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Why did ABO blood group patterns fail to support conventional racial categories?

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Overhead view of a diverse group of people standing in a circle, holding each other's hands in a gesture of unity and teamwork. — Race Skin color is one of the variables that has been used to classify people into different racial categories.

The ABO blood group system failed to support conventional racial categories because it did not provide a biological basis for distinguishing between them. When scientists initially discovered that blood groups followed Mendelian inheritance, they hoped these stable, inherited traits would serve as markers to identify and separate human races.1

However, research revealed that while populations vary in the frequency of A, B, and O types, these patterns do not correlate with traditional racial classifications. Instead, genetic markers are distributed in subtle gradations across geographic space rather than showing abrupt changes between groups. Furthermore, internal genetic variation within any given population is often greater than the variation between different populations.1

Ultimately, the attempt to categorize humanity by blood type failed because human physical and genetic traits are inherited independently of one another, and no evidence exists to support the division of humans into distinct, biologically defined races.1