How Many DNA Markers Are Tested in a Paternity Test (And Why It Matters)

Not all paternity tests are created equal, and one of the most important differences between providers is the number of DNA markers they analyze. The marker count directly affects the statistical power of the test, which in turn determines how conclusive the result is. If you are comparing paternity testing services, understanding what markers are, how they work, and why quantity matters will help you choose a test that delivers a result you can rely on without reservation.

What Are STR Markers?

The markers used in paternity testing are called short tandem repeats, or STRs. An STR is a stretch of DNA where a short sequence, typically two to six nucleotides long, repeats in a row. For example, the sequence AGAT might repeat 8 times at a particular location on one chromosome and 12 times at the same location on the other chromosome. The number of repeats is what varies between individuals, and this variation is what makes STRs useful for identification and relationship testing. At each STR locus, you inherit one allele (one repeat count) from your biological mother and one from your biological father. By comparing the alleles at multiple loci between a child and an alleged father, the lab determines whether the inheritance pattern is consistent with a biological parent-child relationship.

The Evolution from 13 to 20+ Markers

In the early days of forensic and paternity DNA testing, the standard panel included 13 STR loci, known as the CODIS core loci in the United States. This panel was sufficient for most cases but occasionally produced ambiguous results, particularly in cases involving close relatives of the alleged father, populations with reduced genetic diversity, or situations where the mother's sample was unavailable. Starting around 2017, the FBI expanded the CODIS core to 20 loci, and leading paternity testing laboratories followed suit by adopting commercial kits that amplify 21 to 24 or more markers simultaneously. The Promega PowerPlex Fusion System analyzes 24 loci, while the Thermo Fisher GlobalFiler kit targets 24 loci as well, including the amelogenin sex-determination marker. Some advanced panels go even further, testing 27 or more STR markers for maximum discriminatory power.

Why More Markers Produce Better Results

The statistical strength of a paternity test increases multiplicatively with each additional marker. At each locus, the lab calculates an individual paternity index (PI) based on how common the shared allele is in the general population. If the child and alleged father share a rare allele at a given locus, the PI for that marker will be high. If they share a common allele, the PI will be lower. The combined paternity index (CPI) is the product of all individual PIs across every tested locus. With 13 markers, a typical inclusion produces a CPI translating to a probability of paternity around 99.9% to 99.99%. With 20 or more markers, the probability routinely exceeds 99.9999%, making a false inclusion virtually impossible. The difference matters most in edge cases: when participants share ethnic background, when the true father might be a close relative of the tested man, or when the mother is not available for testing. Extra markers provide the statistical cushion that eliminates ambiguity in these challenging scenarios.

What Happens When Markers Do Not Match

If the alleged father is not the biological father, mismatches will appear across multiple loci. With a 13-marker panel, exclusion is typically definitive when three or more loci show mismatches, because a single mismatch could theoretically result from a rare mutation. With a 20-plus marker panel, the lab expects to see mismatches at five or more loci in a true exclusion, and mutations at a single locus can be identified and accounted for statistically rather than creating doubt. Labs report what is called the "number of exclusionary loci," and a higher marker count gives the analyst more data points to distinguish a genuine mutation from a true exclusion. This is why the AABB accreditation standards now effectively require 20 or more markers for relationship testing.

Comparing Testing Providers: What to Look For

When shopping for a paternity test, ask how many STR loci the provider tests. Any accredited lab should be testing at least 20 markers. Be cautious of budget providers that still use legacy 13-locus or 16-locus panels. The difference in laboratory cost between running 16 markers and 24 markers is minimal because modern multiplex PCR kits amplify all loci in a single reaction, so there is no legitimate reason for a reputable lab to offer a reduced panel. Also check whether the lab includes the amelogenin locus for sex verification, which serves as a built-in quality-control check confirming that the samples labeled as male and female produce the expected XX or XY result.

Starting with an Affordable Preliminary Assessment

Before committing to a full STR-based laboratory test, many people choose to start with the TrueDadz AI facial recognition assessment. At $14.99, it uses machine learning to compare inherited facial features between the child and the alleged father, providing a resemblance probability score within minutes. It operates on a fundamentally different principle than genetic marker analysis, examining phenotypic similarity rather than genotypic data, but it offers a fast, affordable, and private data point that can inform your decision about whether to proceed with formal DNA testing. If you do move forward with a lab test, prioritize a provider that tests 20 or more STR markers with full AABB accreditation.

The Bottom Line on Marker Count

The number of DNA markers tested is not a marketing gimmick. It is the single most important technical specification that determines the statistical power of your paternity test. A 20-plus marker panel ensures that inclusions are supported by overwhelming statistical evidence, exclusions are clear and unambiguous, and edge cases involving close relatives or absent mothers are handled without compromising certainty. When the answer matters this much, the science behind it should leave no room for doubt.

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