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:
| Father's Blood Type | ||||||
| A | B | AB | O | |||
| Mother's Blood Type |
A | A or O | A, B AB, or O |
A, B, or AB |
A or O | Child's Blood Type |
| B | A, B, AB, or O |
B or O | A, B, or AB |
B or O | ||
| AB | A, B, or AB |
A, B, or AB |
A, B, or AB |
A or B | ||
| O | A or O | B or O | A or B | O | ||
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.
IDENTIGENE
DNA Testing Center: 2495 South West Temple | Salt Lake City, UT 84115
888-404-GENE (888-404-4363) | Direct: 801-462-1401 | Fax: 801-462-1403
careers at Identigene | careers at Identigene | dna paternity test kit | dna paternity test questions | about IDENTIGENE | paternity test community | site map