In paternity testing, an indirect exclusion occurs when a child does not express an antigen that he should have expressed based on the father’s presumed genotype. An example is when a father who has the phenotype Fy(a+b-) is evaluated for paternity of a child that is Fy(a-b+), when the mother types as Fy(a-b+). The father’s presumed genotype in that setting is FYA/FYA, so he should have transmitted an FYA to the child. The lack of Fya antigen in the child constitutes an indirect exclusion. Indirect exclusions, in general, are less convincing evidence of a lack of paternity than direct exclusions (especially since the father in this case could easily carry a “null” allele instead of two copies of FYA). Both direct and indirect exclusions have been replaced by DNA-based testing methods.
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