The ABO blood group is determined genetically. As a result, its distribution varies among ethnic groups. Globally, the most common blood type is O, but the most common blood type of Europeans is A. The following map shows the distribution of the O, A, B, and AB blood types in the population by country in Europe (with colouring indicating the most common type, which in itself is not a reliable indicator of anything else; the actual figures are more important).
The map is based on an article on Wikipedia, which compiles data from many different sources. The data must be, therefore, taken with a big pinch of salt. They come from different years (which are often impossible to establish), and some of them may be outdated. The map should be understood as a tool allowing us to notice general trends, as small differences are unlikely to be statistically significant.
To see how exactly the types are distributed, take a look at the following maps. Blood type O is clearly more prevalent in Western Europe than in Eastern Europe:
Prevalence of type A, on the other hand, is fairly constant, except a significant drop in Ireland and Iceland:
A trend opposite to type O can be seen in type B, which is more prevalent in Eastern Europe (and even more so in Asia):
The values for the type AB are too small to be meaningfully visualized, but in general, the prevalence of type AB is highly positively correlated with the prevalence of type B.
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