The Basics of DNA Genealogy Explained

This post doesn’t encapsulate everything about DNA Genealogy but serves to cover some of the basics for people interested in taking a test.

So this is my attempt at explaining DNA Genealogy. These days it all makes sense to me but I remember it was a massive learning curve as I got my head around Haplogroups, Segments, Centimorgans, Genomes etc. I’m still learning new things about it even now and there are definitely a lot more people out there who really delve deep into it, but I will try and explain the basics to help you understand how it works and what you can expect.

There are currently 3 main DNA testing companies out there at the moment, that deal with the genealogy aspect of it. There are more but these 3 are the most well known. (autosomal test only) (autosomal test only) and (autosomal, yDNA (direct paternal) & mtDNA (direct maternal)).
There is a 4th company MyHeritage who have just started  DNA testing, but at this point in time I wouldn’t recommend them until they iron out some kinks and get a larger database.
All have their pro’s, con’s and various price differences.

A lot of people also upload their results from these 3 companies to which is a free site that allows people to compare tests against others from different companies and also has some pretty good tools to use.

Ancestry & 23andMe tests involve you spitting into a special tube and FamilyTreeDNA involves taking a cheek swab and sending it off to the company, which can be useful with the elderly or frail.

y-DNA & mt-DNA

A quick lesson in DNA before we get into the nitty-gritty. Males inherit a Y chromosome from their father and a X chromosome from their mother. Females only inherit two X chromosomes from their mother.


Family Tree DNA offer Y-DNA and mtDNA testing. So Y-DNA refers to the Y chromosome. Because females do not inherit a Y chromosome, only males can do this test which explores the direct paternal line. So if you are female you will need to get a blood brother, father, or paternal grandfather/uncle/cousin from an uncle, basically a male in your family with the same surname as yourself, to offer a saliva sample for this test.
For the mtDNA test, you’re looking at the mitochondrial DNA which is passed down the direct maternal line.

When it comes to exploring genealogy matches resulting from the yDNA test, generally speaking, if you find a match with the same surname as you (unless it’s a very common surname) there is a high probability that you share a mutual direct paternal ancestor at some point. If you do not share the same surname then it usually means that either it’s just a coincidence that you are matched OR there’s been a discrepancy along the way in that someone was adopted into the family or a female ancestor diddled with the milkman or historical equivalent. The test cannot confirm who and when so you would have to refer to other matches to triangulate where a discrepancy could have occurred. This can be quite time consuming.
FamilytreeDNA have surname projects on its website that you can join and compare your results with other people of the same surname.
Another thing to note with DNA is that it mutates over time. Some genes mutate at a faster rate than others so you will often find that surname pockets around the world will have similar mutations in the faster mutating genes.
Y-DNA tests come in different degrees of accuracy. The most basic is a 12 marker test, these 12 markers usually take a very long time to mutate. There are then 37, 67 and 111 marker tests. The higher the number the more marker results and the more you can match with others resulting in higher accuracy.


I personally wouldn’t bother with just the 12 marker test as any matches could be anywhere within approximately the past 30 generations (that’s nearly 1000 years). At least the 37 brings you to within 8-10 generations (still hard to trace), 67 around 6 generations (hard but doable) and 111 within a much easier 4 generations. If you can afford it and are conducting the test to help with genealogical roadblocks within the past 300 years then you’d want the 111 marker test, but at least get the 67 and you can upgrade it later.

When receiving the Y-DNA results you’ll get the amount of markers you bought, say 111. Each marker is called a STR: Short Tandem Repeat. Each marker is given a name, such as DYS393. Each marker has a numerical value, for example, 12. It’s these numbers that you use against each marker to determine your matches. The more that match, the more likely you have a shared ancestor. Faster mutating markers may show a 1 or 2 digit difference to your own results. Depending on the marker you can figure out how this affects the match’s accuracy. For example if on one particular marker, DYS393 you have 12 but a match has exactly the same in every other marker but 13 for DYS393 then that particular marker has only mutated once. If it’s 14, it’s mutated twice. This can also go the other way with 11, 10 etc. The higher the difference in the number, the more times it has mutated and as such, more generations would have passed. Websites are around showing the estimated rate of mutation for each marker. Some are likely to mutate every 4 generations, others may go through 20 generations before mutating.

But before you head into checking out your ancestral matches, you’ll probably see your Haplotype group. If you’re male you’ll have a paternal haplotype and a maternal one. This is inherited directly down each line. Females however will only have a maternal haplogroup as they do not receive the fathers Y chromosome.
So what is a Haplogroup? Well this refers to your ancient point of origin (prior to sea travel and colonisation but also includes thousands of years ago.
When humans started migrating out of Africa they split off into groups as they wandered the earth and produced pockets of population resulting in everyone getting similar DNA sequences on the markers that rarely mutate.
The Haplogroups for Y-DNA  are given a letter from A-T. The A’s and B’s are most likely to be of recent African descent, originating from populations who never left the continent. C’s are most likely to be of Asian, North American or Oceanic descent. F includes parts of Asia, Europe and South America and so on and so on.

Each Haplogroup then splits off into subclades of a more refined population group. Some can even be quite specific with R1b1c7 belonging to a 5th Century Irish King who fathered so many children that now around 90% of men in some pockets of north-west Ireland and Scotland have this same haplogroup and as such can trace this King as their ancestor.

This map shows some of the more common haplotype groups around Europe.


Autosomal Tests

Then you have autosomal testing, which is what 23andme and Ancestry specialise in and what FamilyTreeDNA offers along with y-DNA and mt-DNA.
23andme also provide basic health reports too which are quite interesting.

The good thing about autosomal is that both males and females can take the test. It also shows DNA from every single line of your tree, not just your direct maternal or paternal. This also results in a lot of matches with other people who have tested (hopefully close matches, but more often than not, distant) and you have to figure out which branch the mutual ancestor might be on. Unless the match has a detailed tree going back many generations, it’s often very hard to figure out where the match is. Although if you figure out one match you can then use that person to triangulate with other matches you may suspect are related on the same line. It’s not definite as we all inherit different parts of DNA from out parents and ancestors, but if you find the two people match each other then it can confirm the branch.
Most people will get a number of 4th cousin matches and if you’re lucky 3rd cousins or closer.

While you can get results with just one person testing, it becomes a lot easier if you can test other family members. This makes it a bit easier to pinpoint what line your matches are on, especially when you test grandparents. For instance, if Joe matches both yourself and your grandmother, then you know Joe is descended from one of your grandmother’s ancestors – the amount of DNA you share with Joe can give an indication of how long ago your mutual ancestor is (see green chart below).
When testing 1st cousins for example, you have to remember that while you may both descend from the same grandparents, you won’t inherit the exact same segments of DNA. First cousins on average share about 12.5% DNA with each other. So in theory, Joe might match your cousin and your grandmother, but not yourself if you didn’t inherit that particular segment of DNA that Joe has… confused yet?

This visual might help.


                             An example of how you can inherit DNA from great grandparents.                                Source: 

It’s important to test as many family members as possible if you have the funds, especially older generations. Start with them first and if you can test both parents then it’s not as important to test yourself, unless you want to know your particular ethnicity mix. If you’re unable to test parents then test as many children from the one family to cover as much of the parents DNA as possible, as each child inherits different segments from parents.

This chart gives you an idea about how much DNA certain relationships generally share.
Up to a 3rd cousin level it’s usually predictable but past that you can share some or none.


Source: Christa Stalcup –

So I hope that gives you a brief introduction into DNA Genealogy. It can be very useful and eventually the more people that test, the more matches you’ll get and the more you’ll get out of it. It’s a big learning curve but highly interesting when learning about your own deep, dark, ancestral roots!

It may sound like a whole load of gobbledegook at the moment, the best thing to do is to take a test and then learn as you go when your results come in. There are also lots of helpful people in the Facebook groups listed below to answer your questions, although most can be found by doing a quick Google search.

For more reading, check out

And if you’re on Facebook, join a DNA group.
‘Using DNA for Genealogy – Australia and NZ’ is a great source of information for those in Australia and NZ or try DNA Detectives‘ for the US/Worldwide, which specialises in adoption DNA.
Check the pinned posts up the top of the Facebook groups for further reading and group rules.


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