Blood type is determined genetically.

The antigen molecules on the surface of our red blood cells are made up of two different enzymes.

The enzymes are encoded by different alleles of the same gene. Everyone inherits two alleles of the same gene (one from each parent) and the specific combination decides our blood type.

 

It is Mendelian trait because we inherit the two alleles when determining blood type.

A and B alleles are both dominant. If you were to recieve the A blood type allele from one parent and the B blood type allele from the other in your genotype, both blood types wouls be expressed in your phenotype. The O blood type allele is recessive.