Question

What are the three alleles responsible for the ABO blood system?

Answer 1

The first genetic variation or polymorphism, which was discovered in humans is the ABO blood system. It was discovered by Landsteiner. Allele genes are the responsible ones for the grouping of ABO blood systems. The ABO blood group system is categorized based on the inherited properties of red blood cells. The ABO blood group system is the major example of multiple allelism in human beings.

Complete answer:
Alleles are different forms of a gene. Multiple alleles denote the occurrence of three or more alleles for a particular gene. More than two genes are present in the same locus.
In human beings, the ABO blood system is regulated through three alleles, namely ${I^A}$, ${I^B}$and ${I^0}$. $I$ is referred to as isohemagglutinin. ${I^A}$ and ${I^B}$ are codominant to each other, which produce antigens of type A and type B respectively. They are transferred to the surface of red blood cells. That is, they code for surface antigen of red blood cells. ${I^0}$ is a recessive allele, which does not participate in production of any antigen. The interaction of these alleles determines the type of blood group for a particular person.
${I^A}$ codes for antigen A, ${I^B}$ codes for antigen B and ${I^0}$ codes for absence of antigen.
In A blood group, alleles are (${I^A}$, ${I^A}$) or (${I^A}$, ${I^0}$)
In B blood group, alleles are (${I^B}$,${I^B}$) or (${I^B}$, ${I^0}$)
In AB blood group, alleles are (${I^A}$, ${I^B}$)
In the O blood group, alleles are (${I^0}$,${I^0}$).

Note:
Multiple allelism can also be regarded as the presence of three or more alleles in the population for a specific gene. In the same time, inheritance for humans is possible only for two alleles from their parents. One allele is obtained from male parent and another allele is obtained from the female parent. On the surface of RBC, oligosaccharide chains, proteins and lipids are present to which ABO blood group antigens are attached. The sequence of the oligosaccharide chain determines the type of antigen.