Sickle cell is an inherited disease caused by a genetic mutation. Genes are found on structures in the cells of our body called "chromosomes." There are normally 46 total – or 23 pairs – of chromosomes in each cell of our body. The 11th pair of chromosomes contains a gene responsible for normal hemoglobin production.
A mutation or error in this gene is what causes sickle cell disease. This mutation 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 (HbSS) is a genetic disease. A person will be born with sickle cell 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 disease. 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, so that 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.
- Children with sickle cell disease = 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)
Once parents have had a child with sickle cell disease, there is a one in four, or 25 percent chance with each subsequent pregnancy, for another child to be born with sickle cell disease. This means that there is a three out of four, or 75 percent chance, that another child will not have sickle cell disease. There is a 50 percent chance that a child will be born with sickle cell trait, like the parents.