Blood Grouping in Disputed Paternity
Blood grouping can exclude a man from paternity but cannot definitively prove paternity; modern DNA testing has largely replaced traditional blood typing for paternity determination due to its superior discriminatory power and ability to provide conclusive results.
Historical Role and Current Status
Blood grouping systems (ABO, Rh, MNSs, Kell, Duffy, Kidd) were historically used to exclude falsely accused men from paternity, achieving exclusion rates of 22-40% depending on the population tested 1, 2, 3. However, these systems have significant limitations:
Blood typing can only exclude paternity, not prove it - when blood types are compatible between alleged father, mother, and child, this merely indicates the man could be the father, not that he definitively is 1, 4.
Combined blood group systems achieved only 92-95% probability of exclusion for falsely accused men, meaning 5-8% of non-fathers could not be excluded even with comprehensive testing 4, 3.
DNA testing has rendered blood grouping obsolete for paternity determination - modern forensic DNA evidence provides conclusive identification rather than mere exclusion 5.
How Blood Grouping Works for Exclusion
When blood typing was used, the principle relied on Mendelian inheritance patterns:
Direct exclusions occurred when the child possessed a blood group antigen that could not have been inherited from either the mother or alleged father (e.g., child is type AB, mother is type O, alleged father is type O) 1, 2.
Indirect exclusions (possible only when mother's blood is available) occurred when the child lacked an antigen that should have been inherited from the alleged father based on his phenotype 4, 2.
Testing without the mother present significantly reduced discriminatory power, with combined probability of exclusion dropping to approximately 95% for whites and 92% for blacks using all available blood group systems 4.
Racial and Population Differences
Blood group polymorphism varies significantly across populations, affecting test performance:
No significant difference in exclusion rates was observed between Caucasian and Black populations (approximately 26-30% exclusion rate) 1, 2.
Lower polymorphism in Black populations resulted in significantly fewer non-excluded men achieving ≥95% probability of paternity compared to Caucasians, necessitating additional genetic markers 1, 3.
HLA typing provided the most powerful discrimination, accounting for 92% of exclusions when used alone, with red cell antigens contributing an additional 8% of exclusions 2, 3.
Modern DNA-Based Approach
Current paternity testing relies exclusively on DNA analysis using microsatellite markers or single nucleotide polymorphisms (SNPs), which provide:
Definitive inclusion or exclusion with probability of paternity exceeding 99.9% when the alleged father is the biological father 5.
PCR-based techniques using informative markers trace transmission of chromosomes from each parent to determine biparental inheritance patterns 5.
Testing requires DNA samples from the child and alleged father at minimum; maternal DNA increases discriminatory power but is not essential 5.
Critical Considerations
Blood grouping should not be used for modern paternity determination - it has been superseded by DNA testing that provides conclusive rather than probabilistic results 5.
Misattributed paternity, adoptions, and unknown family structures can complicate genetic testing and should be considered when interpreting results 5.
Proper sample collection and chain of custody are essential for forensic applications to ensure admissibility in legal proceedings 5.
Laboratory accreditation and proficiency testing are mandatory for facilities performing paternity testing to minimize errors 5.