How DNA Tests Work: From Spit Tube to Ethnicity Estimate

A consumer DNA test feels like magic. You spit in a tube, mail it back, and a few weeks later a website tells you that you are 23% Scottish with a third cousin in Ohio. The actual chain of events is less mysterious. It is a sample, a lab, a chip, and a lot of statistics.

Step 1: collecting your sample

Most kits collect saliva. You either spit into a tube up to a fill line (AncestryDNA, 23andMe) or rub a cheek swab (FamilyTreeDNA, most pet tests). Saliva contains epithelial cells shed from the inside of your mouth, and those cells contain your DNA. Eating, drinking, smoking, or brushing your teeth in the half hour before collection can contaminate the sample with food particles or bacterial DNA, which is why every kit warns against it.

The sample is stabilized in a buffer solution inside the tube. That buffer preserves the DNA at room temperature for weeks, which is what makes mail-in testing possible at all.

Step 2: extracting the DNA

At the lab, your sample goes through DNA extraction. The cells are broken open, proteins and other cellular material are washed away, and what remains is purified human DNA. The lab measures concentration and quality. If your sample is too degraded or too low in volume, you get the dreaded “we need another sample” email.

Step 3: genotyping versus sequencing

This is the step that separates the two big technical approaches in consumer DNA testing.

SNP genotyping is what AncestryDNA, 23andMe, MyHeritage, FamilyTreeDNA, and Living DNA use. The lab places your DNA on a microarray chip that checks specific positions in the genome called single nucleotide polymorphisms, or SNPs. These are spots where humans commonly differ from one another by a single letter. According to 23andMe’s product technology documentation, their custom array reads roughly 600,000 to 700,000 SNPs. That is a tiny fraction of your full genome, but it is the fraction with the most useful variation for ancestry, traits, and many health markers. Genotyping is cheap, fast, and highly accurate at the positions it reads.

Whole-genome sequencing is the alternative. Rather than checking 700,000 chosen positions, sequencing reads your entire genome of roughly three billion base pairs, usually multiple times over for accuracy. Sequencing.com is the active consumer whole-genome option as of early 2026, after Nebula Genomics shut down in February 2025. Sequencing produces vastly more data, costs significantly more, and is overkill for most ancestry questions. See our explainer on what whole-genome sequencing is for when it actually pays off.

Step 4: turning data into reports

The raw output of either method is a list of letters. Your A/T/C/G readings at hundreds of thousands or billions of positions. By itself this is not a report. The company’s algorithms do the rest.

Ancestry estimates compare your genotype to a reference panel of people with documented ancestry from specific regions. Your DNA gets divided into chunks, each chunk is matched to the region whose reference samples it most resembles, and the percentages on your dashboard are the totals of those assignments. This is statistical, and the panel changes, so the estimate changes. We cover that in detail in our guide on DNA test accuracy.

Relative matching compares your DNA to every other customer who has tested with the same company. Shared chunks of DNA, measured in centimorgans, predict how closely two people are related. The bigger the company’s database, the more matches you will see. AncestryDNA’s roughly 25 to 30 million customers explain why it dominates this category.

Health reports, where offered, look at specific variants the FDA or the company has linked to a condition or trait and report your genotype at those positions. 23andMe holds FDA authorizations for several of these. We do not interpret health results for individuals in our guides. If a result concerns you, talk to your doctor or a board-certified genetic counselor via the NSGC directory.

What the lab does not do

A consumer DNA test does not sequence your whole genome unless you specifically buy a whole-genome product. It does not read every gene. It does not interpret results in your medical context. And it is not the same thing as a clinical genetic test ordered by a physician, which uses different methodology and is held to a different regulatory standard.

For the bigger picture, see our guide to choosing a DNA test. For the specific differences between test types, read autosomal vs Y-DNA vs mtDNA.