heterozygous dominance: "The allele responsible for sickle-cell anaemia is autosomal recessive and can be found on the short arm of chromosome 11. A person who receives the defective gene from both father and mother develops the disease; a person who receives one defective and one healthy allele remains healthy, but can pass on the disease and is known as a carrier. If two parents who are carriers have a child, there is a 1-in-4 chance of their child developing the illness and a 1-in-2 chance of their child just being a carrier. Since the gene is incompletely recessive, carriers have a few sickle red blood cells at all times, not enough to cause symptoms, but enough to give resistance to malaria. Because of this, heterozygotes have a higher fitness than either of the homozygotes. This is known as heterozygote advantage. Due to the evolutionary advantage of the heterozygote, the illness is still prevalent, especially among people with recent ancestry in malaria-stricken areas, such as Africa, the Mediterranean, India and the Middle East." Source and further information: http://en.wikipedia.org/wiki/Sickle-cell_anemia#Genetics Further information: http://en.wikipedia.org/wiki/Pleiotropy http://en.wikipedia.org/wiki/Epistasis http://en.wikipedia.org/wiki/Heterozygote_advantage