Classification of alleles depends on how the phenotype is defined
In all the examples given in this module, the alleles are described based on the phenotype(s) they elicit in the organism.
Although we will commonly describe alleles as dominant or recessive (or incompletely dominant, codominant, etc), what we actually mean when we use these terms is that the resulting phenotype is dominant or recessive. We’ve seen this already in the case of recessive lethality. For example, the agouti phenotype in mice is dominant (heterozygotes for the agouti allele have the agouti phenotype), but the allele confers recessive lethality. The phenotypes associated with a pleiotropic allele may be all be dominant, all recessive, or a mixture.
Because of this, there may be multiple ways to classify an allele. A classic example of this is the HbS allele associated with sickle cell disease (Figure 9). Depending on how the phenotype is identified, the HbS allele can be considered completely recessive, dominant, incompletely dominant, and codominant. Ready for things to get complicated? Let’s look:
- The HbS allele can be considered recessive: sickle cell disease only affects people with two copies of the HbS allele, which makes SCD a recessive disease.
- The HbS allele can be considered dominant: People heterozygous for the HbS allele are said to have Sickle cell Trait because their red blood cells can sometimes sickle under extreme conditions that lower blood oxygenation. They do not typically have the health problems associated with SCD. But if the ability of the blood to sickle is what “counts”, then HbS is dominant because only one allele must be present.
- The HbS allele can be considered incompletely dominant: People with Sickle cell Trait do not have the severe health problems associated with Sickle cell Disease, but under rare conditions they may experience a health crisis because of sickling. This could be considered a much milder form of disease, in which case the allele could be classified as incompletely dominant since the heterozygote has an intermediate phenotype.
- The HbS allele can be considered codominant: The HbS and HbA alleles of the hemoglobin beta gene produce slightly different versions of hemoglobin. The different forms of the protein can be detected via lab techniques like chromatography or gel electrophoresis (Figure). People with sickle cell trait have the genotype HbS/HbA. Both versions of the protein are produced, so the HbS allele can be considered codominant.
It is common to describe an allele as dominant or recessive – geneticists use this shortcut in language all the time. Instead of saying “the allele that causes the dominant phenotype”, we’ll just say “the dominant allele”. The terms are even used that way in this textbook. But keep in mind that this shortcut of vocabulary is imprecise: it is the phenotype, not the allele, that is dominant or recessive. This is especially important when talking about pleiotropic alleles.
