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Why did scientists study blood groups as possible markers of race?

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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.

Following the rediscovery of Mendelian inheritance in 1900, scientists began focusing on genes to identify the hereditary basis of physical traits. Researchers studied blood groups—such as the ABO blood group system, the MNSs system, and the Rh system—because these features are inherited and not susceptible to environmental influences.12

Experts hypothesized that these genetic markers could finally provide a biological method to identify and distinguish human races. They expected that specific populations would exhibit distinct frequencies of these genes, effectively marking them off from other groups.12

However, when scientists attempted to correlate blood group patterns with conventional racial categories, they found no evidence to support such distinctions. Research revealed that human genetic variation is expressed in subtle gradations across geographic space rather than in abrupt changes between races. Ultimately, these studies demonstrated that internal variations within populations are greater than those between them, and that the concept of race holds little meaning at the genetic level.123