Variations of Dominance: Videos & Practice Problems

Complete dominance occurs when a dominant allele completely masks the effect of a recessive allele in a heterozygous individual. This means that the phenotype of a heterozygous individual is identical to that of a homozygous dominant individual. In contrast, incomplete dominance results in a phenotype that is intermediate between the two homozygous phenotypes. For example, if a red flower (homozygous dominant) is crossed with a white flower (homozygous recessive), the offspring will be pink (heterozygous) under incomplete dominance. This intermediate phenotype occurs because the single dominant allele does not completely mask the effect of the recessive allele.

Codominance and incomplete dominance are both forms of non-Mendelian inheritance, but they differ in how the alleles are expressed. In codominance, both alleles in a heterozygous individual are fully expressed, resulting in a phenotype that shows both traits equally. A classic example is the AB blood type in humans, where both A and B alleles are expressed, resulting in the AB blood type. In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes. For instance, crossing a red flower with a white flower results in pink offspring. Thus, codominance shows both traits simultaneously, while incomplete dominance shows an intermediate trait.

Codominance in human blood types is exemplified by the ABO blood group system. The A and B alleles are both dominant and are represented as I^A and I^B. When an individual inherits both I^A and I^B alleles, both alleles are expressed equally, resulting in the AB blood type. This means that both A and B antigens are present on the surface of red blood cells. If an individual inherits two I^O alleles (recessive), they will have type O blood, which lacks A and B antigens. Thus, codominance allows both dominant alleles to be expressed simultaneously in the phenotype.

An example of incomplete dominance in plants is seen in the snapdragon flower. When a red-flowered snapdragon (homozygous dominant, RR) is crossed with a white-flowered snapdragon (homozygous recessive, rr), the resulting offspring (heterozygous, Rr) have pink flowers. This pink color is an intermediate phenotype, demonstrating incomplete dominance. The single dominant allele (R) does not completely mask the effect of the recessive allele (r), resulting in a blend of red and white, which appears as pink in the heterozygous plants.

In complete dominance, the phenotype of a heterozygous individual is identical to that of a homozygous dominant individual. This means that the dominant allele completely masks the effect of the recessive allele. For example, in pea plants, a heterozygous plant with one allele for tallness (T) and one for shortness (t) will be tall, just like a homozygous tall plant (TT). In incomplete dominance, the phenotype of a heterozygous individual is intermediate between the two homozygous phenotypes. For instance, in snapdragons, a heterozygous plant with one allele for red flowers (R) and one for white flowers (r) will have pink flowers, an intermediate phenotype between red and white.