Based on Mendel’s work, science first answered paternity questions by looking at blood type – a trait with inheritance patterns similar to that of flower color in pea plants. Before DNA, blood offered the best clues for answering paternity test questions and included methods like ABO blood typing, Serology, and HLA testing.
ABO Blood Types
Paternity testing took another leap forward in the early 1900s when scientists discovered four different human blood types based on antigens (proteins) in the blood: A, B, AB, and O. By the 1920s, scientists recognized that ABO blood types were genetically linked, that is they were passed from parents to children.
Using the Mendelian idea of inheritance, scientists developed a type of Punnett square for blood types:
Researchers learned that the genes (alleles) for ‘O’ blood type are recessive while the genes (alleles) for A or B blood types are dominant. For example, ‘type A’ blood means a ‘type A’ phenotype. But the genotype, the actual DNA, may be either ‘AA’ or ‘AO’ when analyzed, since the ‘O’ type is recessive. Similarly, ‘type B’ blood must have a genotype of ‘BB’ or ‘BO.’
However, a person with a phenotype of ‘AB blood’ must show a genotype ’AB’ (or ‘BA’) because the ‘A’ and ‘B’ alleles are both dominant (co-dominant). A person with ‘AB’ blood must have both the ‘A’ and the ‘B’ alleles for this type. A person with ‘type O blood’ must have the genotype ’OO’ because the ‘O’ alleles are recessive.
A paternity test by blood type became possible with this discovery and was used with some success. For example, if a child has a blood type of O, he is pure-recessive for the ‘O’ allele. That means his biological parents must both have blood type ‘O’ because they must carry only the recessive ‘O’ alleles; if the alleged father has any other blood type, he is excluded as the father.
However, if a child has blood type A, and the mother has blood type A, the father may have blood type A, AB, or O — many possibilities! Thus, the power of exclusion (the power to eliminate a possible father) for ABO paternity tests is only 30% – not at all conclusive for paternity testing results.
The 1930 paternity test relied on the newly discovered science of serology. Science found that, in addition to ABO types, blood could be separated into other groups using the Rh, Kell, and Duffy blood group systems.
In these systems blood could also be typed as positive (dominant) or negative (recessive). However, even with serology based paternity tests, this only increased the power of exclusion to 40%.
HLA Paternity Testing
In the 1970s paternity testing again advanced as a science. The Human Leukocyte Antigen (HLA) appears in virtually every cell of the human body everywhere except red blood cells. HLA types vary greatly from person to person, providing a more unique measurement for a paternity test.
The HLA paternity test looks at both blood and tissue to find antigens that could be traced to a potential father. With a power of exclusion as high as 80-90%, the HLA test became much more effective than blood tests alone in answering questions about paternity. However, HLA testing is very expensive and time consuming. HLA testing also requires large samples of blood and tissue and is therefore thought unsafe for infants under six months.