What is Incomplete Dominance?
Incomplete dominance is when the two alleles inherited from the parents are neither dominant nor recessive, but blend together to give a physical trait that is somewhere between the two.
What is Codominance?
Codominance is quite similar to incomplete dominance in that neither parental trait is dominant nor recessive. However, with codominance, the two traits both appear in the offspring, often showing up in different parts of the plant or animal.
Examples of Incomplete Dominance and Codominance in Plants
In plants, snapdragons exhibit incomplete dominance for color traits. When a red snapdragon and a white snapdragon are crossed (mated), the color of the offspring is neither white nor red. Instead, the resulting snapdragon will be pink.
With codominance in flowers, on the other hand, the resulting offspring between red and white parents would not be pink. Instead, they would be red with white spots or white with red spots, the result of both colors being codominant.
Example of Incomplete Dominance and Codominance in Humans
The most well-studied example of incomplete dominance in humans occurs in the genes for curly hair. Inheriting a gene for curly hair from one parent and a gene for straight hair from the other parent will give a hair texture that is a blend of the two, wavy hair.
A codominant genetic trait in humans occurs with blood types. There are three different alleles for blood type: A, B and O. While the O type is actually the absence of either A or B proteins in the blood, the A and B blood types are codominant. If someone inherits one A allele and one B allele, they will not have a blood type that is in between those two, but will instead have a blood type AB, which expresses both A and B proteins in the blood.
