Dear student,
Genes contain and carry genetic information from one generation to another. Genes that code for pair of contrasting characters are known as alleles. When more than two alleles control a character, as in human blood groups then they are known as multiple alleles.
ABO blood group system can be studied using Multiple alleles concept for a population:-
Mendel stated that for a particular trait there are two alleles but in case of ABO blood group there are multiple alleles for blood group. Multiple allele in case of blood group IA, IB, IAB, IO .
Regards