<P> Such a balance is seen more simply in sickle - cell anaemia, which is found mostly in tropical populations in Africa and India . An individual homozygous for the recessive sickle hemoglobin, HgbS, has a short expectancy of life, whereas the life expectancy of the standard hemoglobin (HgbA) homozygote and also the heterozygote is normal (though heterozygote individuals will suffer periodic problems). The sickle - cell variant survives in the population because the heterozygote is resistant to malaria and the malarial parasite kills a huge number of people each year . This is balancing selection or genetic polymorphism, balanced between fierce selection against homozygous sickle - cell sufferers, and selection against the standard HgbA homozygotes by malaria . The heterozygote has a permanent advantage (a higher fitness) so long as malaria exists; and it has existed as a human parasite for a long time . Because the heterozygote survives, so does the HgbS allele survive at a rate much higher than the mutation rate (see and refs in Sickle - cell disease). </P> <P> The Duffy antigen is a protein located on the surface of red blood cells, encoded by the FY (DARC) gene . The protein encoded by this gene is a non-specific receptor for several chemokines, and is the known entry - point for the human malarial parasites Plasmodium vivax and Plasmodium knowlesi . Polymorphisms in this gene are the basis of the Duffy blood group system . </P> <P> In humans, a mutant variant at a single site in the FY cis - regulatory region abolishes all expression of the gene in erythrocyte precursors . As a result, homozygous mutants are strongly protected from infection by P. vivax, and a lower level of protection is conferred on heterozygotes . The variant has apparently arisen twice in geographically distinct human populations, in Africa and Papua New Guinea . It has been driven to high frequencies on at least two haplotypic backgrounds within Africa . Recent work indicates a similar, but not identical, pattern exists in baboons (Papio cynocephalus), which suffer a mosquito - carried malaria - like pathogen, Hepatocystis kochi . Researchers interpret this as a case of convergent evolution . </P> <P> G6PD (Glucose - 6 - phosphate dehydrogenase) human polymorphism is also implicated in malarial resistance . G6PD alleles with reduced activity are maintained at a high level in endemic malarial regions, despite reduced general viability . Variant A (with 85% activity) reaches 40% in sub-Saharan Africa, but is generally less than 1% outside Africa and the Middle East . </P>

Describe the advantage of studying human blood types