Standard paternity tests identify the direct biological father, but they are ambiguous when comparing a child to a grandfather. When the alleged father is unavailable, specialized kinship testing methods must be used to resolve the genetic overlap. Specialized tests use known inheritance patterns to statistically determine the likelihood of a grandparent-grandchild versus a parent-child relationship.
Understanding DNA Inheritance
Human genetic material is organized into 23 pairs of chromosomes, with one set inherited from each parent. The main portion of the genome is the autosomal DNA (chromosomes 1 through 22), which is inherited roughly 50% from the mother and 50% from the father. The process of recombination shuffles the genetic material from the four grandparents, meaning a grandchild shares about 25% of their autosomal DNA with each grandparent.
The Y-chromosome is particularly relevant for males. It is passed almost exclusively from father to son along the direct male lineage. A father and his son share the exact same Y-chromosome profile, as do a grandfather and his grandson, because they belong to the same paternal line.
How Standard Paternity Tests Work
A standard paternity test works by comparing the DNA profiles of the child and the alleged father. The test focuses on specific locations on the chromosomes called Short Tandem Repeats (STRs), which are short, repeating sequences of DNA. Laboratories analyze between 16 and 25 STR markers for a conclusive result.
For each STR marker, a person has two alleles, one inherited from each biological parent. The child’s profile is compared to the alleged father’s profile to see if the child inherited one of the father’s two alleles at every single marker. If the profiles match across all tested markers, a Paternity Index (PI) is calculated. This probability must exceed 99.9% to confirm paternity.
The Challenge of Genetic Overlap
The father and the paternal grandfather present a challenge in standard testing due to their close biological relationship. A father is the obligate source of half the child’s autosomal DNA, resulting in a 50% match across all markers. The grandfather, however, is the indirect source of the father’s half, meaning the child shares about 25% of their autosomal DNA with the grandfather.
If a standard paternity test were performed between a grandchild and a paternal grandfather, the grandfather would show a high degree of relatedness. The grandfather’s profile would appear compatible with a close relative, but the lower percentage of shared alleles would prevent the test from reaching the conclusive 99.9% threshold for fatherhood required for a direct parent-child relationship.
Specialized Kinship Testing Methods
When the alleged father is not available, specialized kinship tests are used to resolve the ambiguity. One method for male children is Y-Chromosome STR testing (Y-STR). This test compares the Y-chromosome profiles of the male child and the paternal grandfather. Since the Y-chromosome passes down the male line, a father, a son, a paternal grandfather, and a paternal uncle will all share the same Y-STR profile.
A Grandparentage Test primarily uses autosomal DNA markers. Testing both paternal grandparents allows the laboratory to statistically “reconstruct” the missing father’s DNA profile by combining the alleles contributed by the grandfather and the grandmother. The child’s DNA is then compared to this reconstructed profile. Including the child’s mother in the test further increases the statistical power by allowing the lab to immediately discount the alleles the child inherited maternally.