The concept of complete dominance explains that a dominant version of a gene, otherwise known as a dominant allele, is always expressed over a recessive version or allele. If a person inherits at least one dominant allele, then they will have the dominant trait. However, a person must inherit two recessive alleles to have the recessive trait. Despite this clear-cut explanation, many physical traits are not a result of complete dominance.
Codominance
In some instances, neither allele is dominant over the other. This is called codominance and it results in the simultaneous expression of two physical traits (a physical trait is known as a phenotype). A common example of two phenotypes being expressed at once due to codominance is the AB blood type. Having the AB blood type means that one of the two alleles that an individual has inherited results in an A antigen on the red blood cell while the other results in a B antigen. Instead of A being dominant over B or vice versa, they are both expressed on the red blood cell.
Incomplete Dominance
Other examples of phenotypes that do not result from complete dominance are those that result from incomplete dominance. A phenotype in incomplete dominance is a mix of influence by both alleles. This is because the influence of the dominant allele does not completely override that of the recessive allele. So, if one allele (either one) would have otherwise resulted in a dog having white fur and the other would have resulted in the dog having black fur, then the dog would instead have gray fur.
