What DNA Paternity Testing Can (and Can’t) Do for You

A student recently asked us, ‘What are some advantages and disadvantages for using DNA profiling for paternity testing?’ An excellent question! DNA testing (or ‘DNA profiling’) is universally accepted as the best way to answer paternity questions. But there are some cases where DNA testing can have limitations.

On the list of DNA testing advantages, we’d include genetic fingerprinting (extremely high accuracy), a high power of exclusion, and double-blind confirmation. For our limitations list: partial DNA profiles, closely-related possible fathers (or identical twins), and medical transplants or transfusions. Another limitation might be identification (verifying the ID of each person in the test), although using a chain-of-custody process really turns this into a DNA testing advantage.

DNA Paternity Test Advantages

Let’s take a look at the ‘advantages’ of DNA testing in more detail and learn a bit about the special cases and conditions that limit the usefulness of DNA data. Along the way, we’ll learn a little bit about how DNA paternity testing works at the HomeDNA Paternity lab and why you can trust HomeDNA Paternity to accurately answer your paternity questions.

Genetic Fingerprinting (extremely-high accuracy)

Perhaps the greatest advantage of DNA profiling is its extreme accuracy. Each person in the test – a child, possible father (and mother if she choose to participate) – collects a DNA sample using a simple, painless cheek swab (no blood or needles necessary – definitely another advantage!). Once your swabs arrive at our lab, we extract a single DNA profile from each swab with total accuracy.

What about contamination? There’s no chance that a chemical or speck of dust or other form of contamination can change your DNA profile and somehow make it inaccurate. Our lab methods extract very specific parts of human DNA from the swab. Anything else – dust, chemicals, pet DNA, etc. – is literally washed away in the process. If something does damage a swab sample, we won’t find different DNA; in fact, we won’t find any DNA at all or the sample will be considered too degraded to test. In this case, we simply ask for a new sample (we even pay for  shipping and handling of fresh swabs).

Sometimes one person touches or uses the swab and the same swab gets touched or used by another person in the test. We can easily see if more than one person has used the same swab – we call it a ‘mixed’ DNA profile. If this happens, we again ask for new swabs.

This means that the DNA profile on a paternity test report is accurate – it’s the one and only profile that came from the uncontaminated swab we tested. And it’s unique to you. We call it your ‘genetic fingerprint’ and, once we have it, we can use it to test for paternity.

The Power of Exclusion

Using these completely accurate genetic fingerprints, we then look for a match between the possible father and the child. In general, the possible father and child must match at every location (each genetic system we test).  If a possible father does not match the child, then we can state with certainty he is not the biological father of the child.

Side note: We sometimes find naturally-occurring mutations when comparing related DNA profiles. This means that there can be small differences in the DNA profiles between a man who really is the biological father and his child. How can we tell the difference? Trained molecular geneticists carefully review each case. We may also ask for the mother’s sample in these situations to help confirm test results. 

Double-Blind Testing

HomeDNA Paternity tests are done twice before we say that a man is NOT the father of a child. This means we take two sets of swabs and send them through the lab separately (using bar-code serial numbers and not names). Two teams of scientists review the data  – and both have to get exactly the same result before we issue a report. This double-blind process is just one of many controls we use to ensure that paternity reports are completely accurate.

DNA Testing Limitations

When a man is excluded (he is not the father), we know this with absolute certainty. That’s right. We can be confident that a man is NOT the father when his DNA profile doesn’t match the child’s profile. But, when he can’t be excluded, how confident are we that the tested man is the father?

Partial DNA Profiles

Using genetic frequencies and statistics, we can identify a likely father by using as few as six (6) genetic systems, though on most reports you will see somewhere between 15 and 17 genetic systems listed. When the profiles are compatible, we can calculate the probability of paternity. It’s usually something like 99.99%.

Why not a 100% Probability of Paternity?

Each person has literally millions of genetic markers. But, to test all of them would be much more costly and time-consuming (imagine a paternity test that takes weeks or months and costs more than $10,000!). Using about 15 genetic markers gives a very high (and very trustworthy) probability. But it’s theoretically possible that the tested man is not really the biological father, though this is extremely unlikely. In fact, most paternity reports show the odds (a combined parentage index, or CPI) as something like 10,000 or more to 1.

With less than a 100% probability, how can we trust a paternity test? Think of it like fingerprints. Most people would say that each human on earth has a unique fingerprint. But, do we really know that? Is there a database somewhere that holds all fingerprints for all people on planet Earth-living and dead? Of course not. So, even though it’s theoretically possible that two people somewhere, somehow, someday might share the exact same fingerprint, we haven’t seen it yet. And, detectives and forensic scientists have enough experience with fingerprints (and genetic fingerprints) to know that the odds of this happening are very, very, very unlikely.

Side note: even if two people did share the same fingerprints, or the same DNA profile, how likely is it that they would also be at the same place and time where they could be suspected of committing the same crime or fathering the same child? This scenario is virtually impossible!

Closely-Related Possible Fathers

Of course, if two possible fathers are related (they might be cousins, or brothers, or father and son), it’s more likely that their DNA profiles could match with a child who is not their child. The child might be their grandchild, niece, nephew, or cousin. It’s very important for the lab to know if two possible fathers are related. The best option is to test both related fathers. The lab can tell which man is the biological father (and which is the uncle, grandfather, or cousin). Even if only one of the fathers is available, the lab can use more testing (look at more genetic systems) to identify the most likely father with the same high probability (99.99%) as any other paternity test.

Identical Twins

Identical twins share the same DNA profile (after all, they’re identical!). If both twins are possible fathers, DNA testing cannot identify which one is most likely to be the father (though DNA testing can exclude BOTH men if they don’t match the child). Unfortunately, there currently is no other type of paternity test available that can help identical twins in this relatively rare situation.

Blood Transfusions and Bone-marrow Transplants

What if a possible father or child had a recent blood transfusion? Could that change their DNA? The short answer is no, a person’s DNA does not change-ever. However, for a short time after a transfusion, a person is literally carrying around TWO sets of DNA: their own DNA and the donor’s DNA. In these cases the lab might pick up a mixed profile (two sets of DNA) and ask the patient to wait at least three (3) months before testing. After a few weeks or months, the donor’s DNA gets used up (and leaves the body) and the possible father can participate in a paternity test using his (and only his) DNA. The same basic rules apply for a bone-marrow transplant recipient.

Identification

Knowing that the science works, and assuming there isn’t a special case like identical twins, recent blood transfusion or bone-marrow transplant, there’s really only one question left: How can we be sure that the DNA sample we get at the lab really came from the person whose name is on the envelope? This is a very important question, and the answer is worth reading about.

For personal or peace-of-mind paternity tests, we can’t be certain that the swabs we get really come from the people whose names are on the envelopes. We have to take their word for it. In cases where the participants trust each other to be completely honest, this process gives convenient, quick, and true answers to paternity questions. If there’s any chance that someone might be tempted to cheat and send in someone else’s swab as their own, we recommend one of two options.

Option 1: Test together. Make sure everyone takes the test together so they can watch each other fill out the envelopes and swab their cheeks. Then, go to the post office or secure U.S. mailbox together to send the samples to the lab. (Don’t let one person handle the mailing – they could do something dishonest with the samples or paperwork!)

Option 2: Upgrade to a legal paternity test. When using a DNA paternity test for any legal purpose, such as child support or child custody or immigration cases, courts require this option. (It’s also a great option to provide peace-of-mind about each person’s identity even if you don’t plan to take your results to court.)

DDC schedules a DNA collection appointment where a third-party person (a Collector) verifies each person using a government-issued photo ID and/or by taking a picture. The Collector uses a Chain-of-Custody document and ships the samples directly to the lab – the people being tested are not allowed to handle the other swabs or paperwork or to mail the samples themselves. This Chain-of-Custody process  documents each person’s identity and provides a completely accurate and reliable paternity test report. Please keep in mind that the legal process requires an additional fee.

The Bottom Line

With DNA testing (profiling), we can answer paternity questions more accurately, reliably, and affordably than ever before. The science and technology make DNA testing the gold standard preferred by courts, governments, and immigration agencies for paternity questions. Even though there are some limitations, the reality is that DNA testing works-and works very well-for just about anyone.

We understand how important and life-changing a paternity test can be. That’s why our nationally- and internationally-accredited laboratory undergoes routine inspections just to make sure the technology is working perfectly. And that’s why our expert staff, including friendly Client Support Representatives  and highly-trained geneticists and molecular biologists, is available and happy to answer your questions and help with your specific case.