Posted by Julie Granka on August 19, 2014 in DNA, Science

Research into matching patterns of over a half-million AncestryDNA members translates into new DNA matching discoveries 

Among over 500,000 AncestryDNA customers, more than 35 million 4th cousin relationships have been identified – a number that continues to grow rapidly at an exponential rate.  While that means millions of opportunities for personal discoveries by AncestryDNA members, it also means a lot of data that the AncestryDNA science team can put back into research and development for DNA matching.

At the Institute for Genetic Genealogy Annual Conference in Washington, D.C. this past weekend, I spoke about some of the AncestryDNA science team’s latest exciting discoveries – made by carefully studying patterns of DNA matches in a 500,000-member database.


Graph showing growth in the number of 4th cousin matches between pairs of AncestryDNA customers over time
Graph showing growth in the number of 4th cousin matches between pairs of AncestryDNA customers over time

DNA matching means identifying pairs of individuals whose genetics suggest that they are related through a recent common ancestor. But DNA matching is an evolving science.  By analyzing the results from our current method for DNA matching, we have learned how we might be able to improve upon it for the future.


Life cycle of AncestryDNA matching research and development
Life cycle of AncestryDNA matching research and development

The science team targeted our research of the DNA matching data so that we could obtain insight into two specific steps of the DNA matching procedure.

Remember that a person gets half of their DNA from each of their parents – one full copy from their mother and one from their father.  The problem is that your genetic data doesn’t tell us which parts of your DNA you inherited from the same parent.  The first step of DNA matching is called phasing, and determines the strings of DNA letters that a person inherited from each of their parents.  In other words, phasing distinguishes the two separate copies of a person’s genome.


Observed genetic data only reveals the pairs of letters that a person has at a particular genetic marker.  Phasing determines which strings of letters of DNA were inherited as a unit from each of their parents.
Observed genetic data only reveals the pairs of letters that a person has at a particular genetic marker. Phasing determines which strings of letters of DNA were inherited as a unit from each of their parents.

If we had DNA from everyone’s parents, phasing someone’s DNA would be easy.  But unfortunately, we don’t.  So instead, phasing someone’s DNA is often based on a “reference” dataset of people in the world who are already phased.  Typically, those reference sets are rather small (around one thousand people).

Studies of customer data led us to find that we could incorporate data from hundreds of thousands of existing customers into our reference dataset.  The result?  Phasing that is more accurate, and faster.  Applying this new approach would mean a better setup for the next steps of DNA matching.

The second step in DNA matching is to look for pieces of DNA that are identical between individuals.  For genealogy research, we’re interested in DNA that’s identical because two people are related from a recent common ancestor.  This is called DNA that is identical by descent, or IBD.  IBD DNA is what leads to meaningful genealogical discoveries: allowing members to connect with cousins, find new ancestors, and collaborate on research.

But there other reasons why two people’s DNA could be identical. After all, the genomes of any two humans are 99.9% identical. Pieces of DNA could be identical between two people because they are both human, because they are of the same ethnicity, or because they share some other more ancient shared history.  We call these pieces of DNA only identical by state (IBS), because the DNA could be identical for a reason other than a recent common ancestor.

We sought to understand the causes of identical pieces of DNA between more than half a million AncestryDNA members.  Our in-depth study of these matches led us to find that in certain places of the genome, thousands of people were being estimated to have DNA that was identical to one another.

What we found is that thousands of people all having matching DNA isn’t a signal of all of them being closely related to one another.  Instead, it’s likely a hallmark of a more ancient shared history between those thousands of individuals – or IBS.


Finding places in the genome where thousands of people all have identical DNA is likely a hallmark of IBS, but not IBD.
Finding places in the genome where thousands of people all have identical DNA is likely a hallmark of IBS, but not IBD.

In other words, our analysis revealed that in a few cases where we thought people’s DNA was identical by descent, it was actually identical by state.  These striking matching patterns were only apparent after viewing the massive amount of matching data that we did.

So while the data suggested that our algorithms had room for improvement, that same data gave us the solution.  After exploring a large number of potential fixes and alternative algorithms, we discovered that the best way to address the problem was to use the observed DNA matches to determine which were meaningful for genealogy (IBD) – and distinguish them from those due to more ancient shared history.  In other words, the matching data itself has the power to help us tease apart the matches that we want to keep from those that we want to throw away.

The AncestryDNA science team’s efforts – poring through mounds and mounds of DNA matches – have paid off.  From preliminary testing, it appears that these latest discoveries relating to both steps of DNA matching may lead to dramatic DNA matching improvements. In the future, this may translate to a higher-quality list of matches for each AncestryDNA member: fewer false matches, and a few new matches too.

In addition to the hard work of the AncestryDNA science team, the huge amount of DNA matching data from over a half-million AncestryDNA members is what has enabled these new discoveries.  Carefully studying the results from our existing matching algorithms has now allowed us to complete the research and development “life cycle” of DNA matching: translating real data into future advancements in the AncestryDNA experience.

Julie Granka

Julie has been a population geneticist at AncestryDNA since May 2013. Before that, Julie received her Ph.D. in Biology and M.S. in Statistics from Stanford University, where she studied genetic data from human populations and developed computational tools to answer questions about population history and evolution. She also spent time collecting and studying DNA using spit-collection tubes like the ones in an AncestryDNA kit. Julie likes to spend her non-computer time enjoying the outdoors – hiking, biking, running, swimming, camping, and picnicking. But if she’s inside, she’s baking, drawing, and painting.


  1. sheila wood

    I am a UK Ancestry member –and submitted my DNA in 2008 but now in 2014 excluded from submitting DNA for further testing because I live in the UK—I would like to have my results explained further. I took the mtDNA and seem to have a run of 7 C’s with gaps on the reference person.??? I think DNA projects are excellent ways of extending are family tree research

  2. I do look forward to the new matching algorithms being applied to my accounts. I am one of those fortunate genetic genealogist who have tested both his parents. In working my matches between the three of us, I have discovered a lot of colonial pedigree collapse which has resulted in a bunch of deep autosomal DNA matches. It will be interesting to see how many of those the system says are IBS or IBD and how this will be handled by AncestryDNA. I truly hope it is a better launch than the new search engine on the website. I’m excited to have the new tool and hope to get it by the time my college DNA class starts in October, Not only have I been a 14 year subscriber to Ancestry, but I was one of the first atDNA testers. I’m a bit skeptical in the same vein as excited what this research will uncover. Make me a believer Julie. I hope to get that email real soon so I can check what I have against my six atDNA tests at Ancestry.

  3. Brian Palmer

    Fascinating! It’s exciting to be a part of this exciting research by submitting my DNA.

    So am I understanding correctly that submitting my parents DNA will help grow your reference data set?

  4. Byron Roff

    I did my AncestryDNA test and was extremely happy with the results. I was able to disprove two family stories, but proved a “brick wall” which I have been searching for the past 37 years.

  5. Does your blog have a contact page? I’m having trouble locating it but, I’d like to shoot you an e-mail. I’ve got some creative ideas for your blog you might be interested in hearing. Either way, great blog and I look forward to seeing it develop over time.

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