Sickle cell anemia is an inherited disease caused by a genetic mutation in the DNA material responsible for creating the body's hemoglobin. Hemoglobin exists inside a red blood cell and its job is to carry oxygen throughout the body.
The gene mutation that causes sickle cell anemia is thought to have originated in areas of the world where malaria was common, since people with sickle trait tolerate malaria better and have a reproductive advantage in areas with malaria. The sickle trait actually protects them from the parasite that causes malaria, which is carried by mosquitoes. Malaria is most often seen in Africa and in the Mediterranean area of Europe.
Sickle cell anemia is a genetic disease. A person will be born with the disease only if two HbS genes are inherited-one from the mother and one from the father. A person who has only one HbS gene is healthy and said to be a "carrier" of the disease. They may also be described as having sickle cell trait. A carrier has an increased chance to have a child with sickle cell anemia if he or she has a child with someone who also has a sickle cell gene. This type of inheritance is called autosomal recessive.
Autosomal means that the gene is on one of the first 22 pairs of chromosomes that do not determine gender; males and females are equally affected by the disease.
Recessive means that two copies of the gene, one inherited from each parent, are necessary to have the condition.
In this couple, there is a risk of having:
- Children with sickle cell anemia = S S (one in four, or 25 percent)
- Children who are carriers of the gene like their parents = A S S A (two in four, or 50 percent have sickle cell trait)
- Children who do not get the gene from either parent: A A (one in four, or 25 percent)
For parents who are each carriers of a sickle cell gene, there is a one in four, or a 25 percent chance with each subsequent pregnancy, for another child to be born with sickle cell anemia. This means that there is a three out of four, or a 75 percent chance, that another child will not have the disease. There is a 50 percent chance that a child will be born with sickle cell trait, like the parents.