Sickle Cell Disease
Sickle Cell Disease

Sickle Cell Disease Causes and Risk Factors

Sickle cell disease is inherited , meaning that it runs in families. People who have sickle cell anemia inherit two faulty hemoglobin  genes — hemoglobin S — from each parent. Sickle cell disease can also occur when a child inherits one hemoglobin S gene from one parent and another faulty hemoglobin gene, such as beta (β) thalassemia, hemoglobin C, hemoglobin D, or hemoglobin E, from the second parent.

A person has sickle cell trait when they inherit hemoglobin S gene from one parent and a normal hemoglobin gene — hemoglobin A — from the other. They are called a carrier of the hemoglobin S gene because they can pass it on when they have a child. People who have sickle cell trait are generally healthy.

Sickle cell disease is a genetic disorder caused by mutations in the beta globin genes that leads to faulty hemoglobin protein, called hemoglobin S. Hemoglobin S changes flexible red blood cells into rigid, sickle-shaped cells. These sickle cells can block blood flow, and result in pain and organ damage. Medical Animation Copyright © 2023 Nucleus Medical Media, All rights reserved.

sickle cell

Inheritance pattern for sickle cell disease

In the image above, each parent has one normal hemoglobin A gene and one hemoglobin S gene, which means each of their children has:

  • A 25%, or 1 in 4, chance of inheriting two normal hemoglobin A genes. This child does not have sickle cell trait or disease.
  • A 50%, or 1 in 2, chance of inheriting one normal hemoglobin A gene and one hemoglobin S gene. This child has sickle cell trait.
  • A 25%, or 1 in 4, chance of inheriting two hemoglobin S genes. This child has sickle cell disease.

It is important to keep in mind that each time this couple has a child, the chances of that child having sickle cell disease remain the same. In other words, if the first child has sickle cell disease, there is still a 25% chance that the second child will also have the disease. Both boys and girls can inherit sickle cell trait, sickle cell disease, or normal hemoglobin.

If a person wants to know whether they carry a sickle hemoglobin gene, a healthcare provider can order a blood test to find out.

What should you do if you are a carrier or have the disease?

People who do not know whether they carry a faulty hemoglobin gene can ask their provider for a blood test.

Couples who are planning to have children and know that they are at risk of having a child with sickle cell disease may want to meet with a genetic counselor. A genetic counselor can answer questions about the risk and explain the choices that are available.

What is a “sickled cell”?

Red blood cells with normal hemoglobin are disc-shaped and flexible so that they can move easily through large and small blood vessels to deliver oxygen throughout the body.

Sickled hemoglobin is not like normal hemoglobin. Stiff strands can form within the red blood cell of sickled hemoglobin. These stiff strands can change the shape of the cell, causing the sickled red blood cell that gives the disease its name. Sickle-shaped red blood cells are not flexible and can stick to vessel walls, which can block or slow blood flow. 

The sickle-shaped cells cannot deliver oxygen to the rest of the body. This can cause attacks of sudden severe pain, called pain crises. These pain crises can occur without warning, and a person who has them often needs to go to the hospital to be treated.

Because sickle cells cannot easily change shape, they also tend to burst apart. Normal red blood cells live about 90 to 120 days, but sickle cells last only 10 to 20 days. The body is always making new red blood cells to replace the old cells. However, in sickle cell disease, the body may have trouble keeping up with how fast the cells are being destroyed. Because of this, the number of red blood cells is usually lower than in people without sickle cell disease. This condition, called anemia, can cause a person to have less energy.


Normal red cells and sickle red cells.
Normal red cells and sickle red cells. Figure A shows normal red blood cells flowing freely in a blood vessel. The inset image shows a cross-section of a normal red blood cell with normal hemoglobin. Figure B shows abnormal, sickled red blood cells blocking blood flow in a blood vessel. The inset image shows a cross-section of a sickle cell with abnormal (sickle) hemoglobin forming abnormal stiff rods.
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