Main contributor: Richard Hill
Photo of a man
A father.

When a child is born, there is rarely any question about the mother’s identity as the birth is typically witnessed and recorded by medical personnel. The father, on the other hand, played his role many months earlier. Weeks pass before pregnancy is observed and the identity of the biological father may be uncertain. Paternity tests compare the DNA of a child with the DNA of a possible father to confirm or deny fatherhood.

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Short Tandem Repeats

The genetic markers used in a paternity test are specific chromosome locations that contain a repeating pattern of a DNA sequence. These are known as a Short Tandem Repeats (STRs). The paternity test counts the number of times the sequence repeats at each marker. Paternity tests typically look at CODIS markers and sometimes more. CODIS (Combined DNA Index System) is a set of 20 DNA markers used in forensic DNA analysis. Each DNA marker has two alleles (one from each parent). This is a pair of numbers such as (8, 11), (13, 13), or (15, 23).

Required values

STR markers work well for paternity testing because the close parent-child relationship has required values. For example, if a child has the alleles 8 and 11 at a given marker, she must have received the 8 from one parent and the 11 from the other. If the man does not have one of the same alleles as the child on every tested marker, he will be excluded as the father. Conversely, if there is a match on every tested marker, the probability of the tested man being the biological father of the child is 99.99% or higher. The probability cannot be 100%, since every man in the world is not being tested. But such a high probability eliminates virtually all other men except for an identical twin.

Testing the mother

Testing the mother is not necessary. Yet testing the mother can make this easier since her contribution to each pair of alleles will then be known. In the case of the child with the numbers 8 and 11 the father could not be eliminated if he had either one of those values. Knowing the mother contributed the 8 means that the father must have an 11.

Legal Paternity Tests

physician
Paternity tests are conducted by a professional.

While at-home paternity tests can provide personal peace of mind, they are not legally acceptable as proof. That’s because a court cannot know for certain who provided the sample. Certain legal situations involving custody, child support, inheritance or immigration require proof of fatherhood. In such cases you need a legal paternity test.

Such testing follows a strict protocol to ensure the results are admissible in court. A neutral third party, such as a healthcare professional or court-approved collection facility will collect the samples; the identities of all parties are verified, and the DNA samples follow a documented chain of custody.

Kinship testing

Photo of siblings
Some companies offer "sibling tests."

Most companies that sell paternity tests also offer what they call kinship or sibling tests. Using the same STR technology they attempt to confirm other relationships such as siblings and grandparents. For relationships more distant than parent and child, however, there are no required values. For example, let’s look at one marker: let’s say the mother has alleles 5 and 13. The father has alleles 9 and 12. One child could be 5, 12 and another child 9, 13. They may not match at all on any given marker.

Test labs can only rely on statistics and calculate the probability of a certain relationship. As a result, kinship test results are often inconclusive and sometimes wrong. A better option in such cases is autosomal DNA testing. These tests check more than a half million DNA locations and measure how much DNA two people have in common. This can confirm any relationship out to a second cousin.

Explore more about Paternity tests

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Contributors

Main contributor: Richard Hill
Additional contributor: Maor Malul