Paternity testing can already feel emotional and overwhelming, but when the potential fathers are biologically related the situation can seem even more complex. Whether they are brothers, cousins, or even father and son, many people worry that shared family DNA could make it impossible to determine paternity accurately. The good news is that accurate paternity testing is still possible in many of these cases.
At NorthGene™, our experienced laboratory team uses advanced testing methods designed to support complex family relationship cases and deliver reliable results wherever possible.
Can a paternity test be done if the potential fathers are related?
Yes – paternity testing is still possible when the alleged fathers are related. However, close relatives share portions of the same DNA, which is why more comprehensive testing and analysis is required than a standard paternity test.
It is extremely important to inform the laboratory beforehand if the potential fathers are biologically related so that the appropriate testing and interpretation methods can be used. At NorthGene™, we carefully review each case to ensure the correct analysis is carried out and reduce the risk of inconclusive results.
H3: Understanding the relationships
DNA is inherited through family lines, meaning relatives naturally share genetic markers with one another – the closer the biological relationship between the potential fathers, the more DNA they are likely to share:
| Relationship | Average % shared DNA |
| Identical twins | 100% |
| Parent/child, full siblings | 50% |
| Grandparent/grandchild, half siblings, aunt/uncle to niece/nephew | 25% |
| First cousins | 12.5% |
As a result, distinguishing between closely related individuals can sometimes be more challenging, particularly if only one potential father is tested.
What if the potential fathers are identical twins?
Identical twins (also known as monozygotic twins) can share up to 100% of their DNA because they develop from a single fertilised egg. Due to this, it is usually not possible to distinguish between identical twins using standard paternity DNA testing methods alone.
Improving the likelihood of a conclusive result
Because related individuals share more DNA than unrelated individuals, additional steps are often recommended to improve the chance of obtaining a conclusive result.
- Test both potential fathers wherever possible: including both potential fathers in the testing process provides a clearer DNA comparison and significantly reduces ambiguity.
- Include the mother in the test if possible: testing the mother’s DNA allows us to isolate the DNA inherited from the biological father, allowing for a more accurate interpretation of the child’s results.
- Choose an experienced laboratory: our experienced team of scientists use advanced analysis to provide the clearest and most reliable results possible.
While these additional steps can improve the likelihood of a conclusive result, they are not always essential. A conclusive result can often still be achieved even if one or more of these recommendations cannot be followed.
However, it is essential that you inform the laboratory if the tested father is biologically related to another potential father (for example, if they are brothers or other close relatives). All paternity testing is normally conducted on the assumption that no relative of the tested father could also be the biological father. This information determines the hypotheses and statistical analysis used by our scientists. Without it, interpreting the results becomes significantly more difficult and increases the risk of an incorrect inclusion (false positive) being reported.
Navigating complex paternity testing
Paternity testing involving related potential fathers can be more complex, but with the right laboratory expertise, accurate and reliable results are still achievable in most cases.
At NorthGene™, we understand that every family situation is unique and often sensitive. Our experienced team provides professional, confidential support throughout the process, helping you understand the testing options available alongside compassionate support at every stage. We deliver clear, reliable results when it matters most.
If you are unsure which test is right for your situation, get in touch with our expert team to discuss your circumstances in confidence, or order your paternity test today.




