There are 4 main blood types: A, B, O and AB. Our blood group represents contributions from both parents of the ABO and H genes that encode glycosyltransferase - an enzyme that will transfer monosaccharides to polysaccharide chains. In most cases, an individual will have the same blood type for their whole life, however it is possible for it to change due to things such as infections or autoimmune diseases.
The ABO gene is located on chromosome 9 and has the alleles: A, B and O. The A and B alleles are both dominant to the O allele, but are co-dominant to each other (thus allowing the AB blood type) however the O allele is recessive and when expressed, results in the production of a non-functional protein.
It is thought that the O allele results in a trucated and non-functional protein due to a deletion in exon 6. This causes a frameshift mutation and so the resulting protein is non-functional.
O blood group is the basic oligosaccharide structure for the different blood groups. It contains 2 galactose, a fucose and a N-acetylglucosamine. A blood group has the basic O structure but with the addition of N-acetylgalactosamine and B blood group has the basic O structure with another galactose present. AB blood group humans have both the genes for the type A and type B glycosyltransferases therefore they synthesise both blood types hence the name AB.