Illustration of genetic diversity in blood groups. Blood groups are determined in a gene from chromosome 9. This gene expresses a red corpuscle protein. There are 3 different alleles for this gene, A which gives A protein, B which gives B protein, and O which doesn抰 give anything. Therefore : -two A alleles (red) : group A -and A allele and an O (white) : group A -two B alleles (blue) : group B -a B allele and an O : group B -an A allele and a B : group AB -two O alleles : group O