How do you use autosomal DNA testing to enhance your genealogical research without having to take a PhD level course? This is a question several of my cousins have asked me, so here is my attempt to answer.
- First of all, get as many relatives to test as you can. The more data you have, the easier it is to make useful comparisons and sort new DNA relatives into their related family lines. The closer family members shown in my chromosome map above are all cousins I convinced to test.
- Secondly, make sure you know how to use a spreadsheet: sorting, deleting rows, inserting columns; you only need to know the easy stuff.
- Third, check your understanding of how DNA works. Perhaps read my basics page – http://blog.kittycooper.com/2013/04/the-basics-at-23andme/ – and follow up with whichever lessons, books or videos appeal to you among those that I suggest or that you find listed in the ISOGG wiki – http://www.isogg.org/wiki/Autosomal_DNA
Also lots of beginner questions are answered in the FAQ I keep for the DNA-NEWBIES mailing list on yahoo; a copy is on this blog – http://blog.kittycooper.com/dna-testing/newbie-faq/ – so check there when something is confusing.
- Fourth, bookmark a page with an explanation of all the acronyms – this is a good one: http://www.isogg.org/wiki/Abbreviations – a key term is cM. You do not need to understand the definition of a centimorgan (cM), hardly anyone does; just accept that it is the best measure of the importance of a DNA match, the larger the better.
Now to the practical application of all this, using shared DNA segment data to find relatives, preferably those 3rd and 4th cousins that your family no longer knows of. Click on success stories on this blog to read about some of the cousins I have found with DNA.
When people are shown as matches to you or your relatives, the testing company will make a guess as to how closely related they are. Beyond 3rd cousins, it is not possible for them to make an accurate estimate because of the random nature of DNA inheritance.
If the expected relationship ends in distant cousin, the common ancestor is likely too far back to find, unless both of you have deep trees (on GEDmatch this would apply to anyone greater than 5 or even 4.5 generations distant). So I recommend only contacting the more distant folk when you see a common surname or common locality. The reason is that once you get past 3rd cousins, the amount of DNA you share with relatives can be fairly random or even none due to the vagaries of DNA inheritance.
See the chart and tables at the ISOGG wiki for the expected amount of DNA shared with relatives
To find possible relatives you want to look at how much DNA, how many segments, and how many big segments you share with your matches. The most promising matches are those with more than one segment and preferably at least one of those segments larger than 10cM. Third cousins will usually match you with one or more large segments but 10% of the time they will not match you at all and about half your fourth cousins will share no DNA with you unless you come from an endogamous group. If your ancestors were endogamous, raise the guidelines above and read my post on Ashkenazi DNA.
Another problem is that sometimes when you share two good sized segments (or one very large one which could be two next to each other), they come from two different ancestors, so the close cousin prediction is wrong.
- At 23andme you go to Tools, DNA relatives, DNA. See http://blog.kittycooper.com/2015/11/initial-report-on-the-new-23andme-for-the-more-advanced-users/ (but if you still have the old version you go to ancestry tools, family inheritance advanced: see http://blog.kittycooper.com/2013/04/the-basics-at-23andme/ )
- At Family Tree DNA. you use the chromosome browser, see
- At GEDmatch you use the one-to-one see
One proviso, if person A matches you at the same place as person B, they do not necessarily match each each other. That is because every chromosome is a pair and the tests only know the spot where you match not which of the two chromosomes it is on: the one from your mom or the one from your dad. Even worse sometimes a person listed as a match is not really matching you at all, but rather matching bits from each parent which looks like a match to the computers. See this post for more on false matches: http://blog.kittycooper.com/2014/10/when-is-a-dna-segment-match-a-real-match-ibd-or-ibs-or-ibc/
Triangulation, when person matches you and another at the same location, is how you confirm a real match. Having a parent and child tested can really help with this. For further reading on triangulation try this blog post: http://blog.kittycooper.com/2015/02/triangulation-proving-a-common-ancestor/
Here is my suggested further reading on finding relatives with DNA: http://blog.kittycooper.com/2013/01/finding-distant-relatives-with-autosomal-dna-testing/
Also you can click the tag DNA spreadsheets to go even deeper into that topic. Plus I have many tools including my own listed on my tools page. The picture above is from my segment mapper tool; I used the track column to move the matches around so it looked better.
If you are an adoptee please go to DNAadoption.com and check out their methodology.
Here are some statistics on my own matches with the X removed (since it is so different between the sexes) . These are some of the matches shown in the chromosome picture above. I do have a 4th cousin once removed (maria) who I share 33.6 cM on the X with but no autosomal DNA. I know it is a good match not just from the size but because my Dad has the same match.
|Known Relationship||total cMs > 7||Longest Block||# of segs > 7|
|Triple 4th 1R||27.90||19.10||2|
|5th + half 6th||17.57||17.57||1|
|Half dbl 3rd cousin||93.60||30.70||5|
Those of you curious about what I mean by a half double 3rd cousin, we share 3 gg-grandparents; my g-grandmother fixed up her sister with her husband’s half brother.