DNA results are not always what they seem at first glance.
I was walking through the exhibit hall at the National Genealogical Society conference in Salt Lake City in 2010 when I came across Bennett Greenspan, President of Family Tree DNA, explaining the then-brand-new Family Finder autosomal DNA test. When he told me it could test DNA across genders — no more only being able to test a son of a son of a son or a daughter of a daughter of a daughter — I bought a test kit on the spot. I could practically retire on what I’ve spent on other family test kits since then… and the whole process has been one of the most exhilarating — and most frustrating — experiences I’ve had as a genealogist.
I’ve had enough successes that I understand the promise of autosomal testing.1 But I’ve also had what I personally consider to be more than my fair share of results showing a close relationship the other person and I can’t trace on paper. (I have such a bone to pick with the parents of my second great grandfather George Washington Cottrell (1821 Madison County, KY – 1891 Wichita County, TX), who didn’t even leave footprints as they passed through this world.)
I’ve learned that there are a lot of tools that can help us understand autosomal DNA results… and I’ve learned how very easy it can be to be misled by what looks — in some tools — like a very very close match indeed. Especially if you’re like me and only read the instructions after you’ve spun your wheels for a while…
Case in point: take a look at this screen capture of some of my results in a tool FTDNA calls the chromosome browser. The first thing to keep in mind is that each of the colors represents an area on each of the listed chromosomes where my DNA results match another person’s DNA results. So what we really have on the chart is results for four people: me; my older brother (in orange); my uncle (in blue); and my first cousin (in green).
The second thing to keep in mind is that these are very close relatives on this chart. Each of these three people matches me very closely, and we share a TON of DNA. On average, siblings share as much as 50% of their DNA; an uncle and niece will share as much as 25% of their DNA; and first cousins will share as much as 12.5%.2 The vast majority of folks I match don’t light up the chromosome browser like a Christmas tree the way these very close relatives do.
The third — and most important — thing to keep in mind about this chart is that it really doesn’t show what it looks like it does. Yes, it shows that my brother, uncle and cousin are very closely related to me. What it does NOT show is that my brother, uncle and cousin are very closely related to each other.
Despite those huge overlapping areas where the orange and blue and green all line up so nicely and neatly on this chart, the fact is, my brother is not related to my uncle and my cousin. That’s because he and I are both children of our father, but we don’t have the same mother. And the uncle on this chart is my mother’s brother; the cousin is the daughter of my mother’s sister. While my mother’s family has been in America since before the Revolution,3 my brother’s ancestors on both sides didn’t hit U.S. soil until the 1880s or later. Our father and grandparents arrived at Ellis Island on 6 February 19254 and his mother’s grandparents were all born in Sweden.5 No cross-pollination between the two sides at all.
Even some of the overlaps for my maternal uncle and cousin turn out to be areas where I match both but they don’t match each other. That can happen because of the way a “match” is calculated. Every area sampled has two paired markers called alleles. I could be AG in those areas, my uncle could be AA in those areas and my cousin GG. Since only half of the pair has to match me to be called a match,6 my A would match my uncle and my G would match my cousin — and they wouldn’t match each other at all.
So take a lesson from somebody who still has to be told, routinely, to RTFM.7 The chromosome browser is a great tool. But, like everything else, it has limitations that are explained in the frequently asked questions on the Family Finder test. We may all be related as descendants of some ultimate Adam and Eve… but maybe not as closely as the pretty colored chart makes it seem.
- Judy G. Russell, “Autosomal DNA testing,” National Genealogical Society Magazine, October-December 2011, 38-43. ↩
- Ibid., chart, 41. ↩
- November 1680, order as to Nicholas Gentry, York County, Virginia, Deeds, Orders, Wills (1677 – 1684) 6: 268; York County Microfilm reel 3, Library of Virginia, Richmond. ↩
- Manifest, SS George Washington, Jan-Feb 1925, p. 59 (stamped), lines 4-6, Geissler family, 4; “New York Passenger Lists, 1820-1957,” digital images, Ancestry.com (http://www.ancestry.com : accessed 6 Jan 2012); citing National Archive microfilm publication T715, roll 3605. ↩
- See 1920 U.S. census, Cook County, Illinois, Chicago, population schedule, enumeration district (ED) 2285, p. 250A (stamped), dwelling 7, family 7, Edward F. Anderson household; digital image, Ancestry.com (http://www.ancestry.com : accessed 21 Jan 2012); citing National Archive microfilm publication T625, roll 318. ↩
- See generally ISOGG Wiki, “Identical By Descent segment” (http://www.isogg.org/wiki/Identical_By_Descent_segment : accessed 21 Jan 2012). ↩
- Wikipedia (http://www.wikipedia.com), “RTFM,” rev. 2 Jan 2012. ↩