation, the genotypes were in a ratio of 1:2:1: there was one homozygous dominant individual, having two alleles for green pods, to every two heterozygous individuals, having one allele each for green and yellow pods, to every one homozygous recessive individual, having two alleles for yellow pods. The F2 generation phenotypes were in a ratio of 3:1: there were three plants with green pods, expressing the total number of plants with the dominant characters, to every one plant with yellow pods.
Codominance is the condition in heterozygous genotypes when neither allele is
