There are four different blood groups – A, B, AB and O. The ABO blood group system was discovered by Karl Landsteiner in 1900 and Felix Bernstein later figured out how blood groups are inherited in 1924. The inheritance of ABO blood groups is similar to eye and hair colour. The only difference is that blood groups are products of a single gene rather than multiple genes.
A child receives one of two ABO genes from each of their biological parents. The genes for the A and B blood groups are co-dominant, while the gene for the O blood group is recessive. This means that in order to have group O blood you need to inherit an O gene from both of your biological parents. If you have group A blood, you would have received A genes from both your parents, or an A gene and an O gene from each (the same concept would apply for group B blood expression). A person with an AB blood group would have received an A gene and a B gene from each of their parents, which are co-dominantly expressed. There are rare cases where a child inherits a recessive gene, so blood group information is not a reliable method to determine your parentage.
The table below explains the parent-child blood group possibilities: