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