A child's blood group can be deduced from both parents' blood groups. The blood test result is shown in the means of probabilities, which denote the chance of the child having a particular blood group.

For instance, the result from two parents with blood group AB and O shows 50% each for blood group A and B. This indicates that there is 50% probability for the child to have blood group A and group B respectively.

Note that the probability calculated is independent of one another, meaning the chance of the second child from the same parents to have blood group A is also 50%.