DNA testing has gone mainstream. Between 23andMe, AncestryDNA, and all the holiday gift kits, millions of people have spit in a tube and gotten ancestry results back. It feels like everyone has done one at this point.
But paternity testing and relationship DNA testing are completely different things. The technology is different, the purpose is different, and the results look nothing alike. They are not interchangeable.
If you need to figure out whether someone is a biological parent, sibling, or grandparent, you need to understand what STR DNA testing is and why it exists separately from ancestry kits. Here is how both types work so you know what you are actually dealing with.
What Is Genealogy DNA Testing?
Genealogy DNA testing is what companies like 23andMe and AncestryDNA offer. When you order one of those kits, you provide a saliva sample. The lab runs it through something called a SNP array.
SNP stands for Single Nucleotide Polymorphism. In plain terms, your DNA has billions of individual building blocks. At certain positions, people carry different versions of a single building block. Those variations are called SNPs. A typical ancestry test scans hundreds of thousands of these positions across your entire genome.
The lab compares your SNP pattern to reference databases from populations around the world and estimates your ethnic background -- what percentage of your DNA traces back to Western Europe, West Africa, East Asia, and so on. These are statistical estimates, not exact measurements. The test also compares your DNA to other customers in the company's database. If someone shares enough SNPs with you, it flags them as a potential relative.
This is useful for exploring your heritage and finding family connections across a broad range. But it was not built for answering a specific question like "Is this man the biological father of this child?"
What Is STR DNA Testing?
STR stands for Short Tandem Repeat. Instead of scanning hundreds of thousands of positions across your genome, STR testing focuses on specific locations where short sequences of DNA repeat in a pattern.
At certain spots in your DNA, a short sequence like AGAT might repeat several times in a row -- maybe 8 times, maybe 12, maybe 15. The number of repeats varies from person to person. You inherit one copy from your mother and one from your father.
During a paternity test, the lab looks at these STR locations in both the alleged father and the child. At every location, the child should carry one repeat number from the mother and one from the biological father. If the alleged father's numbers match at every tested location, the evidence for paternity is strong. If they do not match at multiple locations, he is excluded.
This is the same technology used in forensic crime labs and the FBI's CODIS database. It has been used for decades and it is the standard method for proving biological relationships.
Reputable laboratories test 20 or more STR markers. US Diagnostics Center analyzes up to 28 genetic markers, well above the industry standard. More markers means more data points and stronger confidence in the result.
Key Differences Between STR and Genealogy DNA Testing
| Factor | STR DNA Testing (Paternity/Relationship) | Genealogy DNA Testing (Ancestry) |
|---|---|---|
| Primary purpose | Confirm or exclude a specific biological relationship (parent, sibling, grandparent) | Estimate ethnic background and find distant relatives |
| Technology | Short Tandem Repeat (STR) analysis | SNP microarray genotyping |
| Markers analyzed | 20 or more targeted STR loci (USDC tests up to 28) | 500,000 to 700,000+ SNP positions |
| Sample type | Cheek swab (buccal) | Saliva tube |
| Accuracy for close relationships | 99.99% probability for confirming paternity; 100% for exclusion | Can suggest close relatives, but not designed for definitive verification |
| Turnaround time | 2–3 business days after samples arrive at the lab | 2–8 weeks depending on the company |
| Legal admissibility | Yes (with chain of custody collection) | No |
| Typical cost | $79–$300+ depending on test type and provider | $99–$199 for most consumer kits |
The short version: genealogy testing scans a huge chunk of your DNA looking for population-level patterns. STR testing looks at a small number of very specific markers to answer one question -- are these two people biologically related?
Why Paternity Tests Use STR and Not SNP
If ancestry tests look at hundreds of thousands of DNA positions, why would a paternity test only look at 20 to 28 markers? It seems like more data would be better. But it comes down to what each test is actually measuring.
SNP arrays detect population-level patterns. They estimate where your ancestors came from and spot shared DNA segments that suggest a common ancestor. But they were not built to give a definitive yes-or-no answer about whether a specific man fathered a specific child.
STR markers were chosen specifically because they are highly variable between unrelated people and follow clear inheritance patterns from parent to child. At each STR location, a child must carry one allele from the mother and one from the father. Either the alleged father's alleles match or they do not. When you test 20 or more of these locations, the math adds up fast. The odds of a random unrelated man matching at all tested locations are essentially zero.
That is why a confirmed paternity result carries a probability of 99.99% or higher, and an exclusion is 100% definitive. Ancestry tests were not built for that kind of certainty. Different tool, different job.
What to Expect from an At-Home STR Paternity Test
If you have decided that an STR-based paternity test is what you need, here is how the process works with US Diagnostics Center.
Ordering the Kit
You order the kit online. It ships to your door in discreet packaging. Standard delivery takes three to five business days, or you can choose express shipping for two to three day delivery. Nobody looking at the box would know what is inside.
Collecting Your Samples
Inside the kit you will find cheek swabs, labeled collection envelopes, instructions, consent forms, and a prepaid return mailer. Collection takes less than a minute per person -- just rub the swab along the inside of the cheek. No needles, no blood, no clinic visit.
Both the alleged father and child need to provide samples. Including the biological mother's sample is optional but recommended because it helps the lab isolate the paternal DNA contribution more clearly. A mother's sample can be added during checkout.
Mailing the Samples Back
Once samples are collected, seal everything in the provided envelopes and drop the return mailer in the mail. Transit time to the lab is typically one to three business days depending on your location.
Lab Analysis
Technicians extract the DNA and analyze up to 28 STR markers, comparing the repeat patterns between the alleged father and child at each location. The lab calculates a Combined Paternity Index and converts it to a probability of paternity. Processing takes 2–3 business days after samples arrive. If you need results faster, express options are available during checkout.
Getting Your Results
Results are delivered through a secure online portal. You get an email when the report is ready. The report shows either that the alleged father is not excluded as the biological father (99.99% or higher probability), or that the alleged father is excluded (0%). There is no gray area.
From the time you place your order to the time you view your results, the typical timeline is 7–10 business days.
When You Might Need STR Testing
STR DNA testing is not just for paternity questions. Here are the most common reasons people order this kind of test.
Paternity Testing
The most common use. You need to know whether a specific man is the biological father of a child. The Home Paternity Test Kit starts at $79 with lab fees included.
Maternity Testing
Less common but sometimes necessary in cases involving surrogacy, adoption, or hospital mix-up concerns. The Home Maternity Test Kit starts at $129.
Sibling Testing
When the alleged father is unavailable or unwilling to test, comparing the DNA of two potential siblings can help establish whether they share a biological parent. Both full sibling and half sibling kits are available starting at $139.
Grandparent Testing
If the father is deceased, missing, or otherwise unavailable, testing his parents can provide evidence of a biological connection to the grandchild. The Grandparent DNA Test starts at $139.
Aunt and Uncle Testing
Similar to grandparent testing, this option uses the DNA of a sibling of the alleged father to help establish a biological link. The Aunt/Uncle DNA Test Kit starts at $139.
All of these tests use the same STR analysis method. The lab compares the markers between the people tested and runs the numbers to see how likely the claimed relationship is.
What an Ancestry Test Cannot Tell You
An ancestry test is not a substitute for a paternity test. Ancestry platforms might flag someone as a "parent/child" match in their database, but those predictions are based on shared DNA segments compared against their customer pool. They are not verified through the marker-by-marker comparison that STR testing provides, and they carry no legal weight.
If the question is "Where did my ancestors come from?" an ancestry test is the right tool. If the question is "Is this person my father?" you need an STR-based relationship test.
Frequently Asked Questions
Can I Use My 23andMe or AncestryDNA Results as a Paternity Test?
No. Ancestry services use SNP arrays designed for estimating ethnicity and finding distant relatives. Those results are not validated for confirming a direct parent-child relationship. You need an STR-based test from a relationship testing laboratory.
Does US Diagnostics Center Offer Ancestry Testing?
No. US Diagnostics Center specializes in relationship DNA testing -- paternity, maternity, sibling, grandparent, and aunt/uncle. We do not offer ancestry or genealogy testing.
How Many Markers Does USDC Test?
US Diagnostics Center analyzes up to 28 genetic markers. The industry standard is 20 or more. Testing more markers provides greater statistical confidence, especially when participants share a common ethnic background.
How Fast Do STR Paternity Test Results Come Back?
Lab processing takes 2–3 business days after your samples arrive at the laboratory. The total timeline from ordering your kit to receiving results is typically 7–10 business days. Express processing options are available during checkout if you need results sooner.
Is STR Testing the Same Method Used by Law Enforcement?
Yes. STR analysis is the same technology used by forensic crime labs and the FBI's CODIS database. It has been used in court cases and criminal investigations for decades. The same method works for confirming whether two people are biologically related.
This article is part of our Understanding DNA Testing: How It Works and What to Expect guide.
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