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When it comes to determining the biological father of a child, DNA paternity tests are considered highly reliable. But what happens when two potential fathers are closely related such as brothers or cousins? Can a standard paternity test still provide a conclusive answer ?
This article explains how paternity testing works in cases involving men from the same family, and what factors can affect the accuracy of the results.
How Does a Paternity Test Work ?
A paternity test compares the DNA profile of the child with that of the alleged father. Since a child inherits half of their DNA from each biological parent, the father should share around 50% of specific genetic markers with the child.
These markers are located at precise positions on the chromosomes and are unique for each individual, except in the case of identical twins.
When the tested man shares enough of these genetic markers with the child, paternity is confirmed. When there are too many differences, the man is excluded as the biological father.
What If the Alleged Fathers Are Related ?
The situation becomes more complex when two or more potential fathers are biologically related for example, brothers, cousins, or even a father and son. Since they share parts of their DNA, this can lead to false positives or inconclusive results if the laboratory is not informed in advance.
- Brothers share about 50% of their DNA.
- Cousins share less, around 12.5%.
- A father and his son share 50%, and the son will pass 25% of that DNA to his own children.
Because of this shared genetic background, the test may detect a high percentage of similarity with either alleged father, making it harder to determine which one is truly the biological parent.
What Can Be Done in These Cases ?
Despite the difficulty, DNA testing can distinguish between relatives, if the laboratory uses extended analysis.
To improve accuracy:
- Test all potential fathers if possible. Comparing both men’s DNA with the child’s allows the lab to see which one matches more closely.
- Inform the lab of any known biological relationship between alleged fathers. This allows the lab to use additional markers (often 24–35 or more) for a more precise comparison.
- Include the mother’s DNA. While not mandatory, it helps the lab identify which parts of the child’s DNA came from the mother allowing them to focus solely on the paternal contribution.
Fraternal Twins vs Identical Twins
Fraternal twins (non-identical) are like any other siblings and share around 50% of their DNA. In this case, testing both individuals is still effective though more markers may be needed.
However, in the case of identical twins, DNA testing cannot distinguish between them. They share 100% of the same DNA, which makes it scientifically impossible with current technology to determine which twin is the biological father through standard genetic testing.
The Risk of False Positives
In cases where alleged fathers are closely related, there is a greater risk of a false positive where the test mistakenly identifies the wrong man as the father due to the shared genetic traits.
This is why it’s essential to:
- Provide the lab with full information.
- Choose a laboratory experienced with complex family cases.
- Consider legal-grade DNA testing for added reliability in court-related matters.
Paternity testing is a powerful tool, but it becomes more complex when the potential fathers are members of the same family. Brothers, cousins, and even father-son cases require additional care and advanced analysis.
Fortunately, with proper information and expert laboratories, it is still possible to get accurate and conclusive results even in challenging family structures.
If you are considering a paternity test involving related individuals, be transparent with your testing provider and ask whether extended marker analysis is available. It could make all the difference in finding the truth.
