True.
Sickle cell disease is caused by a mutation in the HBB gene, which provides instructions to make beta-globin, a protein component of hemoglobin in red blood cells. Sickle cell disease is an autosomal recessive disorder, which means that an individual must inherit two copies of the mutated gene (one from each parent) to have the disease.
When both parents are carriers of sickle cell disease, there are four possible outcomes for their offspring:
The offspring inherits a normal HBB gene from both parents and is not a carrier of sickle cell disease.
The offspring inherits one normal HBB gene from one parent and one mutated HBB gene from the other parent and becomes a carrier of sickle cell disease.
The offspring inherits one mutated HBB gene from one parent and one normal HBB gene from the other parent and becomes a carrier of sickle cell disease.
The offspring inherits two mutated HBB genes (one from each parent) and has sickle cell disease.
Therefore, the probability of a child inheriting two mutated HBB genes (i.e., having sickle cell disease) when both parents are carriers is 25% (1 in 4).