Genetics encompasses many types of traits that follow a relatively simple inheritance pattern.
When dealing with genetic traits that follow these rules, remember that an organism receives two copies of each gene, one from each of its parents.
The genes that follow Mendelian genetics come in several versions or alleles. A particular allele or trait is dominant, recessive or co-dominant, depending on how it interacts with other alleles.
If an organism has an allele of a type X, it will show the characteristics of this trait. For example, in humans, dark hair is dominant over blond hair. If any of a person’s genes is the allele for dark hair, that individual will have dark hair, even if the other allele is for blonde hair.
Dominant alleles show their effect even if the individual only has one copy of the allele (also known as heterozygotes). For example, the allele for brown eyes is dominant, therefore, you only need one copy of the “brown eye” allele to have brown eyes (although, with two copies, you will still have brown eyes).
Recessive genes are the opposite of dominant genes. As dominant features mask them, an organism can only have the characteristics of a recessive trait if it has two alleles of the gene.
For example, a person with blond hair should have two blond alleles, one from each parent. This also means that heterozygotes (organisms with two different alleles) can be “carriers” of these traits.
To continue with the example of dark hair / blonde hair, two parents may have dark hair, but if both carry the gene for blond hair, their offspring may have blond hair.
If an allele is co-dominant, it means that a heterozygote will show characteristics of two different alleles. In humans, the ABO part of our blood group shows this type of organization.
The allele for blood type A and blood type B can be co-dominant with each other. This means that if a person receives the type A blood allele from one parent and blood type B from the other, it will have type AB blood.
The blood of that individual will have type A and type B blood antigens. However, type O blood is recessive for both type A and type B.
This is because type O blood does not have antigens, so if a person has the type A or type B blood allele, it masks the presence of type O blood. Within this three allele system, type A and type B may be co-dominant, but O is always recessive.
Polygenic features are traits that are governed by the interactions of a group of genes. Polygenic traits do not follow simple Mendelian rules of genetics, since the interaction of multiple genes complicates the inheritance of these traits.
For example, many different genes influence height. This concept is related to the idea of qualitative traits versus quantitative traits. If you can describe a trait in a discrete category, for example, green eyes or blue eyes, the trait is a qualitative trait.
Qualitative traits often obey the rules of Mendelian traits, and usually fall into categories such as dominant, co-dominant and recessive.
Quantitative traits are traits, such as blood pressure, that occur in a range of values and can be measured. Quantitative traits are usually polygenic and are very difficult to describe in terms of dominant and recessive.
Sickle cell disease is a hereditary disease that causes pain and damage to organs and muscles. Instead of having round, flat red blood cells, people with the disease have hard, sickle-shaped cells.
The long, pointed blood cells are trapped in the capillaries, where they block blood flow. Muscle and organic cells do not get enough oxygen and nutrients, and they start to die.
The disease has a recessive inheritance pattern, only individuals with two copies of the sickle cell allele have the disease. People with only one copy are healthy.
In addition to causing disease, the sickle cell allele makes people who carry it sick and resistant to malaria, a serious disease transmitted by mosquitoes.
Resistance to malaria has a dominant inheritance pattern, only one copy of the sickle cell allele is sufficient to protect against infection. This is the same allele that, in a recessive inheritance pattern, causes sickle cell disease!
Now let’s look again at the shape of blood cells. People with two copies of the sickle cell allele have many sickle red blood cells. People with two copies of the “normal” allele have disc-shaped red blood cells.
People with a sickle cell allele and a normal allele have a small number of sickle cells, and their cells have the disease more easily under certain conditions.
Then we could say that the shape of the red blood cells has a co-dominant inheritance pattern. That is, individuals with a copy of each allele have an intermediate phenotype.
So, is the sickle cell allele dominant, recessive or co-dominant? It depends on how you look at it.
If we look at the proteins that encode the two alleles, the image becomes a little clearer. The affected protein is hemoglobin, the oxygen-transporting molecule that fills the red blood cells.
The sickle cell allele encodes a slightly modified version of the hemoglobin protein. The modified hemoglobin protein still carries oxygen, but in low oxygen conditions, the proteins bind.
When a person has two sickle cell alleles, all of their hemoglobin is the adhesive form, and the proteins form very long, stiff fibers that distort red blood cells.
When someone has a sickle cell allele and a normal allele, only part of the hemoglobin is sticky.
Non-adhesive hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle cell allele (each cell has a copy of both alleles). The binding effect is diluted, and in most cells, the proteins do not form fibers.
The protist that causes malaria grows and reproduces in red blood cells. Exactly how the sickle cell allele leads to malaria resistance is complex and not fully understood.
However, it seems that the parasite reproduces more slowly in blood cells that have some modified hemoglobin. And infected cells, because they easily deform, are removed more quickly from the circulation and destroyed.