SOLVED: 4. Sickle cell disease is an umbrella term used to describe a group of genetic diseases that affect hemoglobin: Hemoglobin (image at right) is the protein in red blood cells that transports oxygen from the lungs through the arteries to the body's tissues. Hemoglobin is a tetramer composed of two types of subunits, alpha and beta subunits. Each hemoglobin molecule has two copies of both the alpha and beta subunits, making a total of four subunits in hemoglobin (thus making it a tetramer).