What are incomplete dominance and co-dominance?

Incomplete Dominance: Usually if an allele is dominant over another allele, the dominant trait is the only one that shows in the offspring. Sometimes there is no completely dominant trait. This type of inheritance is called incomplete dominance. In cases of incomplete dominance, offspring will show a blending of traits from each parent.

Incomplete dominance can be seen in many organisms. In four o’clock flowers, red and white are possible flower colors. Neither allele is completely dominant over the other. Homozygous red plants are given the symbol RR. Homozygous white plants are given the symbol WW. The capital letter R stands for red petal color. The capital letter W stands for white petal color. Figure 3-20 shows what can happen when homozygous red-flowering plants are crossed with homozygous white-flowering plants.

Co-dominance: A different type of inheritance patter occurs when both alleles of a certain trait appear in the offspring. This is called co-dominance. As in the case of incomplete dominance, neither allele is dominant over the other. However, co-dominance is different from incomplete dominance because it does not involve blending. The alleles from each parent both appear in the offspring.

Co-dominance in Chickens: There is a kind of chicken that has a gene for black feathers and a gene for white feathers. Neither of these genes is dominant over the other. The capital letter B stands for black feathers. The capital letter W stands for white feathers. A chicken that is homozygous for black feathers has the symbol BB. A chicken that is homozygous for white feathers has the symbol WW. When a homozygous black chicken and a homozygous white chicken are mated, all of the offspring have the gene combination BW. These heterozygous offspring have both black and white feathers.

Heterozygous Crosses: Recessive genes do not disappear. They show up again when heterozygous organisms are mated. If two heterozygous black and white chickens are mated, there will be a 25 percent chance of the offspring being BB, or homozygous black. There will be a 50 percent chance of the offspring being BW, or heterozygous black and white. There will be a 25 percent chance of the offspring being WW, or homozygous white.