The ABO Blood Group

We are all familiar with traditional blood groups (A+, B-, etc.). In terms of notation, the letter of the blood group gives a person's phenotype on the ABO blood group gene, while the plus (+) or minus(-) sign gives the person's phenotype on the Rhesus gene. We will ignore Rhesus for now to concentrate on the ABO system.

The ABO gene is located near the end of the long arm of chromosome 9. There are three main alleles at this locus—the A allele, the B allele, and the O allele. (There are actually more alleles but, to keep matters simple, we will not consider them.) The A allele codes for a polypeptide chain that resides on the membrane of the red blood cell. If we take blood from a person and place into it a checmical specific to the A polypeptide, the blood will react and form "clumps" visible to the eye. The B allele also produces a polypeptide, but one that is different than the one produced by the A allele. It will not react to the A chemical, but it will react to its own specific B checmial by clumping the blood. The O allele apparently does not make a functional polypeptide and hence will not react to either the A or the B chemical.

When someone is blood typed, a drop of their blood is placed on a slide and the A chemical is added. If the blood clumps, then we know that the person has at least one A allele; if the blood does not clump, then the person does not have an A allele. Another drop of blood is placed on another slide along with some B chemical. If this clumps, then the person has at least one B allele, and if it does not, then the person does not have a B allele. Using this logic, one can then construct the blood group phenotypes and their genotypes from the table below.

The ABO blood group genotypes and phenotypes.

 

Results from:

 

 

A antigen

B antigen

Genotype(s)

Phenotype

clumping

no clumping

AA or AO

A

no clumping

clumping

BB or BO

B

clumping

clumping

AB

AB

no clumping

no clumping

OO

O

In terms of gene action, you should be able to see why alleles A and B are dominant to allele O. You should also see that alleles A and B are codominant with each other.