Our red blood cells (and some tissues) have got chemical substances called antigens on their surface and the ability to form these antigens is governed by genes inherited from parents. The pattern of genes we have is called genotype and the form in which it is expressed is called phenotype. It is the genotype (genetic make-up) which decides what the blood group (phenotype) will be.
In the ABO system there are three alleles (one member of a pair or series of genes that occupy a specific position on a specific chromosome): Ia - producing the antigen A, Ib - producing the antigen B and Io - producing neither antigen. IA and IB are co-dominant i.e. if both alleles are present, both antigens (A and B) will be formed on the surfaces of red blood cells and the individual will have blood group AB. Io is recessive and produces neither antigen and thus an individual will have blood group O if two of these alleles are present. The genotypic make-up can have 6 possibilities resulting in the following phenotypes:
Ia Ib - AB, Ia Ia - A, Ia Io - A, Ib Ib - B, Ib Io - B, and Io Io – O
|Blood group of parents ||Blood group of child |
|A and A ||A, O |
|A and B ||A, B, AB, O |
|A and AB ||A, B, AB |
|A and O ||A, O |
|B and B ||B, O |
|B and AB ||A, B, AB |
|B and O ||B, O |
|AB and AB ||A, B, AB |
|AB and O ||A, B |
|O and O ||O |
However, my advice to you would be to get the blood groups of the parents and the children re-tested at a reliable, quality-assured lab before speculating or jumping to any conclusion.