Sickle cell disease is an example of a recessive genetic disease. This means that both copies of an individual’s HBB gene must carry the mutation for that person to be affected; if only one copy has the alteration, the nonmutated gene can produce enough normal hemoglobin to overcome the negative effects of the mutated hemoglobin. People with only one mutated copy of the HBB gene are still carriers of the sickle cell trait, however, and while they’re usually unaffected, they can still pass the mutated gene on to their offspring.
