ABO blood group system is example of multiple allelism in which 3 alleles are there to control the blood group. These 3 alleles are A (Dominant), $\mathrm{B}$ (Dominant) and O (Recessive). This blood grouping has 4 types of phenotypes (as 4 blood groups) which are $\mathrm{A},\mathrm{ }\mathrm{B},\mathrm{ }\mathrm{O}$ and $\mathrm{A}\mathrm{B}$ . This grouping has total 6 types of genotypes which are $\mathrm{A}\mathrm{A},\mathrm{ }\mathrm{A}\mathrm{O},\mathrm{ }\mathrm{B}\mathrm{B},\mathrm{ }\mathrm{B}\mathrm{O},\mathrm{ }\mathrm{A}\mathrm{B}$ and $\mathrm{O}\mathrm{O}$ .