A person receives the sickle cell genes or not only at the time of conception. Therefore, neither the sickle cell trait nor sickle cell disease can be contracted. By the same token, people cannot lose their sickle cell genes over time. A person born with sickle cell trait (one sickle cell gene) will always have sickle cell trait. The same is true of sickle cell disease (two sickle cell genes).

Sickle cell disease produces illness, while sickle cell trait usually does not. The severity of sickle cell disease can change over time. The change in severity is not due to a change in the sickle cell genes over time. Rather, a number of other biological factors, most of which are not understood, change to alter the severity of sickle cell disease.

People who inherit two genes for sickle haemoglobin (one from each parent) have sickle cell disease. With a few exceptions, a child can inherit sickle cell disease only if both parents have one gene for sickle cell haemoglobin. The most common situation in which this occurs is when each parent has one sickle cell gene. In other words, each parent has sickle cell trait.

A one-in-four chance exists that a child will inherit two normal genes from the parents. A one-in-four chance also exists that a child will inherit two sickle cell genes, and have sickle cell disease. A one-in-two chance exists that the child will inherit a normal gene from one parent and a sickle gene from the other. This would produce sickle trait.

These probabilities exist for each child independently of what happened with prior children the couple may have had. In other words, each new child has a one-in-four chance of having sickle cell disease. A couple with the sickle cell trait can have eight children, none of whom have two sickle genes. Another couple with sickle trait can have two children each with sickle cell disease. The inheritance of sickle cell genes is purely a matter of chance. These probability odds cannot be altered.