Blood provides an ideal opportunity for the study of human variation without cultural prejudice. It can be easily classified for many different genetically inherited blood typing systems. Also significant is the fact that we rarely take blood types into consideration in selecting mates. In addition, few people know their own type today and no one did a century ago. As a result, differences in blood type frequencies around the world are most likely due to other factors than social discrimination. Contemporary Japan is somewhat of an exception since there are popular Japanese stereotypes about people with different blood types. This could affect choice in marriage partners for some Japanese.
All human populations share the same 29 known blood systems, although they differ in the frequencies of specific types. Given the evolutionary closeness of apes and monkeys to our species, it is not surprising that some of them share a number of blood typing systems with us as well.
When we donate blood or have surgery, a small sample is usually taken in advance for at least ABO 
and Rh systems typing. If you are O+, the O is your ABO type and the + is your Rh type. It is possible to be A, B, AB, or Oas well as Rh+ or Rh- . You inherited your blood types from your parents and the environment in which you live cannot change them.
ABO Blood Type System
We have learned a good deal about how common each of the ABO blood types is around the world. It is quite clear that the distribution patterns are complex. Both clinal and discontinuous distributions exist, suggesting a complicated evolutionary history for humanity. This can be seen with the global frequency patterns of the type B blood allele (shown in the map below). Note that it is highest in Central Asia and lowest in the Americas and Australia. However, there are relatively high frequency pockets in Africa as well. Overall in the world, B is the rarest ABO blood allele. Only 16% of humanity have it.
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| Distribution of the B type blood allele in native populations of the world |
The A blood allele is somewhat more common around the world than B. About 21% of all people share the A allele. The highest frequencies of A are found in small, unrelated populations, especially the Blackfoot Indians of Montana (30-35%), the Australian Aborigines (many groups are 40-53%), and the Lapps, or Saami people, of Northern Scandinavia (50-90%). The A allele apparently was absent among Central and South American Indians.
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| Distribution of the A type blood allele in native populations of the world |
The O blood type (usually resulting from the absence of both A and B alleles) is very common around the world. About 63% of humans share it. Type O is particularly high in frequency among the indigenous populations of Central and South America, where it approaches 100%. It also is relatively high among Australian Aborigines and in Western Europe (especially in populations with Celtic ancestors). The lowest frequency of O is found in Eastern Europe and Central Asia, where B is common.
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| Distribution of the O type blood in native populations of the world |
Other Blood Type Systems
The majority of the people in the world have the Rh+ blood type. However, it is more common in some regions. Native Americans and Australian Aborigines were very likely 100% Rh+ before they began interbreeding with people from other parts of the world. This does not imply that Native Americans and Australian Aborigines are historically closely related to each other. Most African populations are around 75% Rh+. Europeans have the lowest frequency of this blood type for any continent. They are 60% Rh+. The lowest known frequency is found among the Basques of the Pyrenees Mountains between France and Spain. They are only 47% Rh+.
The distribution patterns for the Diego blood system are even more striking. Evidently, all Africans, Europeans, East Indians, Australian Aborigines, and Polynesians are Diego negative. The only populations with Diego positive people may be Native Americans (2-46%) and East Asians (3-12%). This nonrandom distribution pattern fits well with the hypothesis of an East Asian origin for Native Americans.
Conclusion
These patterns of ABO, Rh, and Diego blood type distributions are not similar to those for skin color or other so-called "racial" traits. The implication is that the specific causes responsible for the distribution of human blood types have been different than those for other traits that have been commonly employed to categorize people into "races." Since it would be possible to divide up humanity into radically different groupings using blood typing instead of other genetically inherited traits such as skin color, we have more conclusive evidence that the commonly used typological model for understanding human variation is scientifically unsound.
The more we study the precise details of human variation, the more we understand how complex are the patterns. They cannot be easily summarized or understood. Yet, this hard-earned scientific knowledge is generally ignored in most countries because of more demanding social and political concerns. As a result, discrimination based on presumed "racial" groups still continues. It is important to keep in mind that this "racial" classification often has more to do with cultural and historical distinctions than it does with biology. In a very real sense, "race" is a distinction that is created by culture not biology.
