Blood groups are due to carbohydrates on RBC surfaces
produced by blood group gene with 3 alleles:  A,  B,  &  i
 i allele adds no sugar on RBC cell surfaces
 A allele adds sugar galactosamine to RBC surface
 B allele adds sugar galactose to RBC surface
 A and B allele each produce a sugar
                              galactosamine & galactose.