Which antigen phenotype frequency is the lowest in the ABO system?

In the ABO blood group system, the phenotypes represent four main blood types: A, B, AB, and O. The blood type AB is characterized by the presence of both A and B antigens on the surface of red blood cells.

When considering the frequency of these phenotypes in various populations, type AB is the least common. This is due to the fact that both A and B alleles must be inherited from the parents, making it statistically less likely compared to the other blood groups. Blood type O, which lacks A and B antigens, is the most common phenotype in many populations, followed by blood types A and B, which are more prevalent than AB.

In summary, the frequency of the AB antigen phenotype is lower than the others because of the specific genetic combination required for its expression, making it the rarest among the ABO phenotypes.