Can a DNA Paternity Test Distinguish Between Possible Fathers from the Same Family?

A DNA paternity test is used to determine who a child’s biological father is by comparing the presumed father’s DNA with the child’s DNA. When the possible fathers belong to the same family, the comparison becomes more complex, but it is still often possible depending on the situation.

How does a DNA paternity test work?

The principle is straightforward: the child’s DNA is compared with that of the presumed father.

If a sufficient amount of shared DNA is found, the test concludes that the child is his.

This similarity is assessed by comparing multiple genetic markers, and each person has a unique combination of these markers.

Related alleged fathers: why it is more difficult, but not always impossible

When two possible fathers are biologically related, part of their DNA is naturally similar. This makes interpretation more difficult, because genetic compatibility may appear high with more than one person from the same family line.

Even so, this does not automatically prevent a reliable result: a laboratory can often differentiate two DNA profiles and identify which one is truly consistent with paternity.

The key point is this: the more distant the family relationship, the easier the distinction usually is. For example:

• two brothers share more genetic characteristics;

• two cousins generally share fewer.

So, when two cousins are being compared in a paternity context, the probability of obtaining a decisive result is usually higher.

Paternity doubt: father or grandfather?

In some cases, the two possible fathers are linked by a father-son relationship, for example the presumed father and his own father.

• A father passes on 50% of his DNA to his child.

• A grandchild inherits around 25% of their DNA from a grandfather.

This genetic proximity increases the risk of a false positive, meaning a result that appears positive when it should not be interpreted that way.

What to do to improve reliability

To reduce that risk, it is important to:

• tell the laboratory that the possible fathers are biologically related, so that additional analysis can be planned;

• include both possible fathers, where feasible;

• also include the biological mother’s DNA, which is strongly recommended.

Paternity doubt between brothers

If the alleged fathers are brothers, each has inherited:

• 50% of their DNA from their mother;

• and the other 50% from their father.

However, brothers do not necessarily inherit exactly the same DNA segments, because genetic transmission is random. In practice, two brothers share roughly:

• about 25% of their shared genes through their father;

• and about 25% through their mother.

The best testing configuration

To identify with certainty which of the two is the biological father, the best approach is to test both brothers, so the laboratory can determine which one shares the greatest number of relevant genetic markers with the child.

What if only one brother can take part?

If only one brother is available, or agrees to be tested, there is still a good chance of obtaining a precise result.

The mother’s participation is not mandatory, but it remains advisable because it makes it easier to interpret the child’s genetic profile. This is also why our paternity test page specifically recommends including the mother when the possible fathers come from the same family.

Paternity doubt between twins: identical or fraternal?

When the possible fathers are twins, the first step is to determine whether they are:

• identical twins;

• or fraternal twins.

Identical twins: a major limitation of paternity testing

Identical twins share the same genetic profile because they come from the same fertilised egg and received the same genes from their parents.

As a result, it becomes extremely difficult to identify the biological father, because the child’s DNA can appear compatible with both men.

In this very specific situation, a DNA paternity test may not establish paternity with certainty and reliability. If the doubt concerns two identical twins, a standard paternity analysis is generally not enough to distinguish them.

Fraternal twins: the same logic as for brothers

Fraternal twins have a genetic mix comparable to that of ordinary siblings. In other words, the same rules that apply to brothers also apply here.

The biological relationship can then be studied by comparing:

• the DNA of both twins;

• the child’s DNA;

• and the biological mother’s DNA.

If there is any doubt about twin type, a twin DNA test can clarify whether the twins are monozygotic or dizygotic before moving forward with a paternity analysis.

Asking for an extended DNA test: more markers, more precision

A standard paternity test is generally based on comparing the participants’ alleles across around 16 to 21 markers.

For the conclusion to be robust, the tested markers must show a fully coherent pattern between the father and the child. It is also important to remember that mutations can occasionally affect the comparison.

When the possible fathers belong to the same family, it is advisable to ask the laboratory to increase the number of genetic markers analysed. Expanding the number of compared alleles improves the precision of the result and helps the laboratory distinguish between closely related profiles.

How should you read the results of a DNA test?

The results are based on the comparison of the tested genetic markers and on the consistency of the observed matches.

To understand a report more easily, you can read our guide on how to interpret DNA test results.

How to do a paternity test in practice

1) Tell the laboratory if the possible fathers are related

Before anything else, if the possible fathers are from the same family, the laboratory must be informed. This allows it to anticipate possible interpretation issues and, where necessary, to plan additional analysis.

2) Order and carry out the test

A home DNA test can be ordered directly from a laboratory website, for example through a paternity test service.

Participants may complete the testing:

• together or separately;

• even if they are in different countries.

3) Collect and send the samples

Saliva collection material is usually sent by post and returned in the same way.

It is also possible to use non-standard samples such as hair, nails or a toothbrush. To understand which samples may be suitable, see our guide on which sample can be used for a DNA test.

4) Results turnaround time

• With buccal swabs for all participants: results are usually sent by email within 3 to 5 days.

• If one participant provides a non-standard sample: the turnaround is more likely to be 10 days or more.

5) Private test or legal procedure

The test may be requested:

• on a private basis within a family context;

• or for a legal reason, for example in judicial or administrative proceedings.

In England, a DNA test intended for court use must be carried out by an accredited laboratory, as explained in the official GOV.UK guidance on getting a DNA test for court.

Why a Y-chromosome DNA test does not work when the possible fathers are from the same family

When the doubt concerns men from the same paternal line, it is not possible to rely on a Y-chromosome DNA test.

The reason is simple: the Y chromosome is highly similar among men from the same paternal lineage. That means the test cannot distinguish between the alleged fathers in this situation. This limitation is also reflected in our Y-chromosome test explanation.

Conclusion

Yes, a DNA paternity test can often distinguish between two possible fathers from the same family, but the level of difficulty depends on the biological relationship involved: cousins, brothers, father and grandfather, or especially twins.

To maximise reliability, it is essential to tell the laboratory that the possible fathers are related, to consider testing both men whenever possible, and, where relevant, to request an extended analysis with more genetic markers. Lastly, choosing the right type of test, and avoiding Y-chromosome analysis in this context, helps produce a result that is far more useful and interpretable.

For the scientific basis of STR comparison and the distinction between monozygotic and dizygotic twins, Oxford University Hospitals also explains that these markers can distinguish individuals and that identical twins are expected to show identical profiles across all loci tested.