When both alleles for a particular trait are identical, the organism is said to be homozygous for that trait. When the alleles differ, the organism is heterozygous. The presence of two different alleles raises the question of whether one or the other, or both, will determine the phenotype of the organism. For his experiments on peas, Mendel chose traits for which one allele of each pair had a decisive effect, completely determining the phenotype even in the presence of the other allele. He called the determining allele "dominant" and the other allele "recessive." Only when the organism is homozygous for the recessive allele does the phenotype show the recessive trait. For instance, the albino skin pigmentation allele is recessive to other pigmentation alternatives.
While some sets of alleles do show a complete dominance-recessiveness relationship, most allele sets do not. Instead, each allele contributes to the phenotype. Such a relationship is called codominance. In humans, the A and B blood group alleles are codominant, and a person inheriting both will have blood type AB. Incomplete dominance is another variant in this system. In this case, the phenotype of the heterozygote is intermediate between the two extremes. Skin color in humans often shows this pattern.
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