ABO system of blood grouping in humans is based on the chemical difference due to presence of antigens on the surface of the RBC and epithelial cells as follows:
(i) Bernstein discovered that, the inheritance of different blood group in humans is determined by a number of multiple allelic series.
(ii) The three autosomal alleles located on chromosome 9 are concerned with determination of blood group.
(iii) The gene controlling blood type is labelled as 'L' or I. The gene (isoagglutination) I exists in three allelic forms `I^(A), I^(B) " and " I^(o)`
(iv) `I^(A)` specifics A antigen, `I^(B)` allele determined B antigen and `I^(O)` allele specifies no antigen.
(v) Each allele (IA and IB) produces a transferase enzyme. IA allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and IB 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.
(vi) `I^(A) " and " I^(B)` are dominant to `I^(O)` but co - dominant to each other `(I^(A)=I^(B))`. The dominance hierarchy is given as `(I^(A)=I^(B) gt I^(O))`.
(vii) A child receives one of three alleles from each parent giving rise to six positive genotypes and four possible blood type (phenotypes)
Genetic basis of the human ABO blood groups
