Normal blood cells live for about 120 days and they are then replaced by new blood cells from the bone marrow. Sickle cell disease creates abnormally shaped cells that only live about 16 days so bone marrow cannot keep up with the needed production of blood cells. The trait for Sickle cell disease is a recessive gene so that most carriers of that gene do not exhibit the disease. Normally a disadvantageous trait like this would eventually die out but this one continues. The organism that causes malaria lives part of its life in the red blood cells and scientists have proposed that this recessive gene which also keeps out malaria organisms so that the carriers are highly resistant to malaria. The term ‘balanced polymorphism’ described this situation where one disadvantageous trait is balanced against an advantageous trait.
(The Great Courses – Understanding Genetics: DNA, Genes, and Their Real-World Application)