Previously we learned about different modes of inheritance – complete dominance, incomplete dominance, and polygenic. We wrap-up with one last type – codominance. In many ways, codominance is very much like incomplete dominance except that traits do not blend. This is why a person can have type AB blood.

Key terms

Codominant (adj.) – relationship between two versions of a gene where neither is masked; both are fully expressed in the phenotype

Glycoprotein – biomolecule made of a combination of carbohydrates and proteins

Details

Red blood cells are coated by hundreds of chemicals that can be recognized by our immune system. This is the basis of a “blood type” – the chemical signatures found on red blood cells. There are about 100 different blood types, but we will just focus on three: ABO blood groups, Rh factor, and sickle-cell trait.

ABO blood group

On the outer surface of red blood cells are molecules called glycoproteins. These glycoproteins (Figure 6-9) are constructed by the cell and modified by an enzyme (glycosyltransferase) which comes in three varieties. So far, we have only looked at characteristics with two alleles, but the ABO blood group has three. Here is what each allele does:

A – this adds a chemical group to make a glycoprotein we call “type A.” This is a dominant allele.
B – this adds a chemical group to make a glycoprotein we call “type B.” It’s also a dominant allele.
O – this does not modify the glycoprotein, essentially leaving it “blank.” This is a recessive allele.

Diagrams depicting glycoproteins on surface of red blood cells for ABO blood group.

FIGURE 6-9. Blood glycoproteins. The surface of red blood cells have glycoproteins on the surface that allow other body cells to recognize them. Glycoproteins come in three types – A, B and O – depending on the mix of chemicals. Type AB blood has both A and B glycoproteins on the surface. ("ABO blood groups" by InvictaHOG is in the Public Domain)

Since two alleles are dominant (codominant) it means they can both express themselves without one masking the other. This allows for more possible phenotypes. Not only that, but there is also a recessive allele – the O allele. We can follow the same rules of inheritance we have used before, but will need to pay more attention to interpreting phenotypes from genotypes. (Table 6a describes this).

Table 6a. ABO blood groups. All possible genotypes are shown for each phenotype.

Phenotype	Possible genotypes
Type A	AA or AO
Type B	BB or BO
Type AB	AB only
Type O	OO only

Stop & Think #14: What blood types could children have if both parents were type AB blood?

This has important implications when it comes to blood transfusions. Since this is one of the ways your body recognizes your own cells, you naturally make antibodies that attack foreign blood types. For example, if you have type A blood then you make antibodies that attack type B glycoproteins. This causes them to cluster together in the bloodstream, which clogs up arteries. This is why it is important to match blood types of donor and recipient.

Stop & Think #15: What types of antibodies would a person with type AB blood have?

Stop & Think #16: Could parents, who are type O and type A, have a child with type O blood?

Rh factor

The other commonly reported blood type is the Rh factor. This one is simple – there are just two alleles, one dominant and one recessive. That is what is happening from the inheritance point of view, but here is what is happening on the surface of blood cells:

Rh positive (dominant) – creates a protein on the surface of red blood cells that triggers an immune response in other people
Rh negative (recessive) – creates one of dozens of proteins that fails to cause an immune response in other people or no protein at all

Blood types are usually given as two phenotypes combined. Blood types, like “A-negative” or “O-positive,” are really reporting on two genes – the ABO blood group and Rh factor. For example, the A-negative phenotype could have a genotype of AArr or AOrr.

Stop & Think #17: What is the probability of having a child with A-positive blood, if parents are AB-negative and O-positive (heterozygous for Rh factor)?

The clinical significance of this has to do with childbirth. If a Rh-negative mother carries a Rh-positive child, her immune system may be exposed to the child’s blood during the birth process. The potential problem is with the next child. If that child is also Rh-positive, the mother’s immune system could attack the child’s blood and harm the developing fetus. Fortunately, mothers can be treated shortly after giving birth with medicines that bind up the baby’s blood cells and prevent the immune reaction from ever taking place.

Sickle-cell

Sickle-cell is hard to categorize in terms of inheritance. Depending on your perspective, it can be considered incomplete or co-dominance. From the broadest perspective, there are three possible phenotypes:

Sickle-cell disease: individuals have mis-shaped red blood cells that stick together causing kidney and heart failure, among other problems. Without medical intervention individuals seldom live past age 2. This is the result of inheriting two recessive alleles.
Sickle-cell trait: individuals have some mis-shaped red blood cells, which cause problems, but those are seldom severe. This is the result of being heterozygous.
Normal: individuals have normal, donut-shaped red blood cells. This is the result of having two dominant alleles.

The molecular root of these disorders is a genetic mutation for hemoglobin, the protein that carries oxygen. A single change to the genetic code causes the change in one amino acid. Normal hemoglobin (HbA) carries oxygen very effectively. Hemoglobin from the mutant allele (HbS) does not bind oxygen as well. HbS also forms a gel, which changes the shape of red blood cells to look sickle-shaped (Figure 6-10).

Artwork showing normal blood cells and sickle cells in capillaries. The sickle cells get clumped together as arteries narrow.

FIGURE 6-10. Sickle cell. Normal and sickle-shaped blood cells shown. ("Sickle-cell anemia" by BruceBlaus is licensed under CC BY 4.0)

Individuals with both alleles (HbA and HbS) make both types of hemoglobin. Those with both types of alleles show mostly normal blood, which becomes sickle-shaped when oxygen levels are low.

If you stop to think about it, the question that will probably come to mind is, how has the sickle-cell allele been able to escape natural selection? It seems like a disease with such devastating effects would have been eliminated by natural selection. In many areas, that is what happened. The allele is very rare in many populations. However, the allele is more common in areas with mosquitoes that carry the microorganism that causes malaria, Plasmodium falciparum. This microorganism is a protozoan (part of the Protist group).

The malaria protozoan invades the liver but periodically leaves the liver to seek red blood cells where they consume the proteins inside. This is part of their reproductive process. However, individuals with HbS hemoglobin, are naturally resistant to malaria because the germs are harmed by this malformed protein.

Stop & Think #18: If mosquitoes that carry malaria went extinct, what would you expect to happen to the frequency of the sickle-cell allele?

The resistance to malaria has caused the sickle-cell trait to become more common in some areas of the world (Figure 6-11). The heterozygous genotype provides the best possible compromise. In these locations, people with normal blood are at risk of death from malaria. Those with sickle-cell disease (homozygous recessive) are likely to die from the disease. But having one sickle-cell allele is a survival advantage. Natural selection in action!

Two maps of Africa.
1. Distribution of malaria germ.
2. Frequency of sickle cell allele.
Both maps show considerable overlap.

FIGURE 6-11. Map of sickle-cell incidence. Left: map of distribution of the Plasmodium microbe that causes malaria. Right: distribution of the sickle-cell allele frequencies. Note: malaria and sickle-cell also occur outside Africa. ("Malaria vs sickle-cell anemia distributions" by Anthony Allison is in the Public Domain, CC0)

Learning objectives

Solve inheritance problems with codominant alleles.
Solve inheritance problems for two blood types (such as A-negative and B-positive)

Stop & Think Questions Explained

14. Two AB parents could have children with any blood type, except Type O.

15. A person with AB blood would have no antibodies. That is why they are universal recipients of other blood types.

16. Yes, they could. However, this is only possible if the type A parent has the genotype AO. If that is the case, both parents could pass on the O allele, resulting in a child with type O blood.

17. Probability of type A blood is 50%. Probability of Rh negative is 50%. Multiplying the two gives a 25% probability of this combination.

18. If malaria-carrying mosquitoes went extinct, then the conditions favoring having sickle-cell trait would go away. In this case, the sickle-cell allele would probably decrease in the population.