Allele Interactions

Allele interaction becomes important when two alleles are present in the same cell - such as in the case of a heterozygote of a diploid organism. When both alleles are present then both products are being expressed and can both act biologically to affect the phenotype. For instance, we might know that allele A₁ is associated with a particular phenotype, for example red hair. And we might know that allele A₂ is associated with a different phenotype, say brown hair. In both cases the phenotype is the result of how the allele product acts biologically: A₁A₁ individuals will have red hair and A₂A₂ individuals will have brown hair.

What we want to know is the phenotype that is produced when both alleles are present, in other words the phenotype of A₁A₂ heterozygotes. How the two alleles interact will determine what this phenotype is. There are many ways that allele products can interact at the molecular level but the general phenotype outcomes can be classified into a few general categories. (Since we are only concerned right now with the phenotype that results from allele interaction, we will not discuss the molecular details of the interaction of the products.) The general categories or outcomes are:

• Dominance: Dominance, and the complementary concept of Recessive, are the form of interaction that Mendel famously introduced. Dominance is the condition under which one allele of the pair in a heterozygote manifests the phenotype, that is it gives the heterozygote its phenotype. The other allele is, by definition, recessive - that is, a dominant allele must be dominant TO something. So, if you refer to a dominant allele you must refer to a recessive allele that it is dominant to. In the example of hair color above, if A₂ is dominant to A₁ then heterozygotes will have brown hair.


• Incomplete Dominance: Another type of interaction is Incomplete Dominance. This is when the phenotype of a heterozygote (e.g. B₁B₂) is intermediate between the phenotype observed in the homozygotes for each allele (B₁B₁ and B₂B₂). For example, if B₁B₁ plants have red flowers, B₂B₂ plants have white flowers and B₁B₂ plants have pink flowers, then it is a case of incomplete dominance.


• Co-dominance: Co-dominant alleles produce independent effects in the heterozygote. The result is a phenotype that has both effects, or both phenotypes. A classic example is that of the A and B blood type alleles. The heterozygous state has AB blood - both A and B phenotypes are produced and act independently. You should not mistake this for incomplete dominance, which is an intermediate phenotype, not one in which the two alleles are both manifested independently. This is explained in more detail here.

Dealing with multiple alleles: This is not a separate category or type of interaction, but we do need to consider the case when there are 3 or more alleles that you are working with for a particular gene. In such a case it is important to know what type of allele interaction occurs between each pair of alleles. The type of allele interaction cannot be assumed in advance and just because you know the type of interaction between, say A₁ and A₂, and the interaction between alleles A₂ and A₃ you cannot assume that you know how A₁ and A₃ will interact. You need to observe the phenotype of a heterozygote. In other words, if A₁ > A₂ (i.e. A₁ dominant to A₂) and A₂ > A₃ you CANNOT conclude that A₁ > A₃.