When the phenotype is the same whether an individual carries one or two copies of an allele the allele is?

Summary

• Genes are the blueprint for our bodies.
• If a gene contains a change, it disrupts the gene message. 
• Changes in genes can cause a wide range of conditions.
• Sometimes a changed gene is inherited, which means it is passed on from parent to child.
• Changes in genes can also occur spontaneously.
• Parents who are related to each other are more likely have children with health problems or genetic conditions than unrelated parents (although most related parents will have healthy children).

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This page has been produced in consultation with and approved by:

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Inheritance patterns

Sickle-cell disease is an inherited condition that causes pain and damage to organs and muscles. Instead of having flattened, round red blood cells, people with the disease have stiff, sickle-shaped cells. The long, pointy blood cells get caught in capillaries, where they block blood flow. Muscle and organ cells don’t get enough oxygen and nutrients, and they begin to die.

The disease has a recessive pattern of inheritance: only individuals with two copies of the sickle-cell allele have the disease. People with just one copy are healthy.

In addition to causing disease, the sickle-cell allele makes people who carry it resistant to malaria, a serious illness carried by mosquitos. Malaria resistance has a dominant inheritance pattern: just one copy of the sickle cell allele is enough to protect against infection. This is the very same allele that, in a recessive inheritance pattern, causes sickle-cell disease!

Now let’s look again at the shape of the blood cells. People with two copies of the sickle-cell allele have many sickled red blood cells. People with two copies of the “normal” allele have disc-shaped red blood cells. People with one sickle-cell allele and one normal allele have a small number of sickled cells, and their cells sickle more easily under certain conditions. So we could say that red blood cell shape has a co-dominant inheritance pattern. That is, individuals with one copy of each allele have an in-between phenotype.

So is the sickle cell allele dominant, recessive, or co-dominant? It depends on how you look at it.

Protein function

If we look at the proteins the two alleles code for, the picture becomes a little more clear. The affected protein is hemoglobin, the oxygen-carrying molecule that fills red blood cells. The sickle-cell allele codes for a slightly modified version of the hemoglobin protein. The modified hemoglobin protein still carries oxygen, but under low-oxygen conditions the proteins stick together.

When a person has two sickle cell alleles, all of their hemoglobin is the sticky form, and the proteins form very long, stiff fibers that distort red blood cells. When someone has one sickle-cell allele and one normal allele, only some of the hemoglobin is sticky. Non-sticky hemoglobin is made from the normal allele, and sticky hemoglobin is made from the sickle-cell allele (every cell has a copy of both alleles). The sticking-together effect is diluted, and in most cells, the proteins don’t form fibers.

The protist that causes malaria grows and reproduces in red blood cells. Just exactly how the sickle-cell allele leads to malaria resistance is complex and not completely understood. However, it appears that the parasite reproduces more slowly in blood cells that have some modified hemoglobin. And infected cells, because they easily become misshapen, are more quickly removed from circulation and destroyed.

To see more examples of how variations in genes influence traits, visit The Outcome of Mutation.

What is it called when you have two copies of the same allele?

What is it called when two alleles are expressed in the phenotype?

What is the phenotype of an individual with one allele?

When two alleles appear in the phenotypic at the same time?