Dominance Revisited

One of Mendel's important contributions to the study of heredity is the concept of dominance—the idea that an individual possesses two different alleles for a characteristic, but the trait enclosed by only one of the alleles is observed in the phenotype. With dominance, the heterozygote possesses the same phenotype as one of the homozygotes. When biologists began to apply Mendel's principles to organisms other then peas, it quickly became apparent that many characteristics do not exhibit this type of dominance. Indeed, Mendel himself was aware that dominance is not universal, because he observed that a pea plant heterozygous for long and short flowering times had a flowering time that was intermediate between those of its homozygous parents. This situation, in which the heterozygote is intermediate in phenotype between the two homozygotes, is termed incomplete dominance.

Dominance can be understood in regard to how the phe-notype of the heterozygote relates to the phenotypes of the homozygotes. In the example presented in I FIGURE 5.2, flower color potentially ranges from red to white. One homozygous genotype, A1 A1, codes for red flowers, and another, A2A2, codes for white flowers. Where the heterozygote falls on the range of phenotypes determines the type of dominance. If the heterozygote (A1 A2) has flowers that are the same color as those of the A1 A1 homozygote (red), then the A1 allele is completely dominant over the A2 allele; that is, red is dominant over white. If, on the other hand, the heterozygote has flowers that are the same color as the A2A2 homozygote (white), then the A2 allele is completely dominant, and white is dominant over red. When the heterozygote falls in between the phenotypes of the two homozygotes, dominance is incomplete. With incomplete dominance, the heterozygote need not be exactly intermediate (pink in our example) between the two homozygotes; it might be a slightly lighter shade of red or a slightly pink shade of white. As long as the heterozygote's phenotype can be differentiated and falls within the range of the two homozygotes, dominance is

Complete dominance

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