You know that person. The one who, the moment you express any interest in what they’re working on, gives you so much more information than you needed or wanted that you end up deeper down a rabbit hole than you ever intended to go? That’s what DNA testing can feel like the first time you log in to see your results. You came for a family tree. What you got was centimorgans, haplogroups, STRs, SNPs, mitochondrial pathways, and depending on which platform you used, a slightly different version of all of it. It’s enough to close the app and put it away, because where do you even start?
Right here. This is the orientation. Not the deep dive. There are plenty of places to go deeper, and we’ll get there. This is just enough science to make the rest of it make sense.
The Three Main Types of DNA in Genealogy
Every major testing platform is measuring DNA, but not all DNA tells the same story. There are three types you’ll encounter, and each one answers a different question about your family.
Autosomal DNA (atDNA)
This is the one you’re most likely starting with. Autosomal DNA is the mixed bag of your ancestry. You inherit half from your father and half from your mother, and they inherited from theirs, and so on back through the generations. Your autosomal results reflect a random assortment of genetic material from all of your ancestors going back roughly five to six generations.
Every major platform — Ancestry, MyHeritage, 23andMe, FamilyTreeDNA — uses autosomal DNA to generate your match list. That list of strangers who are apparently related to you? All autosomal matches. Because it captures all your family lines, it’s the right tool for finding cousins on any branch of your tree.
What is cM?
Next to each DNA match you’ll see a number followed by “cM.” That stands for centimorgans, the unit used to measure how much DNA you share with someone. The higher the number, the more closely you’re related. A parent and child share roughly 3,500 cM. A first cousin might share around 850 cM. A distant fifth cousin might share only 20 to 40 cM, and might not show up at all on some platforms.
Here’s the catch: the same centimorgan count can point to multiple different relationships. 400 cM could mean first cousin twice removed, or half first cousin, or great-grandparent. The number alone does not tell you which. This is why genealogists use a shared cM tool to see the full range of probable relationships for any count. The DNA Painter Shared cM tool is a good example. Enter a centimorgan count and it shows you every likely relationship along with probabilities. It’s most useful when you can pair it with other DNA matches. If you have a mystery match and one of your parents is also in the system, you can often figure out which side of the family that match belongs to. That’s called triangulation, and it’s its own conversation. More: What Is a Centimorgan and Why Does It Matter?
Your ethnicity estimates can change.
If you tested a few years ago and recently logged back in, you may have noticed your ethnicity percentages look different. Over time, this makes sense. As more people test, there’s more data. The reference populations get larger, the science gets more precise, and regions that were once grouped together start to separate into more specific categories. Not because your ancestry changed, but because the resolution improved. Think of it like the difference between an early map drawn from explorer reports and a modern satellite image. The landmass was always there. The detail just keeps getting better.
One note worth knowing: not all countries allow direct-to-consumer DNA testing, which means your match pool from certain regions may be thinner than others.
Mitochondrial DNA (mtDNA)
Here is where it helps to remember a little of what you learned in science class. You have 23 pairs of chromosomes. One of those pairs determines biological sex. Women carry two X chromosomes, men carry one X and one Y. Mitochondrial DNA travels separately from all of that. It passes almost exclusively from mother to child, to both sons and daughters. But here is the key: only daughters pass it on to the next generation. Sons carry their mother’s mitochondrial DNA their entire lives, but it stops with them. It does not get passed to their children.
This creates a single unbroken line running straight up your direct maternal side. Your mother, her mother, her mother’s mother, with no branching, no dilution, no interruption.
For genealogists, that’s valuable precisely because women’s lineage has always been the hardest to trace in most documented record systems. Women changed their names at marriage, were recorded as “wife of” or “daughter of,” and often disappear from the paper trail within a generation. Mitochondrial DNA can follow that line even when the documents cannot. Like any genealogical source, it works best as one piece of a larger picture. It points the direction, but you still need records to meet it there.
Your mtDNA tells you about your direct maternal line only. Nothing about your mother’s father’s side, or any other branch. It is narrow but very, very deep.
Y-Chromosome DNA (Y-DNA)
Y-DNA works the same way, in reverse. It passes from father to son, creating an unbroken line tracing your direct paternal ancestry. Because surnames have traditionally traveled the same paternal path, Y-DNA is a natural fit for surname research and for connecting men who share a name but don’t know how.
There are two types of Y-DNA testing worth knowing, and yes, the names are a mouthful.
STR testing stands for Short Tandem Repeats. This is the entry-level test. It looks at specific repeating patterns in your DNA to find close relatives on your paternal line and confirm whether two men with the same surname actually share a common ancestor. It’s usually where researchers start.
SNP testing stands for Single Nucleotide Polymorphisms. This is the deeper test. FTDNA’s Big Y-700 is the standard. It analyzes hundreds of thousands of specific genetic markers to place you precisely on the Y-DNA tree and define your haplogroup. If STR testing is a neighborhood map, SNP testing is the survey that shows you exactly which lot you’re standing on and when the road was built.
The Jargon You’ll Keep Running Into
Haplogroups
A haplogroup is the branch of the ancient human family tree that your direct paternal or maternal line belongs to, defined by specific genetic markers passed down over thousands of years. It doesn’t tell you about your recent family. It tells you about the deep migration history of the population you descend from.
Here is how they develop: occasionally, a small variation occurs in DNA. Not a flaw, just a change. That variation gets passed down intact through that person’s descendants. Over generations, researchers identify these variations as markers that define a new branch. The more people who test, the more precisely these branches get mapped and named.
Genetic Dating
When a platform estimates when a common ancestor lived, it’s calculating backward from how much DNA two people share and the average rate of genetic change over generations. It is an estimate, not a date, not a fact, not something to take to the bank. Think of it as a prompt to ask a better question, not an answer to stop at. Which, honestly, is the nature of genealogy as a whole.
Neanderthal DNA
If your ancestry includes people from Europe or Asia, you likely carry a small percentage of Neanderthal DNA, typically somewhere between 1 and 2%. This traces back to interbreeding between early modern humans and Neanderthals after populations spread out of Africa. It is a marker of where your ancestors were thousands of years ago. Not a health indicator, not a personality trait, and not the dramatic thing the headlines make it sound like. People with ancestry entirely from sub-Saharan Africa tend to carry little to none of it, not because one group is older or more original, but because their ancestors traveled different paths and encountered different populations along the way.
Traits
Most of us got some version of this in middle school or high school. Mendel, his peas, dominant and recessive genes. Why some traits skip generations. Why two brown-eyed parents can have a blue-eyed child. The traits your DNA testing platform reports are built on that same foundation: eye color, hair texture, whether cilantro tastes like soap to you. Real, measurable, occasionally amusing. Not the same thing as health data, and not something to treat as though it is.
Health Data
Health data is a different conversation, more complicated, more regulated, more consequential, and more actively debated than the traits section would suggest. It ventures into genetic ethics, insurance questions, medical interpretation, and territory where the science is still developing. It deserves its own post, and it will get one.
All of This Is a Better Way to Ask Questions
None of these tests, none of these numbers, none of these categories gives you an answer on its own. Autosomal DNA gets more useful when other family members have tested. Centimorgans tell you more when you can triangulate across multiple matches. Y-DNA and mtDNA get more precise as more people in your lines test. Genetic dating narrows a search but does not close one.
What all of it gives you, when you use it together, is a better set of questions and a stronger theory to take to the evidence. That is how genealogy works. The DNA is one more tool in the process, and a powerful one. But it is always, in the end, pointing you toward something you still have to go find.
This is the first post in the Beyond DNA Basics arc. Next up: What Is a Centimorgan and Why Does It Matter to Your DNA Research?