Can two brothers have different blood group types?
The simple answer to your question is yes. If the father was blood group A his genotype could be AA or AO. The A gene is dominant whereas O is recessive. In the case of a dominant and recessive gene being present the dominant gene is the one that gains expression. Each parent contributes one of these genes to each child therefore the son of this father could inherit either the A or O gene. If by coincidence the mother was A she could give either an A or O to her son. Therefore the sons or daughters of this couple could have the following genotypes; AA resulting in blood group A, AO also resulting in blood group A or OO resulting in blood group O.