Historically, blood typing was used to establish biological relationships when paternity was uncertain. This approach is primarily effective for excluding potential fathers, not for definitively proving paternity. Its limitations stem from the fundamental genetic principles involved.
The Genetics of Blood Types
Blood types are inherited traits, passed from parents to their children through genes. The most widely known system is the ABO blood group, which classifies blood into four main types: A, B, AB, and O. These types are determined by the presence or absence of specific antigens on the surface of red blood cells. The ABO gene has three alleles: A, B, and O. Alleles A and B are codominant, meaning both are expressed if inherited, while the O allele is recessive.
An individual with an AO genotype will have A blood type, and a BO genotype will result in B blood type. However, only an OO genotype will result in O blood type.
In addition to the ABO system, the Rh factor is another important blood group system. Individuals are either Rh-positive (Rh+) or Rh-negative (Rh-), depending on the presence or absence of the Rh protein on their red blood cells. The Rh factor is inherited independently of the ABO blood group, with the Rh-positive allele being dominant over the Rh-negative allele. A person inherits one Rh factor gene from each parent, determining their Rh status.
Blood Typing for Paternity Exclusion
Blood type analysis can effectively rule out a man as a biological father due to predictable inheritance patterns. A child inherits half their genetic material from each parent, so their blood type must be genetically compatible with both the mother’s and the alleged father’s blood types. For instance, if a child has O blood type and the mother also has O blood type, a man with AB blood type cannot be the father. This is because an individual with AB blood type does not carry the O allele to pass on. If the child’s blood type is genetically impossible based on the parents’ blood types, paternity is excluded.
Limitations of Blood Type Paternity Testing
Blood typing can exclude a potential father, but it cannot definitively confirm paternity. The primary limitation is that many individuals share common blood types. If a potential father’s blood type is compatible with the child’s, it only signifies he could be the father, not that he is the father. This lack of uniqueness means multiple men could have a blood type that does not exclude them from paternity.
Blood types are common characteristics, unlike highly specific identifiers such as fingerprints. Only a few genes determine blood type, and variations are limited, so the information is not granular enough for definitive identification. Even if a man’s blood type is compatible, it merely places him within a large group of individuals who could also be the biological father. This commonality prevents blood typing from serving as conclusive evidence for proving paternity.
The Role of DNA Testing
Modern DNA testing has largely replaced blood typing as the gold standard for paternity determination. Unlike blood typing, which analyzes a limited set of antigens, DNA testing examines numerous unique genetic markers, or polymorphisms, that are highly specific to an individual. Each person inherits half of their DNA from their mother and half from their father, making DNA comparison a precise method for identifying biological relationships.
DNA paternity tests achieve an accuracy rate of 99.9% or higher in confirming paternity. This high level of accuracy is why DNA testing is widely accepted in both legal and personal contexts. The technology can provide a virtually conclusive determination, offering a level of certainty that blood typing simply cannot match.