Is it possible to perform investigation of natural paternity, maternity or brotherhood and sisterhood using the ABO blood typing?
By using the ABO blood typing it is possible only to exclude paternity, maternity or brotherhood/sisterhood but it is not possible to conclude positively about these relationships.
For example, if an individual has type O blood, ii genotype, he or she cannot have biological parents of the type AB (IAIB genotype) as necessarily one of his/her alleles has come from the father and the other from the mother. Another example: a couple of individuals of the type O (ii) in their turn can only formed direct offspring of the type O blood, as they do not have alleles that condition antigen A neither antigen B.