When it comes to UK paternity testing, it is not unusual for a customer to tell us there are two possible fathers that need to be tested for paternity and that those two fathers are biologically related. Not all biological relationships cause issues with testing or affect the likelihood of providing false results, but testing two very closely related alleged fathers requires additional care.

Although two possible fathers who are related don’t have exactly the same DNA (unless they’re identical twins), they can share so much DNA that getting conclusive results for a paternity test may be problematic. Although it is a challenge, it is definitely possible for an experienced lab to run extended DNA testing with more in-depth analysis and obtain conclusive results, even if only one of the alleged fathers is available for or agrees to test. Here are some relationship possibilities and how they may affect test results.

Paternity Testing if Alleged Fathers are Brothers

Although they are closely related, full brothers each have DNA profiles that are actually still quite distinct from one another. Some of their DNA is the same, but certainly not all of it. For that very reason, the chances of two brothers who are not identical twins matching a child at each genetic marker for paternity testing would be highly unlikely.

But because the biological relationship between possible fathers is so close, we would still recommend that both fathers’ DNA be tested against the DNA of the child in question. At the very least, it is essential to let the lab know there are two possible fathers and that they are brothers. The lab can then test more genetic markers, if necessary. It’s also important to include the mother’s DNA in testing if at all possible.

Paternity Testing if Alleged Fathers are Twins

Identical Twins

For identical twins, their DNA is as you would expect: exactly the same. Even with the most advanced technology available today, it would still be impossible in any practical or affordable way to genetically differentiate between the two fathers for paternity purposes.

 Fraternal Twins

If the men are fraternal (not identical) twins, the DNA that they share would be the same amount as it would be for non-twin siblings. As with a non-twin brothers’ test, it would be preferable if both brothers could be tested, and wherever possible the child’s mother should also provide a sample. And again, the lab should be advised that the relationship exists so that additional testing is undertaken and a definitive result can be provided.

Paternity Testing if Alleged Fathers are Father and Son

50% of the son’s DNA comes from his father, so if these two men are the possible fathers of a child, there is a high possibility of obtaining a “false positive” result if only one of the men participates in testing. A good UK paternity testing lab would recommend that both men test, if possible. If that isn’t possible, conclusive results can still be obtained if the mother also agrees to contribute her DNA. Additionally, the lab must be notified before testing begins that there is another alleged father with a filial connection. This is an absolutely essential step, because the lab can then test additional genetic markers, if necessary.

Paternity Testing: If Alleged Fathers are Cousins

Even men who are first cousins don’t share enough genetic material in common to cause a “false positive” for UK paternity testing: the genetic connection is just too far removed to affect a test one way or the other.

In Conclusion

If the alleged fathers are closely-related, there are two things to remember when doing UK paternity testing:

  1. Test both fathers, if at all possible. If only one can be tested, then the mother should contribute her DNA as well in order to strengthen results.
  2. If only one alleged father can test, let the lab know ahead of time that there is a different man who could also be the father and what the biological relationship is between the two men.

If you need additional information about a DNA test or want to order, contact our experienced customer-service team on 0800 009 2969.