The question of whether blood type can prove paternity is rooted in forensic and medical history. Blood is categorized primarily using the ABO system (A, B, AB, and O) and the Rhesus (Rh) system, which determines if blood is positive or negative. Before modern genetic analysis, these two grouping systems were the standard tools used in legal and forensic settings to determine if a man could be biologically linked to a child. This method relied strictly on the predictable patterns of inheritance established by genetic science.
Understanding How Blood Types Are Passed Down
Blood type is a physical characteristic, or phenotype, determined by the genes inherited from both parents. The ABO system is governed by a single gene locus with three primary alleles: A, B, and O. Alleles A and B are co-dominant, meaning that if both are present, both are expressed, resulting in the AB blood type.
The O allele is recessive, so it is only expressed if a person inherits an O allele from both the mother and the father (genotype OO). A person with Type A blood, for example, could have the genotype AA or AO, while a Type B person could be BB or BO. Since every person inherits one allele from each parent, the possible combinations are highly predictable.
The Rh factor is inherited independently of the ABO type and is determined by the presence or absence of the D antigen on red blood cells. If a person inherits even one dominant Rh-positive allele, they will be Rh-positive. The Rh-negative trait is recessive and requires the inheritance of two Rh-negative alleles (dd), one from each parent, to be expressed.
Geneticists use the known genotypes of the parents to calculate the possible genotypes and phenotypes of the child. This systematic process of inheritance provides the scientific basis for either confirming a possible parentage or, more definitively, ruling one out.
Identifying Impossible Parental Combinations
The application of blood typing to parentage disputes centers on the concept of “exclusion,” which means definitively proving that a man is not the biological father. This is possible because a child cannot possess a blood group antigen that is not present in at least one of the parents. If the child’s blood type requires an allele that the alleged father does not possess, that man is genetically excluded.
A classic example from the ABO system involves a Type O parent (genotype OO) and a Type AB parent (genotype AB). The Type O parent can only pass on an O allele, while the Type AB parent can only pass on A or B. Therefore, their child can only be Type A (AO) or Type B (BO). If they were to have a Type AB child, the alleged father would be excluded.
Another clear exclusion occurs when both parents are Type O. A child resulting from this pairing must also be Type O, as neither parent possesses the A or B allele. If such a couple were to have a child with Type A, Type B, or Type AB blood, the alleged father would be excluded as the biological parent.
Similarly, in the Rh system, two Rh-negative parents, who both have the recessive genotype (dd), can only pass on the d allele. Consequently, a child of two Rh-negative parents must also be Rh-negative. If their child were found to be Rh-positive, the alleged father would be excluded. These specific combinations highlight the power of blood typing to disprove parentage with certainty.
The Shift from Blood Typing to DNA Testing
Blood typing is no longer the standard for determining parentage because its ability to exclude a falsely accused man is relatively low. The inherent limitation is that common blood types are shared by large segments of the population. For instance, the ABO system alone can only exclude approximately 10 to 20% of men falsely accused of paternity.
Even when combining the ABO and Rh systems with other red blood cell antigen systems, the overall “Power of Exclusion” (POE) typically only reaches about 40%. This means that in six out of ten cases where the alleged father was not the biological parent, blood typing would fail to provide an exclusion.
Modern DNA testing, which analyzes Short Tandem Repeats (STRs) within the human genome, replaced blood typing due to its vastly superior accuracy. STR analysis examines numerous unique markers on an individual’s chromosomes, providing a highly specific genetic profile. This technology offers a probability of parentage that typically exceeds 99.99% when the alleged father is indeed the biological parent.
If the genetic markers do not match, the probability of parentage is 0%, which is a definitive exclusion. This high degree of certainty, which can both prove inclusion and exclusion, has made DNA analysis the definitive and legally accepted standard for parentage testing today.