Another departure from the standard Mendelian situation is codominance. Codominance occurs when two alleles of a gene produce two distinct and detectable gene products. A good example of this is found in human MN blood typing. These blood types are distinct from the better-known ABO blood groups, but the principle is the same. Blood is typed according to what type(s) of antigen (a cellular product that induces antibody formation in a foreign host) are found on the surface of the red blood cells. (Blood type is detemined by reacting the blood with antibody against the antigens. For details of the procedure, refer to Snustad, page 62) Within the MN blood groups, there are are two antigens, M and Nwhose production is determined by a gene with two alleles, L^M and L^N. L^M confers the ability to produce the M antigen, while L^N confers the ability to produce the N antigen. Individuals who have the genotype L^M L^M will have only the M antigen on their red cells, and will be type M. Individuls with the genotype L^N L^N will have only the N antigen on their red cells, and will be type N. Heterozygotes (L^M L^N) produce both antigens, and are type MN.

Note that in the case of codominance, both alleles contribute equally to the phenotype. This is the major difference between codominance and incomplete dominance. With incomplete dominance, the two alleles did not contirbute equally to the phenotype: the W allele contributed red pigment, but the w allele contributed nothing to flower color.

Multiple Alleles

Mendel's concept of inheritance dealt with only two different alleles for a particular gene. After the rediscovery of Mendel's work, it didn't take long before genes were discovered that had more than two possible alleles. Some genes have three, four, or more alleles. An extreme example of this is the white gene, one of the genes controlling eye color in Drosophila. So far, over 100 different alleles of this gene have been identified! Of course, any individual fruit fly would only have a maximum of two of those alleles.

Understandably, having more than two alleles makes genetic investigation more complicated. We can still use Mendel's basic rules to understand inheritance of the trait specified by a multiallelic gene. Let's use coat color in rabbits as an example. The gene for coat color has four alleles:

Which of the alleles is dominant, and which is recessive? Obviously, this is not as straightforward as it was with two alleles. In this case, there is a heirarchy of dominance. The albino allele is recessive. The himalayan allele is dominant to albino, but recessive to everything else. Chinchilla is partially dominant to himalayan and albino, but recessive to wild type. Wild type is dominant to all of the other alleles. This can be shown in the following way:

An explanation for this is that the gene in question is responsible for producing pigment in the fur. The wild-type allele is fully functional, the chinchilla and himalayan alleles are partially functional, and the albino allele is non-functional.

The phenotypes can be determined from the genotype using Mendelian principles, because any individual will only have two of the alleles. For example, a rabbit with the genotype c⁺ c^h would have a wild-type phenotype, because the wild-type allele is dominant to himalayan. A rabbit with the phenotype c^h c would be himalayan, because the himalayan allele is dominant to albino. A rabbit with the genotype c^ch c^h would be chinchilla with himalayan markings, because chinchilla is only partially dominant to himalayan. What phenotype would a c⁺ c^ch rabbit have?

It is also possible to have multiple alleles of a gene that exhibits codominance. The best-known example of this is the human ABO blood group. Everyone knows that there are four blood types within this group: A, B, AB, and O. As with the MN blood group, the different blood types are defined by the presence of different antigens on the surface of the erythrocytes (red blood cells). The gene responsible for producing these cell-surface antigens is called I. This gene has three alleles: I^A, I^B, and I^O. I^A and I^B are codominant, and both are dominant to I^O. Therefore, a person with type A blood can be I^A I^A or I^A I^O, a person with type B blood can be I^B I^B or I^B I^O, a person with type AB blood is I^A I^B, and a person with type O blood is I^O I^O. Practise your crossing skills: what are the possible phenotypes and associated probabilities if a person with AB blood and a person with type O blood have children?