Why Do We Have Different Blood Types?

Quick Answer

Genetic variation in sugar molecules on red blood cell surfaces produces the A, B, AB, and O blood groups.

The Full Story

Blood type is determined by antigens — specific sugar chains — on the surface of red blood cells. The ABO gene on chromosome 9 has three main alleles: A, B, and O. Alleles A and B each code for an enzyme that attaches a distinct sugar, while O codes for a non-functional enzyme, leaving the base H antigen bare. Evolutionary pressures, particularly from infectious diseases like malaria and cholera, appear to have maintained this diversity because different blood types confer different levels of resistance to various pathogens.

Key Facts

1.Type O is the most common blood group worldwide, found in about 44% of the global population.

2.The Rh factor (positive or negative) is determined by a separate gene on chromosome 1.

3.Bombay blood group, found in roughly 1 in 10,000 people in India, lacks even the H antigen.

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