The ABO blood type is determined by different alleles for a gene that codes for an enzyme. How do the mutations in the alleles lead to the different blood types?

Mutations arise as a result of changes in the normal DNA sequence either by mistake or environmental factors. Mutations in the ABO allele can lead to different blood types other than that which is expected as a result of the parental genetic makeup.

Explanation:

The ABO gene resides on chromosome 9 and it has 3 alleles - the A allele, which encodes glycosyltransferase which makes the A antigen, the B allele which encodes a different glycosyltransferase that adds d-galactose producing the B antigen and the O allele which is a deletion that results in unmodified H antigen with no surface molecules

The combination of 2 alleles one from each parent results in different blood types.

Alleles Blood Types

A x A A

A x B AB

A x O A

B x B B

B x O B

O x O O