Ancestry at 23andMe: What Can You Learn?

Genetic testing is not a concept that most people encounter outside of popular TV shows and so when you hear about services such as 23andMe’s, you might be wondering what exactly it is you’re getting. Even after you’ve signed up and have your data back, you might not know where to start exploring or how to make sense of the wealth of information you’ve just received. Since many people are unfamiliar with genetics and 23andMe offers several unique features, we’ll be presenting a series of posts to help both current customers and the curious reader to understand what you can (and can’t) learn from genetics and from 23andMe in particular.

This first post is part of the “Ancestry at 23andMe” series and is an introduction to the types of information we can tell you about your ancestry. (There will also be a “Health at 23andMe” series to cover the features found in the Health portion of our service.) It may be a review for some, but should help orient those new to genetics or 23andMe (for an even more basic primer, check out our videos in Genetics 101). Read on, and let us know if you have a question you would like to see featured in future articles on the 23andMe blog! 

Ancestry at 23andMe: What Can You Learn?


People who have contributed to your DNA.

Using DNA to uncover clues about your ancestry is a tantalizing prospect for many people. Adoptees and people with diverse ethnic backgrounds may be especially interested in finding out more about where they came from. But even those who have relatively thorough knowledge about their family’s roots may be curious about the more obscure branches of their family tree or the deeper ancestry that connects them to all humans on this planet. Fortunately, DNA acts as a biological record that can reveal many of the pieces of story that is us.

With 23andMe’s genetic testing services, individuals can see what their DNA says about where in the world their ancestors came from, what populations they might be a part of, and who else in the world shares that DNA with them.

All of us have three or four of the following types of DNA, defined by where each is found:

Who contributed this DNA if you are… Female Male
Autosomal DNA is found in the 22 pairs of non-sex chromosomes passed down to us equally from our mothers and fathers, one of each pair from each parent. This is about 96% of the DNA that makes you you.
The X chromosome is one of the larger chromosomes and contains a large number of essential genes. Women have two copies, one each from mother and father; men have just one copy, inherited from their mother (except in rare genetic conditions).
The Y chromosome is what makes someone male. Men have one of these, passed down from their father. N/A
All of us also have mitochondrial DNA (mtDNA), found in the chemical factories — the mitochondria — inside each of our cells that convert the food we eat into energy for our bodies. mtDNA is always passed down from mother to child.

Each type of DNA has unique properties that are informative for ancestry. Because your mitochondrial DNA comes all in one piece from your mother — and she received it from her mother, and so on — we can say pretty definitively where that piece of your DNA came from. In fact, scientists can map all mtDNA onto a family tree of sorts, tracing back to a “mitochondrial Eve”, the common female ancestor that gave rise to all variations of mtDNA around today.

This mapping is possible because DNA isn’t always copied perfectly when it’s passed from parent to child. With each successive generation, the changes spread and become the background from which new changes arise, serving both to distinguish different maternal lineages from each other and to connect lineages to each other at some point in genealogical time. So one thing that 23andMe can tell you is where that piece of DNA — from your mother’s mother’s mother’s ancestors — is from. (23andMe customers can see this information in the Maternal Line feature of their account.)

Similarly, because the Y chromosome is passed down exclusively from father to son, we can use it to trace the ancestry of a small part of your DNA if you are male. The same principle of DNA variation spreading and building up through generations allows scientists to construct a tree for all the paternal lineages and allows 23andMe to map a man’s Y chromosome DNA to a particular branch in that tree. (Male 23andMe customers can see this information in the Paternal Line feature of their account.)

Your maternal — and for some, paternal — line gives you an interesting piece of your DNA history, but it is a very small part. Your autosomal and X chromosome DNA make up almost 99% of all your DNA, and is passed down from both parents (except for the X chromosome in males, which comes only from the mother). This means that the DNA can be used to infer more distant relatedness between individuals on any branch of their family tree — 4th cousins, for example, who share common great-great-great-grandparents — based on the percentage of DNA shared. Thousands of people are already making connections with distant relatives through 23andMe’s database. (23andMe customers can explore our DNA Relatives (Relative Finder) and Ancestry Finder Countries of Ancestry features which use these principles.)

More generally, we can compare your autosomal DNA to that of different populations around the world and show you which groups of people you are most similar to genetically. This can be done at a very high level with broad ethnic groups or global populations, or at a finer-grained level with more specific populations, such as Han Chinese or Norwegian. (23andMe customers can see these using the Ancestry Painting Ancestry Composition Global Similarity and Global Similarity: Advanced Global Similarity Map features.)

Unlike maternal and paternal line analysis, autosomal and X chromosome DNA analysis cannot tell us much definitively past a certain number of generations. Because each pair of chromosomes recombines, or mixes, before being passed down from parent to child, the size of possible shared segments gets successively smaller with each generation. Too small, and it’s no longer possible to tell whether the segment truly came from a shared common ancestor, or if it’s identical simply by chance. The information we can infer, however, is much more versatile. And although scientific methods using autosomal and X chromosome DNA are not as firmly established or plentiful as those used for mtDNA and Y chromosome analyses, this should change as more research accumulates.

The cool part is that all of these features we’ve just mentioned are just the tip of the iceberg. One thing that separates 23andMe’s services from most other genetic ancestry services is that we use data about hundreds of thousands of variants in the human genome — places where a single DNA letter can vary between individuals, called single nucleotide polymorphisms, or SNPs (“snips”). This offers greater resolution and versatility than more traditional methods, allowing us to provide a unique and constantly evolving set of tools for exploring your ancestry.

In the next post in this series, we illustrate 23andMe’s ancestry features in more detail by following some (fictional, but based on real) customers as they explore their 23andMe data.

Summary of 23andMe features mentioned in this post:
Maternal Line – Uses mitochondrial DNA passed down from your mother to trace the origins of your mother’s mother’s mother’s ancestors.
Paternal Line – (Possible for men only) Uses Y chromosome DNA passed down to sons from their fathers to trace the origins of their father’s father’s father’s ancestors.
DNA Relatives (Relative Finder) and Countries of Ancestry – Uses autosomal DNA passed down from both parents to connect you to other 23andMe users who may be related to you.
Ancestry Composition and Global Similarity Map - Uses autosomal DNA passed down from both parents to place your DNA in the broader context of populations, continents, and regions of the world.

Other posts in this series:

Introducing Sheridan – We meet Sheridan and she learns about her maternal haplogroups.

Sheridan’s Global Origins – Sheridan compares herself and her friends to various populations around the world.

Sheridan’s Got Relatives – Sheridan responds to a fifth cousin and discovers a predicted second cousin in the 23andMe database.

Finding Connections – Sheridan and Brian bond over their fourth cousin connection.

Old Roots and New Horizons – Sheridan and Brian put together the pieces to sketch out their shared family tree.


  • Mike

    The limitations of autosomal and X chromosome DNA analysis are what what drew us to doing this ‘across the whole of genome’ test at 23andme. But beware while it more broadly reveals one’s ancestry for many that’s more complex than one might have previosuly imagined. Thus one may have to be very persistant and patient to get the most out of it as this type of test is proving to be very challenging to many if not most customers. However if nothing is ventured then nothing is gained so if you want to see if you can crack some of the old family secrets it’s certainly worth a try as the broad and still developing basic Health SNP’s scan has for us has proven to be of good value.

  • Les Lambert

    Can you please define how you distinguish between “identical by chance” and “identical by descent”, in terms of length of a match in genetic distance (centiMorgans) and number of SNPs? How do “no-calls” complicate making this distinction?

    Thanks

    • http://23andme.com Shwu

      @Les The actual process for determining this involved simulating different models with test data that had known relationship. This allowed us to define thresholds at certain segment lengths (roughly by number of consecutive SNPs) that gave good performance in predicting the different degrees of relationship. No-calls do complicate the prediction, but we account for them in the calculation; they do not automatically break up a potential “identical-by-descent” segment.

      Shirley

  • http://linealarboretum.blogspot.com Jim Owston

    Great article – I really appreciate you posting this as it will be helpful for all who read it. I also appreciate 23andMe’s product and its service.

    While I applaud you for having squares and circles correctly represent males and females respectively, I find that 23andMe continues to deviate from standard genealogical practice in ancestral chart arrangement.

    In the genealogical world, males are referenced on the left (or top) and females on the right (or bottom). This is a standard and accepted practice; however, I have seen, with Ancestry Finder and the graphics used in this article, the opposite is being used which may be a source of confusion.

    With Ancestry Finder, the order of maternal grandmother, maternal grandfather, paternal grandmother, and paternal grandfather requires a paradigm shift and creates cognitive dissonance for anyone who has constructed a pedigree chart. It is simply not logical with what the genealogical world uses.

    The accepted practice of enumerating one’s ancestors with both the Ahnentafel System and William Dollarhide’s System start with the individual as #1. In that person’s ancestry, the father is #2 and the mother is #3. With the exception of the #1 person, all males are even numbered and all females are odd numbered. This fits the schema of males on the left or top and females on the right or bottom.

    I find the current arrangement disconcerting and I might appear somewhat obsessive/compulsive in this regard; however, if you are attempting to attract genealogists and family historians, it would seem that you would want to use the same arrangement that has been in use for centuries.

    Simply opening the graphics in Photoshop or Fireworks, flipping the charts, saving, and reposting the images would correct this grievous error in a matter of minutes. Ancestry Finder will require some additional work to get the order in the accepted order to which genealogists are accustomed.

    Thank you in advance for your attention to what might appear to be a minor issue in your eyes but is opposite of what we all are using outside of this site.

    • http://23andme.com Shwu

      Hi Jim,

      Thanks for your comment, and we’re glad you like the service! You’re certainly right about the standard conventions for males and females in genealogical diagrams. We’ve updated those images in the post so that the males (squares) are on the left, and the females (circles) are on the right. We will look into updating the display in Ancestry Finder, but it will likely be subject to current research and product priorities, of which there are many!

      Regards,
      Shirley

  • AB

    How far back in time does shared dna go on the results we get? If I share several small segments on 2 or 3 chromosomes are we looking at more than 300 years to find a common ancestor? is the answer different for Ashkenazi Jewish? for half AJ?

    Thanks for doing this article. I find I need to keep coming back to basic genetics. I’d like to hear more about what we find on the X and on different chromosomes. I know X has fertility and intelligence and pigmentation and represents a large portion of genes, but what else? And what about the others? More discussion on this topic is of interest.

    • http://23andme.com Shwu

      @AB In general, our autosomal relatedness predictions are accurate only up to the 5th-10th cousin range, so the maximum time frame would be somewhere around 300 years (but probably less). If you know the predicted relationship (e.g. the match is listed in your Relative Finder) it might help narrow that time frame. If it’s below our Relative Finder threshold it would be very difficult to estimate. Ashkenazi ancestry does make things more challenging since there is more background sharing in this population. It might not necessarily increase or decrease the time frame, but it could certainly make determining the common ancestor more complicated.

      If you’d like a more detailed explanation, please contact help@23andme.com and reference this post!

  • http://? Harvey Woehlck

    I found some interesting things. From family records and history, I expect my background to be mostly German, with smaller amounts of Czech, scandinavian, and a small amount of Mediterranean. My results from 23 and me confirm this to a large extent. However, I am listed as 0 – .1% Czech. My wife has essentially none. However, our children (whom 23 and me confirm are biologically ours) show large amounts of Czech. Why does the program not identify that in me, as I should have that in my background, and there is no other way for my children to have that in their background, as my wife is not from that part of the world. What is preventing the 23 and me program from identifying that part of the background for me?

    • http://23andme.com Shwu

      Hi Harvey,

      Interesting scenario, and one that is actually not uncommon. There are two things that could be happening here. One is that Ancestry Finder (which is where I’m guessing you’re seeing this information) isn’t updated in real time, so your results and your children’s results might be out of sync temporarily. Another explanation has to do with how segment boundaries are determined. When we are comparing segments between people, if one person has a “TT” at one SNP and the other person has a “CC”, we can definitively call that a mismatch and end the segment there. But if one person has a “TT” and the other has a “CT”, we don’t know if that’s an endpoint or not — the one ‘T’ that the second person has could continue the run. It could be several or many more SNPs later before a definitive mismatch occurs. So if you are the “CC” in the first scenario, you wouldn’t show up as sharing that segment with the other person, but if your children are “CT”, they might. Differences in segment sharing like your case are more likely to happen as the segments get shorter, so if the “Czech” segments your children have are all in the 5-8 cM range, that could be the explanation.

      We might tackle this general situation in a future post, but if you’d like to discuss your question in more detail with one of our scientists, feel free to contact help@23andme.com with your question and as much clarifying information (screenshots, etc) as you can. Thanks!

  • http://ideas.4brad.com Brad Templeton

    Generally a good article but it still (in spite of toning it down) overplays the role that MT DNA and Y-DNA play in ancestry, and in particular genealogy. They play almost no role in genealogy except with your immediate family and very close cousins. If you go back to the “haplomother” of a certain MTDNA group, it is so many generations ago that you have uncounted trillions of slots in your family tree, and the haplomother fills only one of those. (Actually, she also appears in trillions of other slots, like all your ancestors do.) In fact, evidence suggests that when you go back 20,000 years, you are descended many times over from every breeding person who lived back then on your ancestral land mass, and in most cases from every breeding person in the world. The haplomother (and father) are just 2 out of those tens of millions, and no more important than any of them.

    What haplogroups can reveal is likely ethnicities. Most of your ancestors will be from your ethnic group or geographic region of origin, and thus the odds are higher that the haplomother is also from there. But of course, the odds are also decent that she’s not from there, so it’s risky to make really solid conclusions you can only suggest some probabilities.

    The autosomal DNA is where it’s at — this article understands that to some degree, but when it outlines the limitations (only going back a modest number of generations) it sounds like a bug. Haploid DNA does tell you about something hundreds of generations ago, but it tells you one story among hundreds of millions — something fairly meaningless. Autosomal DNA may only tell you about a few hundred years but the result is vastly more meaningful.

    • http://23andme.com Shwu

      Brad, very good points. I’ve tweaked the post to reflect some of what you brought up. We certainly want to get across the fact that Y and mito aren’t everything, and in fact are only a tiny part of the overall picture.

  • http://linealarboretum.blogspot.com Jim Owston

    Shirley:

    Thanks so much. This is one of the things I like about the 23andMe family – you listen to your customer base. I’ve seen this time and time again. Keep up the great work.

    Jim

  • Kate

    I love your service and really appreciate all the information that has been outlined for us.Before testing I had only a minor understanding of my ancestry,now I’ve infered alot more than I even though would have been possible,information I could’nt find through traditional methods.Ancestry finder and Relative Finder has been the most illuminating tools but they are all helpful and useful.I had no idea when I signed up that I would end up with this amount of ongoing info on my ancestry.

  • pammy

    Clarification please–So as a female I am only getting my mom’s side of the family?

    • Scott23H

      No that’s not quite right.
      While women do not inherit a Y chromosome from their fathers thus making identifying a paternal line difficult, they do get the other 22 pairs of chromosomes from both their mothers and fathers. You get half of these autosomes from your mother and half from your father. So when we identify DNA Relatives those will be from both sides of your family. It’s just that it will be harder to determine which side those relative matches from from.

      • pammy

        ok, thanks–this stuff is so confusing…:)

        • fattykim

          You can get the Y for yourself. I you have a father, paternal grandfather, brother, or an uncle (father’s brother) you can have them take the test and use their Y as your own.

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