Can you test Paternity when the possible fathers are related?

Casey Randall Alphabiolabs

By Casey Randall, Head of Genetics at AlphaBiolabs
Last reviewed: 11/10/2023

In this article, we look at how paternity testing is done when two or more potential fathers are related to each other.

What is paternity testing?

A paternity test is a type of DNA relationship test that is used to verify a biological relationship between a child and their potential father.

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To perform the test, DNA samples are collected from the child and any potential fathers using cheek (buccal) swabs: a simple, pain-free method of sample collection that means the test can be performed at any time on a person of any age, including new-born babies and grown-up children/adults.

These samples can then be analyzed and compared in the laboratory to identify matching DNA markers (loci) in the DNA samples.

AlphaBiolabs examines short tandem repeat (STR) markers that are specific locations on chromosomes made up of sequences of repeated DNA. These STR markers are highly variable in length between individuals. Each individual has two copies of each STR marker, known as alleles: one is inherited from the father and the other from the mother. By looking at these specific DNA markers in each sample, it is possible to identify which half of the child’s DNA is inherited from the mother and which half is from the father.

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When testing the biological father of the child, both the man and the child will share DNA at every tested DNA marker to show that they are parent and child.

If they have more than 3 markers that do not match, the man will be excluded as the biological father.

How does a DNA paternity test work?

Our peace of mind paternity tests are performed in the same way, whether you want to verify the paternity of an adult, or of a baby who has recently been born.

Paternity testing works by comparing a child’s DNA profile with that of the potential father(s).

An AlphaBiolabs paternity test looks at up to 45 DNA markers (loci) to identify which half of the child’s DNA is inherited from the potential father.

Every person inherits half of their DNA from each of their biological parents, so a comparison can help us to establish the probability of paternity.

When testing the biological father of the child, both will share identical sections of DNA at each marker. When the man tested is not the biological father, the man and child will not share enough DNA to verify a relationship.

Can more than one possible father be tested?

Yes! The AlphaBiolabs paternity test can be performed with more than one potential father. The standard testing kit provides enough swabs to test one child, one potential father and the mother (if available). However, extra swabs to test up to five potential fathers in total can be requested when ordering the kit. Swabs to test additional children can also be added.

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It is still possible to get an accurate result from paternity testing when two possible fathers are related, even if they are brothers, cousins or father & son.

Depending on the relationship, the amount of DNA shared with a relative will vary as shown in the table below:

Relationship Average % DNA shared
Identical twin 100%
Parent/child, full siblings, fraternal twin 50%
Grandparent/grandchild, aunt/uncle and niece/nephew, half siblings 25%
First cousin 12.5%

If the two potential fathers are relatives then they will share some DNA – up to 50% in the case of full siblings and fraternal twins. However, this also means that around 50% of their DNA is different, which allows our expert scientists to still establish the probability of paternity. The father will share identical sections of DNA at each marker with the child, whereas the relative who is not the biological father will not share enough DNA with the child to verify a relationship.

The only situation where this is difficult is if the potential fathers are identical twins, as they share near-identical DNA profiles (see Can a paternity test differentiate between twins?).

You will need to tell us when ordering your test if more than one potential father is being tested and whether they are close relatives of each other.

Can a paternity test between brothers determine which one is the father?

It is possible to carry out DNA testing on brothers to establish paternity and it will produce accurate results. This is because the amount of DNA that brothers share depends on their relationship to each other, and whether they have one or both biological parents in common.

Full siblings who have the same biological mother and father are known as first-degree relatives and share approximately 50% of their DNA.

Half siblings who have one biological parent in common – their mother or father – are known as second-degree relatives and share approximately 25% of their DNA.

In a paternity test between brothers, these differences in their DNA mean our expert scientists can distinguish which brother is the father of the child being tested.  However, in these cases, we would always recommend that the mother is also included in the testing process wherever possible.

The only situation where this is difficult, is if the potential fathers are identical twins.  This is because they share near-identical DNA profiles (see Can a paternity test differentiate between twins?).

You will need to tell us when ordering your test if more than one potential father is being tested and whether they are close relatives of each other.

Can a paternity test differentiate between twins?

There are two types of twins – non-identical (fraternal) and identical – and which type they are can result in significant differences when taking a paternity test due to their DNA. A paternity test can differentiate between non-identical (fraternal) twins, but not between identical twins.  Whether twins are identical or fraternal depends on how the babies are formed in the womb.

Non-Identical twins

Fraternal (non-identical) twins are known as dizygotic. These twins are the result of two separate eggs, fertilized by two separate sperms.

This means that non-identical twins are no more alike than siblings from different births, and may be the same sex, or one male and one female. As is the case with brothers from separate pregnancies, fraternal twins only have around 50 per cent of their DNA in common with one another.

This means that they also have around 50% of DNA that is different to each other, meaning we can test and accurately determine the probability of paternity between non-identical twins.

Identical twins

Identical twins are known as monozygotic. This means that they are formed from a single fertilized egg which went on to split into two separate embryos.

Identical twins therefore have the same DNA, meaning they share the same genetic makeup and almost identical DNA profiles.

These genetic similarities explain why identical twins look alike, and why they usually have the same characteristics, abilities, and behaviors. A paternity test would be able to confirm whether or not one of the identical twins was the biological father of the child, but it would not be able to distinguish which twin it was.

You will need to tell us when ordering a paternity test if more than one potential father is being tested and they are twins.

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Will a paternity test work if the possible fathers are father & son themselves?

It is possible to carry out DNA testing where the two potential dads are father and son themselves, and paternity can still be established accurately.  This is because the potential fathers will only share 50% of their DNA (the DNA that the father has passed on to his son).

This means that they will also have 50% of their DNA that isn’t the same. In a paternity test, these differences in their DNA mean our expert scientists can distinguish which relative is the father of the child being tested.

You will need to tell us when ordering your test if more than one potential father is being tested and whether they are close relatives of each other.

What about if the possible fathers are cousins?

Potential fathers who are cousins can undergo DNA testing to establish paternity with accurate results. This is because the amount of DNA that first cousins share is only around 12.5%.

This means that a substantial proportion of their DNA is not the same so, in a paternity test between cousins, these differences can be used to distinguish which relative is the father of the child being tested.

You will still need to tell us when ordering your test if the potential fathers are relatives of each other.

What can be done to get more conclusive results?

An AlphaBiolabs paternity test is 100% accurate based on the samples received into the laboratory.

Where there is a complicated DNA relationship, such as two or more potential fathers being relatives themselves, it is better to test samples from all the relevant people.  The laboratory may request that the biological mother’s DNA sample is also submitted for testing, if possible.

Please speak to one of our friendly and discreet Customer Services team on 727-325-2902 (or email info@alphabiolabsusa.com) to discuss the best options for your needs.

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You can order your Paternity Test online now for just $109, with results in 3 business days.

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Casey Randall AlphaBiolabs

Casey Randall

Head of Genetics at AlphaBiolabs
Casey joined AlphaBiolabs in 2012 and heads up both the Genetics and Health testing teams. An expert in DNA analysis and a member of the International Society for Forensic Genetics (ISFG), Casey holds an MS degree in DNA Profiling and a BS degree in Forensic Science. Casey is responsible for maintaining the highest quality testing standards, as well as looking for ways to further enhance the service that AlphaBiolabs provides and exploring new and innovative techniques in DNA analysis.

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