Question

What is the number of alleles for a blood group in an individual ___________.
A. $1$
B. $2$
C. $3$
D. $4$

Answer 1

Landsteiner was the first person to discover the ABO blood group. The first polymorphism discovered in humans was the ABO blood group system. The blood group system is classified on the basis of inherited properties of red blood cells.

Complete Explanation
Option A $1$: This is a wrong answer because the number of alleles for a blood group in an individual is three instead of one. The ABO group is the first genetic polymorphism discovered in humans. There are three alleles: two codominant A and B alleles and one silent and recessive O allele. ABO blood grouping is multiple allelism. So, it cannot be a $1$ allele for a blood group.
Option A is incorrect.
Option B $2$: This is a wrong option because the number of alleles for a blood group in an individual is three instead of two. The ABO blood group system has three alleles two codominant A and B and one silent and recessive O allele. ABO blood group system is the classification of blood groups based on the inherited properties of red blood cells. As we know that the ABO blood group has multiple allelism means the gene has more than two alleles. Hence blood groups in an individual cannot have only $2$ allele.
Option B is incorrect.
Option C $3$: This option is correct. The number of alleles for a blood group in an individual is three. There are three alleles in the ABO blood group system: two codominant A and B alleles and one quiet and recessive O allele. Multiple alleles in ABO blood grouping refers to the fact that they have more than two alleles. Hence the human blood group has $3$ alleles; they are $I^A$, $I^B$, i.
Option C is correct.
Option D $4$: This option is wrong because the ABO blood group system contains only three alleles. The ABO blood group system has three alleles two codominant A and B and one silent and recessive O allele.
Hence, ABO has multiple allelism and means more than $2$ alleles. But the ABO blood group has only $3$ alleles in it. The $3$ alleles are $I^A$, $I^B$, i.
Option D is incorrect.

So, the correct answer is Option C. The number of alleles for a blood group in an individual is three.

Note:
In one individual, there are three alleles for each blood group. The first genetic variation discovered in humans was the ABO blood group. Landsteiner was the first person to discover the ABO blood group. Allele genes are the ones responsible for ABO blood grouping. $I^A$ and $I^B$ alleles are co-dominant, while i is recessive. The phenotype in the human blood groups is type A, type B, type AB, and type O.