Haemoglobin S (HbS) is known to cause sickle cell disease and it is usually fatal if untreated so shouldn't natural selection have eliminated it?    

Instead, the sickle cell gene is common in people of African, Mediterranean and Indian origin, areas with historically high levels of malaria.    A new study says such geographical adds to the hypothesis that the gene, though deadly, avoids disappearing through natural selection by providing protection against malaria.

More than sixty years ago, researchers observed that the sickle cell gene tended to be more common in populations living in, or originating from, areas of high malaria prevalence. This led to the 'malaria hypothesis', which suggested that, although deadly when inherited from both parents, the gene provided a degree of protection from malaria in children inheriting it from just one parent. This protective advantage would be strong enough in areas of intense malaria transmission for the gene to survive, according to the hypothesis.

The malaria hypothesis has gained support in population and laboratory studies but the original observations of a geographical overlap between frequency of the gene and malaria prevalence have never been tested beyond simple visual comparisons at the global scale.

To address this, geographers, biologists and statisticians at the University of Oxford, together with colleagues from the KEMRI-Wellcome Trust Programme in Kenya, produced the first detailed global map showing the distribution of the sickle cell gene by collating all the information currently accessible on the occurrence of the sickle cell gene in native populations worldwide and, using modern mapping techniques, created a map of the global frequency of this gene. The map was then compared with the distribution and intensity of malaria before widespread malaria control.

The results showed that the sickle cell gene is most common in sub-Saharan Africa, the Middle East and India, and that the areas of high frequency of this gene are coincident with historically high levels of malaria, thus confirming for them that the malaria hypothesis is correct at the global scale. 

 Malaria Atlas Project
These are maps showing the distribution of the HbS "sickle cell gene" and the endemicity of malaria.  Credit: Malaria Atlas Project

"This study highlights the first steps in our efforts to create an open-access, online database of the frequency of various inherited blood disorders," says lead author Dr. Fred Piel from the University of Oxford. "Such databases will help improving estimates of their public health burden and guide where resources would be best applied."

Citation: Frédéric B. Piel, Anand P. Patil, Rosalind E. Howes, Oscar A. Nyangiri, Peter W. Gething, Thomas N. Williams, David J. Weatherall, Simon I. Hay, 'Global distribution of the sickle cell gene and geographical confirmation of the malaria hypothesis', Nov 02 2010 Nat. Commun. 1 : 104 doi: 10.1038/ncomms1104 (full article is free to read)