blood antigens

Mutations and multiple alleles (LS)

1. What determines the blood groups?

Blood types are determined by the antigens A and B found on the surface of red blood cells.

Individuals having blood type A have antigen A on the surface of RBC.

Individuals having blood type B have antigen B on the surface of RBC

Individuals having blood type AB have both antigens A and B on the surface of RBC.

Individuals having blood type O don’t have any antigen on the surface of RBC.

2. What is the chemical composition of antigen A and antigen B?

Both antigens are made up of Glycoprotein where glyco is the sugar part.

Antigen A is made up of N-acetyl-galactosamine (sugar) and substance H (protein)

Antigen B is made up of galactose (sugar) and substance H (protein)

3. Where does formation of antigens take place?

It occurs in the erythroblasts (initial cell of RBC) in the bone marrow. RBC are

differentiated cells of erythroblast ( initial nucleated cells of the RBC )

4. How does allele A code for antigens A?

a. Substance H (protein ) is coded by a gene in the erythroblast.

b. Allele A on chromosome 9 in the erythroblast codes for transferase enzyme A which

picks up N-acetyl-galactosamine from blood and adds it to substance H that is found

in the cytoplasm of erythroblast to form Antigen A.

5. How does allele B code for antigen B?

a. Substance H (protein ) is coded by a gene in the erythroblast.

b. On the other hand allele B codes for transferase enzyme B that picks up galactose

from blood and adds it to substance H to form antigen B.

6. How does allele O code for antigen O (no antigen)?

Allele O is mutated allele by deletion and it codes for an inactive or truncated enzyme that is

not able to pick any sugar from blood, so no sugar will be attached to substance H. Then antigen

O is made up of protein H only without sugar.

7. How do mutations in genes interfere in the formation of blood type O?

Mutation by deletion occurs in one of the alleles leads to non sense causes the enzyme to be

inactive (truncated). Then the enzyme will be unable to pick up the sugar from blood and

attach it to substance H.

 Alleles A and B are formed because of 4 substitution mutations of the wild allele at

the same nucleotide of the same codon.