Variation

Principles of Inheritance and Variation antigen and I O allele specifies no antigen. Individuals who possess these antigens in their fluids such as the saliva are called secretors. I A allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and I B allele encodes for the enzyme galactose transferase that adds galactose to the precursor (i.e., H substances) In the case of I O /I O allele no terminal transferase enzyme is produced and therefore called “null” allele and hence cannot add NAG or galactose to the precursor. From the phenotypic combinations it is evident that the alleles I A and I B are dominant to I O , but co-dominant to each other (I A =I B ).

Their dominance hierarchy can be given as (I A =I B > I O ). A child receives one of three alleles from each parent, giving rise to six possible genotypes and four possible blood