Normal red blood cells slide easily through narrow blood vessels. In sickle cell disease, many red blood cells change to a crescent shape like a slender moon. Crescent-shaped blood cells cause blockages in blood vessels. These blockages decrease the transport of oxygen. People with sickle cell disease often have sharp pains due to the lack of oxygen in parts of the body. Some people have one sickle cell gene and one normal gene. They have few defective cells and no symptoms because the sickle cell gene is recessive. How does the human system of reproduction result in people getting one copy of a sickle cell gene and one copy of a normal gene? Describe the process.