Is there any reason for why the O blood group is rear compared to other blood group?

ABC blood group is determined by three alleles: any two alleles are present in each individual. Allele for antigen A, allele for antigen B are co dominant; hence when these come together, AB blood group is expressed. Both allele for antigen A and antigen B are dominant on recessive allele i . O blood group is expressed when there is ii genotype: i.e. in double recessive genetic condition. This is why persons with blood group O are rare in population.

The group O allele produces a lack of an A or B antigen on the red blood cell, which are called blood group alleles. Even though two recessive alleles are needed to produce the O blood group, the O allele is the most common of the ABO blood groups. So most people have the O blood group, followed closely by the A blood group, then the B blood group, then the most rare, the AB blood group.

Dominance should not be confused with allele frequency. The allele for 12 fingers and/or toes is dominant, but the vast majority of people have 10, which is recessive, but the allele frequency for the recessive allele makes up the vast majority of the alleles in the population for number of digits.

The pie chart below shows the percentage of ABO blood groups in a population. Add the Rh positive and negative blood types to get the total percentage of each ABO blood group.

The allele frequency for the A blood group worldwide is 21%, for the B allele, 16%, and the O allele 63%, though variations occurs according to race, ethnicity, and geographic location.
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