If one parent's ABO phenotype is known as well as that of one or more children, then the phenotype of the other parent can often be inferred.


The inheritance of the ABO group is as described in the previous section.


If several children are available for testing, and if we assume that all of them are offspring from the same 2 parents, then the possible ABO phenotype(s) of the unknown parent is the subset of possible ABO phenotypes common to all of the children.


For example, if there are 3 children:

• possible parental ABO phenotype for first child = O, A, B

• possible parental ABO phenotype for the second child = A, AB

• possible parental ABO phenotype for the third child = O, A

then the parental ABO phenotype is A, since it is a candidate for all 3 children.


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