Answer:
A. MUTATION.
Step-by-step explanation:
Sickle cell disease is caused by the “mutation” in the beta-globin (HBB) gene that lead to the production of abnormal version of hemoglobin, known as hemoglobin S. Hemoglobin is one kind of protein responsible for carrying oxygen in RBC ( red blood corpuscles or red blood cells). Abnormal hemoglobin S make red blood cells more fragile and and shape them up to the sickle like. These sickle shaped RBC can block blood flow through the body and can lead to serious problems, like - stroke, eye problems, infections and episodes of pain. Sickle cell disease is a lifelong illness. A blood and bone marrow transplant is currently a the only cure for sickle cell disease.
A child inherit a copy of sickle cell gene from both parents when both of them are carriers of the sickle cell gene or one parent has sickle cell disease and the other is a carrier of it.