Hemoglobin is the protein which carries oxygen from the lungs to the tissues. Its complex molecule includes four polypeptide chains (i.e. chains of amino-acids produced by genes, and characteristic of proteins in general). In any one molecule there are two identical alpha chains and two identical beta chains. It is variants of the beta chains which are important physiologically and anthropologically. Because the functional activity of hemoglobin is so important for efficient living, selection has almost completely weeded out the genes for all but one kind of beta chain, that which characterizes hemoglobin A. With a very few exceptions all other types of beta chain are exceedingly rare. However, in Africa and elsewhere, the gene for hemoglobin S (sickle-cell hemoglobin) is fairly common.() This hemoglobin behaves almost normally in the oxygen-saturated state, but when it gives up its oxygen it becomes insoluble in the red-cell fluid, forming crystals of a sort, which distort the cell and tend to cause its disruption. As explained by Mourant, it is subject to a very important process of natural selection.()
Other beta variants are that for Hemoglobin E, fairly common throughout south-east Asia, and Hemoglobin D, present in the Gujarat region of India and the Punjab. Possible selection processes favouring these are discussed on p. 126.