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Please forgive me in case my question wouldn't make much sense. I was reading about ABO blood groups on Wikipedia, where I learnt that O is a recessive allele, and that it seems the A allele predates the O allele. My question is, therefore, how is it possible that almost half of the human population is group O?

For instance, I would expect (quite naively I reckon), if only alleles A and O existed, that around 75% of the population should be A, and 25% should be O.

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You are wondering because O is recessive and A is older? –  kmm Apr 27 at 12:46
    
Exactly. Unless there were an evolutionary advantage to having group O (which I could find no evidence of by searching the web), it would appear logical to me that there should be more people with group A, for both of these reasons. –  scozy Apr 27 at 12:49

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up vote 7 down vote accepted

There are many different variants of O (all loss-of-function) indicating that this mutation has arisen many times in the human population. The prevalence of O is indeed taken as evidence of balancing selection. Various pathogens use the A or B antigens as receptors. The cited paper presents evidence about the phylogeny of the ABO gene in human populations and reports that there is clear evidence of balancing selection.

We propose several hypotheses for the cause of [balancing selection], which most likely involved interactions with multiple pathogens at different geographic regions and timescales.

Calafell et al (2008) Evolutionary dynamics of the human ABO gene. Human Genetics 124: 123-135

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