Understanding Blood Type Genetics in Paternity Assessment

Blood type has been used as a basic tool for paternity assessment since the early 20th century, long before DNA testing became available. The ABO blood group system is determined by a single gene on chromosome 9 with three alleles: A, B, and O. The A and B alleles are co-dominant, meaning both are expressed when present together, producing blood type AB. The O allele is recessive, so a person must inherit an O allele from each parent to have blood type O. Every person carries two alleles, one inherited from their biological mother and one from their biological father. This means a child's blood type is always a combination that can be traced back to the parental alleles. For example, two parents who are both blood type O (genotype OO) can only produce children with blood type O. If a child in that family has blood type A, it indicates that at least one biological parent must carry an A allele.

How the Rh Factor Affects Paternity Assessment

The Rh factor adds another layer of genetic information to blood typing. The Rh system is primarily determined by the RhD protein, encoded by the RHD gene on chromosome 1. A person is Rh-positive (Rh+) if they carry at least one functional copy of the RHD gene and Rh-negative (Rh-) if they carry two non-functional copies. Rh-positive is dominant over Rh-negative. This means two Rh-negative parents (both genotype dd) cannot produce an Rh-positive child. If both parents are confirmed Rh-negative and the child is Rh-positive, this is strong evidence that the presumed father is not the biological father. However, two Rh-positive parents can produce an Rh-negative child if both are heterozygous carriers (genotype Dd), so an Rh-negative child born to Rh-positive parents does not indicate non-paternity.

When Blood Type Can Rule Out Paternity

Blood type analysis can definitively exclude a man as the biological father in certain combinations, but it can never confirm paternity. For instance, if the mother is blood type O (genotype OO) and the child is blood type AB (genotype AB), then the biological father must have contributed either an A or B allele. A man who is blood type O (genotype OO) cannot be the biological father in this scenario because he has no A or B allele to pass on. Similarly, if the mother is type A and the alleged father is type A, and the child is type B or AB, the alleged father can be excluded unless the mother also carries a B allele. These exclusion scenarios are straightforward applications of Mendelian genetics and are considered scientifically reliable when blood types are accurately determined.

Limitations of Blood Type Paternity Testing

The primary limitation of blood type paternity testing is its low power of exclusion compared to DNA analysis. Because there are only four possible ABO blood types and two Rh types, many unrelated men will share the same blood type as the true biological father. Studies estimate that ABO blood typing alone can only exclude approximately 30% of falsely accused men. Even when combining ABO with Rh and other red blood cell antigen systems like MNS and Kell, the cumulative exclusion power reaches only about 50 to 70%. By comparison, modern DNA STR analysis examines 20 or more independent genetic markers and achieves exclusion powers exceeding 99.99%. Blood type testing is therefore useful as a quick, inexpensive screening tool that can sometimes rule out paternity conclusively, but a compatible blood type result provides very weak evidence of biological relationship.

Blood Type as Part of a Broader Assessment

In modern paternity assessment, blood type genetics serves primarily as educational context rather than a standalone testing method. If you already know the blood types of the mother, child, and alleged father, you can check whether the combination is genetically possible using standard ABO and Rh inheritance charts. An impossible combination is a definitive exclusion. A possible combination, however, tells you very little because the same blood types are shared by a large percentage of the population. For anyone seeking meaningful paternity assessment today, AI-based facial recognition analysis through services like TrueDadz offers a more informative preliminary screening tool, while DNA testing remains the definitive standard. Blood type information can be a useful data point within a broader assessment, but it should never be relied upon as the sole basis for a paternity determination.