What Autosomal STR Markers Actually Measure in a Paternity Test

Frequently Asked Questions

What are autosomal STR markers?

Autosomal STR (Short Tandem Repeat) markers are specific locations on non-sex chromosomes where a short DNA sequence repeats back-to-back a variable number of times. Because the repeat count differs between individuals and is inherited from biological parents, these markers serve as highly precise identifiers for establishing biological relationships.

How many STR markers does USDC analyze in a paternity test?

USDC's laboratory analyzes up to 28 genetic markers per case. The standard panel covers 23 autosomal STR markers. If an initial result returns an inconclusive Combined Relationship Index, the lab can expand the panel by up to 5 additional markers to reach a definitive conclusion.

What does a 99.9% probability of paternity actually mean?

A probability of paternity at or above 99.9% means the statistical calculation, expressed as a Combined Paternity Index, is so strongly in favor of the tested man being the biological father that scientists consider exclusion essentially impossible. A 0% result means the man is definitively excluded as the biological father.

How long does a home paternity test take to process?

Once USDC's laboratory receives your samples, standard processing takes 2 to 3 business days in the lab. Total turnaround from the time you mail your kit is typically 7 to 10 business days, accounting for shipping time. Faster result options are available during checkout.

Is an at-home paternity test result court-admissible?

Home collection kits are designed for personal knowledge. Court-admissible (legal) paternity testing requires a witnessed sample collection and a documented chain of custody, which is a separate process. The underlying lab science, including STR marker analysis, is the same as court-admissible testing.

Can a paternity test produce an inconclusive result?

Yes, though it is uncommon with a multi-marker panel. An inconclusive result occurs when the Combined Relationship Index falls between 0.1 and 10, a range that does not strongly support or exclude paternity. USDC's lab addresses this by extending the panel up to 28 total markers to resolve the ambiguity.

You swab the inside of your cheek, seal the collection envelope, and drop it in the mail. A week or so later, a report lands in your inbox telling you the probability of paternity is 99.9999%. But what is the lab actually doing with those cells? The answer lives inside a class of genetic landmarks called autosomal STR markers, and understanding them turns a mysterious number into a genuinely meaningful scientific conclusion.

The Building Blocks: What "STR" Means

DNA is a long molecule made of four chemical bases arranged in a sequence. Most of that sequence is identical from person to person. The parts that differ, the places where individual variation lives, are what forensic and clinical geneticists care about. Short Tandem Repeats are one of the most useful categories of that variation.

An STR is a motif, typically two to six base pairs long, that repeats consecutively at a fixed location (called a locus) on a chromosome. One person might have the sequence AGAT repeated 11 times at a given locus. Another person might have it repeated 14 times. Neither is abnormal. The repeat count is simply inherited, like eye color, from biological parents.

Because these repeat counts vary widely across the population and are reliably passed from parent to child, they make exceptional biological fingerprints. The National Institute of Standards and Technology (NIST) maintains a comprehensive STR database that underpins population-frequency calculations used in labs worldwide.

Why "Autosomal" Is the Key Word

Humans have 23 pairs of chromosomes. One pair determines biological sex (the X and Y chromosomes). The other 22 pairs are called autosomes. Autosomal STR testing draws its markers exclusively from those 22 non-sex chromosome pairs.

This matters for paternity testing for a practical reason: every child inherits one copy of each autosomal chromosome from the biological mother and one copy from the biological father. At any given STR locus, a child therefore carries exactly two alleles (one per chromosome), one traceable to the mother's side and one to the father's side.

When a lab compares a child's STR profile to an alleged father's STR profile, it checks whether the child's paternal allele at each locus could have come from that man. A single locus match is interesting. Consistent matches across 20, 23, or up to 28 markers become statistically overwhelming.

How Many Markers Are Enough?

The short answer is: more is always better, up to a point of statistical certainty. The DNA community has converged on a minimum standard of 20 or more autosomal STR markers for relationship testing. USDC's laboratory goes further, analyzing up to 28 genetic markers per paternity case.

Here is why the number matters. Each additional locus examined multiplies the statistical power of the result. At 10 loci, you might achieve a Combined Paternity Index of a few hundred. At 20 or more loci, indices routinely reach millions or billions to one, pushing the probability of paternity above 99.9999%. At up to 28 markers, the result is essentially beyond scientific dispute in the vast majority of cases.

The National Institutes of Health (NIH) has published extensive research on STR marker discrimination power, confirming that expanded panels dramatically reduce the already small probability of a coincidental match between an unrelated man and a child.

Inside the Lab: From Swab to STR Profile

DNA Extraction

The buccal (cheek) cells collected during your swab contain nucleated cells loaded with genomic DNA. Lab technicians break open those cells using chemical reagents and isolate the DNA strands, removing proteins, lipids, and other cellular material.

Amplification

The extracted DNA is copied at target STR loci using a process called polymerase chain reaction (PCR). PCR selectively duplicates only the regions of interest millions of times over, producing enough material to detect and measure accurately even from a very small starting sample.

Capillary Electrophoresis and Genotyping

The amplified fragments are separated by size through a thin capillary tube filled with a polymer. Shorter fragments travel faster, longer fragments slower. A laser reads the fragments as they pass a detection window, and software translates the resulting peaks into allele calls: specific repeat counts at each locus. The output is an STR profile, a string of number pairs (one allele from each chromosome) across all analyzed loci.

Reading the Math: CPI and Probability of Paternity

Once the lab has profiles for the child and the alleged father, statisticians calculate a Combined Paternity Index (CPI). At each locus, a Paternity Index (PI) is calculated by dividing the probability of observing the child's allele combination if the tested man is the father by the probability of observing it if a random unrelated man from the same ethnic population is the father. The PIs from all loci are multiplied together to produce the CPI.

The CPI is then converted to a Probability of Paternity using a formula established by the forensic genetics community. A CPI of 100,000 (100 thousand to one) translates to roughly 99.999% probability of paternity. A CPI of zero, meaning the alleged father cannot possibly have contributed the observed alleles, yields a 0% probability, which is a definitive exclusion.

Population frequency databases, validated against large reference samples, supply the denominator in each PI calculation. This is why a paternity report specifies the ethnic reference population used: allele frequencies differ slightly across populations, and accurate science requires matching the calculation to the appropriate group.

What Happens When a Result Is Inconclusive

Occasionally, a standard 23-marker panel returns a CPI between roughly 0.1 and 10, a range that neither strongly includes nor excludes the tested man. This is uncommon but not impossible, especially in cases involving close relatives or unusual allele combinations.

When USDC's lab encounters an inconclusive Combined Relationship Index in this range, technicians extend the panel, adding up to 5 additional markers, for a total of up to 28 genetic markers. In the great majority of cases, this expanded analysis resolves the ambiguity and produces a clear inclusion or exclusion. If you are curious about how kinship cases with similar complexity are handled, see our overview at /blogs/news/what-is-a-relationship-dna-test-your-clear-guide.

At-Home Testing: Same Science, Personal Purpose

USDC's home paternity kit uses the same autosomal STR methodology, the same loci, and the same statistical framework as tests performed in any accredited forensic or clinical DNA laboratory. The underlying lab science is the same as court-admissible testing.

The distinction lies in collection, not analysis. Home kits are designed for personal knowledge: confirming or ruling out a biological relationship for your own peace of mind, family planning, or medical history purposes. Because home collection is not witnessed by a neutral third party and does not follow a documented chain of custody, results from a home kit cannot be submitted as legal evidence. Court and legal proceedings require a separate witnessed collection process.

Understanding this distinction helps you choose the right path from the start. For a deeper look at how the paternity testing process works end to end, visit our complete paternity testing guide.

Turnaround: From Sample Receipt to Your Report

Once your kit arrives at the laboratory, standard processing takes 2 to 3 business days. Factoring in the time your samples spend in transit, most customers receive their results within 7 to 10 business days of mailing. If you need results sooner, expedited processing options are available during checkout.

The AABB (formerly the American Association of Blood Banks) publishes standards for relationship testing laboratories, including quality and turnaround guidelines, which help consumers evaluate what to expect from any DNA testing provider.

Frequently Asked Questions

What are autosomal STR markers?

Autosomal STR (Short Tandem Repeat) markers are specific locations on non-sex chromosomes where a short DNA sequence repeats back-to-back a variable number of times. Because the repeat count differs between individuals and is inherited from biological parents, these markers serve as highly precise identifiers for establishing biological relationships.

How many STR markers does USDC analyze in a paternity test?

USDC's laboratory analyzes up to 28 genetic markers per case. The standard panel covers 23 autosomal STR markers. If an initial result returns an inconclusive Combined Relationship Index, the lab can expand the panel by up to 5 additional markers to reach a definitive conclusion.

What does a 99.9% probability of paternity actually mean?

A probability of paternity at or above 99.9% means the statistical calculation, expressed as a Combined Paternity Index, is so strongly in favor of the tested man being the biological father that scientists consider exclusion essentially impossible. A 0% result means the man is definitively excluded as the biological father.

How long does a home paternity test take to process?

Once USDC's laboratory receives your samples, standard processing takes 2 to 3 business days in the lab. Total turnaround from the time you mail your kit is typically 7 to 10 business days, accounting for shipping time. Faster result options are available during checkout.

Is an at-home paternity test result court-admissible?

Home collection kits are designed for personal knowledge. Court-admissible (legal) paternity testing requires a witnessed sample collection and a documented chain of custody, which is a separate process. The underlying lab science, including STR marker analysis, is the same as court-admissible testing.

Can a paternity test produce an inconclusive result?

Yes, though it is uncommon with a multi-marker panel. An inconclusive result occurs when the Combined Relationship Index falls between 0.1 and 10, a range that does not strongly support or exclude paternity. USDC's lab addresses this by extending the panel up to 28 total markers to resolve the ambiguity.