Paternity Test Mutations: What a Single Mismatch Really Means

A genetic mutation can make a biological father mismatch his child at one DNA marker, but it almost never changes the overall conclusion of a properly interpreted paternity test. Labs expect these rare events, and the standard practice is to look at the whole profile rather than any one marker. A result should be read as an exclusion only when several independent markers disagree, not when a single one does.

This guide explains what a mutation is in paternity testing, how often it happens, how it differs from a false negative caused by a sample or handling problem, and which steps make sense if your report shows one unexpected mismatch.

TL;DR

• A paternity mutation is a rare, spontaneous change in a repeat marker that is passed from parent to child, and it can produce a mismatch at one marker even when the man is the biological father.
• Labs do not normally conclude non-paternity from one mismatch. Published literature describes it as generally accepted that at least two independent exclusions are needed.
• Mutation rates differ by marker and are higher on the paternal side, so a single-marker discrepancy is uncommon but expected across many tests.
• A true false negative more often comes from sample mix-ups, contamination or a very close relative being the real father, which are separate problems from mutation.
• If your report shows one mismatch, ask how the lab interpreted it, and consider testing with additional markers rather than drawing a conclusion from that single line.

Table of Contents

• What a mutation means in a paternity test
• How often mutations occur
• How labs interpret a single mismatch
• Germline mutations, somatic mutations and false negatives
• Why more markers help
• Our perspective
• Steps to take if your result shows one mismatch
• Choosing a test that fits your question
• FAQ
• Sources

What a mutation means in a paternity test

Standard paternity tests compare short tandem repeats (STRs), stretches of DNA where a short sequence repeats a number of times. Each person inherits one version (an allele) of each marker from the mother and one from the father. At every marker, the child’s two alleles should be traceable to the two parents.

A mutation, in this context, is a spontaneous change that happens when egg or sperm cells form. The repeat count in the parent’s sperm or egg differs by a step or two from the parent’s own profile. The child then carries an allele that appears in neither parent’s recorded profile at that marker. The father has not done anything unusual and the test has not malfunctioned: the parent’s cheek-swab profile simply does not show the altered copy that was passed on.

The National Institute of Justice training material on mutation and paternity explains that a meiotic mutation can produce a discrepant result at a locus, and that analysts must account for this when interpreting a case. If you want a refresher on the markers themselves, our article on what autosomal STR markers are and why paternity tests use them covers the basics.

A simple illustration

This example is hypothetical and meant only to show the logic. Suppose a mother has the repeat pair 14 and 15 at one marker, and the alleged father has 12 and 16. The child shows 14 and 17. The 14 comes from the mother. The 17 matches neither of the father’s alleles, but it is one step away from his 16. That pattern, a change of a single repeat unit, is the most common form of STR mutation.

Now suppose the other markers on the report, perhaps up to 28 of them, all agree with the alleged father. A lab looking at that pattern would treat the single discrepancy as consistent with a mutation rather than as evidence the man is not the father.

How often mutations occur

Mutation at STR markers is real but rare for any single marker. The research summary we reviewed puts typical STR mutation rates between roughly 1 in 1,000 and 1 in 100 per marker per generation, depending on the marker. A paternity-testing study of commonly used forensic STR loci reports rates ranging from 0 to about 1.5 x 10^-2 per locus per generation, with several important patterns:

• Paternal mutations are more frequent than maternal ones, sometimes by a ratio as high as 6:1.
• Rates tend to rise as the repeat gets longer.
• Most mutations are single-step, meaning the repeat number changes by just one unit.

Another study, of 10,844 parent-to-child transfers at nine STR loci, found 23 isolated mismatches in cases where parenthood was otherwise highly validated (probability above 99.97%). That is a useful sense of scale: mismatches in true parent-child pairs happen, they are isolated, and the surrounding markers still support the relationship.

Because a modern panel tests many markers, the chance that a true father shows one mutation somewhere in the panel is small but not negligible. The chance that he shows mutations at two independent markers in the same child is far smaller, which is exactly why the two-marker convention exists.

Why the rate differs between markers

Markers are not equal. Some repeat sequences are stable, and others copy less faithfully. Longer repeat tracts and certain repeat structures are more prone to slippage. That is one reason laboratories track mutation rates marker by marker and apply those figures when calculating the statistical weight of a result, rather than treating every marker the same.

How labs interpret a single mismatch

Laboratories do not treat paternity as a yes-or-no checklist at each marker. They calculate a likelihood ratio, which compares how probable the observed DNA profiles are if the man is the father against how probable they are if an unrelated man from the population is the father. Mutation rates are built into that calculation, so one discrepancy lowers the number without necessarily overturning it.

The principle is well documented. A forensic science paper on paternity exclusion and paternal mutations examines how many loci would be expected to show exclusion because of mutation when a parent is otherwise the true parent. A separate case report from disputed-parentage work describes two apparently false exclusions at STR loci in a single child, and it opens by noting that at least two independent exclusions are generally required before concluding a man is not the father. That case is a reminder that unusual results can occur, and that careful follow-up testing exists for them.

What the lab may do next

When only one marker disagrees, a lab may take one or more of these steps:

1. Re-run the sample to rule out a reading or transcription error.
2. Test additional STR markers to see whether the discrepancy stands alone.
3. Use Y-STR markers when the question is about a paternal line, since those markers are inherited together from father to son. Our guide on Y-STR DNA testing and confirming the paternal line explains when that helps.
4. Report the result as consistent with paternity, with a probability calculated to account for the mutation, or as inconclusive if the evidence does not support either conclusion.

The Y-chromosome has its own mutation behavior. A direct-observation study of father and son pairs documents germline mutations at Y-chromosome microsatellites, so Y-STR results are also read with that in mind.

Germline mutations, somatic mutations and false negatives

These terms are often mixed together, so it helps to separate them.

The research summary we reviewed notes that somatic mutations can occasionally produce a third allele or other unexpected pattern in one tissue, and that this pattern may not appear in another tissue such as cheek cells. That is one more reason a swab-based test, taken carefully, is a sound approach, and a reason to recollect if a pattern looks strange.

Is a mutation the same as a false negative?

Not quite. A false negative in everyday language means the test said the man is not the father when he is. A single-marker mutation does not cause that outcome in a correctly interpreted report, because the conclusion rests on the full profile. A real false negative is more likely to trace back to one of these:

• A swab labeled for the wrong person, or samples swapped between participants.
• Contamination, for example when someone ate, drank or smoked shortly before the swab, or when swabs touched each other.
• A different man being the biological father, particularly a close relative of the tested man. We cover this in paternity testing when both possible fathers are related.
• Misreading the report, such as treating a single mismatch line as the final answer.

For a deeper checklist, see our article on false positive paternity results and five checks to verify a test, which applies in both directions: the same collection discipline reduces errors whichever way a result points.

Why more markers help

A larger panel gives a lab more independent evidence, which makes one mutation easier to identify and absorb. Our paternity kits test up to 28 markers. If one of them shows a discrepancy that fits a single-step mutation and the rest line up, the surrounding evidence carries the interpretation. With a small panel, the same discrepancy would be a larger share of the total evidence.

To see how panel size affects confidence, read 28 genetic markers on a paternity test and how many DNA markers a paternity test needs. For the reasoning behind high-confidence conclusions, why paternity testing is conclusive goes through the statistics.

Our perspective

We see the same worry repeatedly: someone opens a report, notices that one marker line does not match, and concludes the result is broken or that the answer must be a no. In our experience, the useful response is to slow down and ask what the rest of the profile shows. A mutation is a documented feature of repeat markers, and a well-designed process expects it.

We also think families deserve plain language here. If a mismatch appears, it should be explained as one data point within a larger pattern, and the next step should be proportionate: confirm the sample handling, look at the other markers, and consider additional testing if the picture is unclear. We do not encourage anyone to make decisions about a relationship, a child or a family from a single line on a report.

Our kits include a prepaid return shipping envelope, and our lab processing typically takes 2 to 3 business days, with a standard order-to-results timeline of 7 to 10 business days. We mention this because when a result raises a question, waiting is stressful, and knowing the timeline helps people plan a follow-up calmly. We do not offer court-admissible or legal testing at this time, so our tests are intended for personal knowledge and peace of mind.

— Dr. Todd Lewis

Steps to take if your result shows one mismatch

If you are unsure what a mismatch means for your report, consider this order of actions:

1. Read the full report. Check how many markers were tested and how many disagree. One disagreement among many agreeing markers has a very different meaning from several disagreements.
2. Check the collection details. Confirm that every swab was labeled with the right person, that no one ate or drank beforehand, and that the swabs were dried and sealed as instructed.
3. Ask the lab how it interpreted the discrepancy. A reputable lab can say whether the pattern fits a single-step mutation and how it affected the probability figure.
4. Consider additional testing. More markers, or a retest with freshly collected samples, can settle the question. Y-STR testing may help when the paternal line is the issue.
5. Consider other explanations. If more than one marker disagrees, think about sample errors or a close relative being the true father.
6. Speak with a genetic counselor or a qualified professional if the result affects medical decisions or family planning, since an unclear result may have consequences beyond the test itself.

Our article on when a DNA test cannot give you an answer describes the situations where testing reaches its limits and what alternatives exist.

Choosing a test that fits your question

If you are simply looking for clarity about whether a man is a child’s biological father, a home paternity kit is a direct route. Our Home Paternity Test Kit is 79 USD, and it uses a cheek swab you collect at home and return in the prepaid envelope. If the father is not available, other relationships can sometimes answer the question: the Home Grandparent DNA Test, Home Half Sibling DNA Test Kit and Home Aunt/Uncle DNA Test Kit are each 139 USD, and the Home Maternity Test Kit is 129 USD. If you are not sure which kit suits your family, our guided test finder walks through your situation, and the Next-Day Results option is available for 100 USD.

For general background on how these tests work, our complete guide to paternity testing is a good starting point, and our page on kinship DNA tests covers sibling and grandparent options. You may also find biological father confirmation useful if your question is about what a confirmation means.

One more point of clarity: the STR tests described here are different from genealogy tests, which look at broader patterns and answer different questions. Our comparison of STR and genealogy DNA testing explains why.

FAQ

Can a genetic mutation cause a false negative on a paternity test?

A mutation can cause a mismatch at a single marker, but a correctly interpreted report does not call non-paternity from that alone. The general convention in the literature is that at least two independent exclusions are needed. True false negatives are more often tied to sample mix-ups, contamination or a close relative being the actual father.

How common are mutations in paternity testing?

They are uncommon for any individual marker. Published STR mutation rates range from near zero up to about 1.5 x 10^-2 per marker per generation, depending on the marker, according to a study of forensic STR loci. Because a panel tests many markers, one isolated mismatch in a true parent-child pair is possible, while multiple independent mismatches are far less likely.

Do mutations come more often from the father or the mother?

Paternal germline mutations occur more frequently than maternal ones, sometimes at a ratio as high as 6:1, as reported in the same STR mutation study. This is one reason a single mismatch between an alleged father and child is a recognized pattern that labs plan for.

What should I do if my paternity test shows one unmatched marker?

Start by reading the whole report and counting how many markers agree. Then check how the samples were collected, ask the lab how it interpreted the discrepancy, and consider testing additional markers or recollecting samples. If the result affects medical or family decisions, a genetic counselor can help you interpret it.

Sources

• Paternity exclusion by DNA markers: effects of paternal mutations (PubMed)
• Two apparently false exclusions of paternity at STR loci in one child (PubMed)
• Mutation and paternity, National Institute of Justice
• Mutation rate at commonly used forensic STR loci: paternity testing experience (PMC)
• Mutation rate in human microsatellites: influence of structure and length of the tandem repeat (PMC)
• Germline mutations at Y-chromosome microsatellite loci in father/son pairs (PMC)

Recommended

• Why Probability of Paternity Never Reaches 100%
• Positive Paternity Test Results: What They Mean & Next Steps
• Paternity Test When Possible Fathers Are Related (2026)
• How to Read Paternity Test Results: DNA Report Explained