It gets even more interesting when a person has both an A and a B allele. In this case, the A allele will make the A proteins, and the B allele will make the B proteins. This leads to red blood cells that have two different protein decorations.
Since both the A and the B alleles are visible in the blood, the trait is codominant. One thing to keep in mind is that dominance is only important in how it affects the trait. Terms like recessive, dominant, codominant, and incomplete dominance all refer to the trait phenotype , not the set of genes we have genotype. The sickle cell version of the hemoglobin gene is a great example of this idea. As you can see below, depending on what trait we look at, the same allele can be dominant, recessive, or codominant:.
Sickle cell anemia is a disease where red blood cells become thin and elongated. If a person has one copy of the sickle cell allele, half of their red blood cells will be misshapen. In this way, the allele is codominant , since both normal and sickled shapes are seen in the blood. But the allele can sometimes look recessive too. The misshapen cells caused by the sickle cell allele are not able to carry oxygen as efficiently which can cause anemia. However, if the person only has one sickle allele, they are able to get enough oxygen from the normal shaped blood cells.
Only people with two copies of the sickle allele get anemia, making this trait recessive. Finally, having the sickle shaped cells increases malaria resistance read more about how here. Since only one copy of the allele is needed to give resistance, this trait is dominant. As you can tell, dominant, recessive and codominance can be a bit complicated. But however complicated, one thing is for sure—there are definitely codominant traits in people.
By Abbey Thompson, Stanford University. People do too! AB blood type is codominant because the red blood cells have the products of both the A and the B alleles of the ABO gene. The polymers get very, very long, and they stretch the cell out of shape. Now your spleen is very good at raking these cells out of the peripheral blood as they come through.
And that's what causes the anemia, and the old name for sickle cell was sickle cell anemia. However, though, short polymers are very, very dangerous. If you think of a red blood cell going through your veins and arteries and capillaries as a water balloon, you'll see how it can squish itself down into a long cylinder to get through a capillary and flatten out like a pancake to get through areas in the spleen that kind of rake out the bad-shape red blood cells, and then to form right back to a regular water balloon shape to go into the arteries and veins.
But if you have a red blood cell full of these small hemoglobin S polymers, it's like having a water balloon full of ice chips, and as that goes through the capillaries and the small veins, it tears up the lining of these things, just as if you had a cut, and it activates your clotting response and micro-clots will form. And sometimes these get to be bigger and bigger clots, and so the real lethality in sickle cell disease is not from the anemia, it's from this vascular disease.
And so it's very unfortunate that about one-third of people who are homozygous for hemoglobin S will have one or more strokes before they're 10 years old. And in those fortunate few that are able to get to be adolescents, these clots can build up in the lungs and give a very severe disease called acute chest syndrome, which is basically emphysema or lung destruction from these obstructions.
About one-third of the patients will live healthily to adulthood, but they have many problems with iron-overloaded organs, and their life span is significantly shorter. Test Crosses. Biological Complexity and Integrative Levels of Organization. Genetics of Dog Breeding.
Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity.
Citation: Miko, I. Nature Education 1 1 Why can you possess traits neither of your parents have? The relationship of genotype to phenotype is rarely as simple as the dominant and recessive patterns described by Mendel. Aa Aa Aa. Complete versus Partial Dominance. Figure 1. Figure Detail. Multiple Alleles and Dominance Series. Summarizing the Role of Dominance and Recessivity. References and Recommended Reading Keeton, W. Heredity 35 , 85—98 Parsons, P. Nature , 7—12 link to article Stratton, F.
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