Blood type is determined by just one gene, not two. The gene, which has the unoriginal name ABO, has three alleles A, B and O, with O being a dysfunctional vari

Blood type is determined by just one gene, not two. The gene, which has the unoriginal name ABO, has three alleles A, B and O, with O being a dysfunctional variant. This means there are nine possibilities: OO, OA, OB, AO, AA, AB, BO, BA, BB. A and B are both dominant over O, and code for blood type AB when they're both present. So there is 1 in 9 possibilities for O, 3 in 9 for both A and B and 2 in 9 for blood type AB. It would be extremely hard to find a statistically significant indicator of increased fitness. Evolution is really good at amplifying a 0.1% advantage over countless generations to dominate a population, but to distill that signal from the noise you'd need a study with hundreds of thousands of participants. Another problem is that the advantage from certain blood types might have only been relevant in prehistoric times, i.e. resistance to diseases that we now can easily treat and avoid. The norovirus mentioned in the article was quite dangerous back then, and still is in developing countries, but is little more than an annoyance here. Lastly, blood type is correlated with ethnicity, which is correlated itself with life expectancy and resistance to various ailments, making it even harder to unentangle statistically.