Grandparent DNA testing is a widely practiced and highly accurate method, especially when the father is unavailable for testing. In such cases, testing the paternal grandparents provides a reliable alternative as the father’s DNA is directly inherited from both the paternal grandmother and grandfather.

Testing with Both Paternal Grandparents

Performing tests with both grandparents allows for a comprehensive reconstruction of the supposed father’s DNA. In every autosomal marker, the alleged father inherits one allele from his mother and one from his father. Subsequently, he transmits one of these alleles to his children. By incorporating both grandparents, the child being tested will exhibit a match with either the grandfather or grandmother on each marker.

Here are two instances of autosomal markers displaying alleles in numerical format. Each example includes the alleged father’s parents, the child, and the mother. The father’s DNA is also included to illustrate the potential inheritance from his parents. It’s important to note that in grandparent tests, the alleged father is not tested; our aim is to provide a visual representation of how DNA is passed down through generations.

In the D3S1358 marker, it is evident that the child shares a match with the mother on the 12 allele. The remaining allele, which is 14, corresponds with the grandmother. Regarding the FGA marker, the child inherited the 23 allele from their mother and aligns with the grandfather on the 45 allele.

Many relationship tests involve 21 or more genetic markers. If the child consistently matches either the grandfather or grandmother on each of these markers, it provides evidence supporting the paternal relationship. Conversely, if there are markers where the child does not align with the grandparents, it indicates a lack of support for that particular relationship.

Testing with One Paternal Grandparent

When only one paternal grandparent is available for testing, the process can still be completed, but there is a potential for inconclusive results. We can explore the recommended approaches for DNA testing when only one grandparent is available.

Testing Male Children with One Paternal Grandparent

For male children, opting to test the paternal grandfather is preferable due to the inheritance of Y chromosomes through the male line. When a male has a son, he transmits all his Y chromosomes to his male offspring, who, in turn, pass them on to their own male descendants. The Y chromosome matching between two males from the same lineage serves as a conclusive indicator of a biological relationship, minimizing the likelihood of inconclusive results.

In the provided example, we are presenting two Y chromosome markers. Notably, the grandmother and the mother of the child exhibit no alleles on the Y chromosome. This is because females do not carry Y chromosomes. Furthermore, the results indicate a complete match between the alleged paternal grandfather and his son (the alleged father), with the child in question also displaying an exact match to both of them. Consistent matching on all Y chromosomes, if sustained, would support the evidence for a paternal connection.

Should the grandfather be unavailable, testing can proceed with the paternal grandmother. Nevertheless, the additional benefit conferred by the presence of Y chromosomes would be absent in such cases.

Testing Female Children with One Paternal Grandparent

On the contrary, if the child is a female, it is recommended to test the paternal grandmother. A female and her paternal grandmother are expected to share at least one match in their X chromosomes.

The father consistently inherits a single allele for his X chromosomes exclusively from his mother. In contrast, females receive alleles for X chromosomes from both their mother and father. Since males possess only one allele in each of their X chromosomes, females inherit their father’s singular allele from each specific X chromosome.

In our last illustration, we present two X chromosome markers, featuring the alleged paternal grandparents, the alleged father, the child, and the mother. It’s evident that the alleged paternal grandfather’s X chromosomes are not transmitted to his son (the alleged father). In both X markers, the female child exhibits a match with her mother on one allele, while the other allele corresponds to the paternal grandfather, aligning with the alleged father as well.

While X chromosome markers may not offer the same robust evidence as Y chromosomes do for males and their paternal grandfathers, they provide an additional layer of testing for this unique relationship with females and their grandmothers. Although not as definitive, this inclusion helps reduce the likelihood of inconclusive results.

If the grandmother is unavailable for testing, it is possible to proceed with testing the paternal grandfather. However, in such cases, the additional advantage provided by the presence of X chromosomes would be lacking.


In conclusion, grandparent DNA testing serves as a valuable and accurate method, particularly when the father is unavailable for testing. In every scenario, whether testing with both grandparents or just one, the careful analysis of genetic markers helps establish or rule out paternal relationships. Grandparent DNA testing is a powerful tool that aids in unraveling the complexities of familial connections when direct paternal testing is not possible.