Thanks Jim. I think my last question reflected some (or much) of my ignorance as to how crossover works. This is something I am still fuzzy about and am rooting through various blogs and archives to find a selection of explanations for. Also I just ordered FF (having done it for both my paretns already) and am hopeful that when I begin phasing in about 6 weeks, this concept will begin to gel. Barton On Wed, Oct 09, 2013 at 05:28 PM, Jim Bartlett wrote: > Barton > > Whenever a Match has more than one shared segment, most of the time it > will be from the same Common Ancestor, but not always. Each IBD > segment should be treated in its own right - as if it came from any of > the Common Ancestors between the two. Often, with Colonial American > ancestry, Matches will actually share several Common Ancestors. > Usually the closest one will be the one who provided the shared > segment, but not always. That too is random. So keep your options > identified and open until you can find another Match on that segment > who agrees on the Common Ancestor = genealogy Triangulation. > > Your second question was how close do segments have to be to make it > likely they are from the same Ancestor? This is an unusual question > for me, because all segments from ancestors abut each other. One SNP > will be from one ancestor, and the next SNP will be from a different > ancestor. Somewhere on each chromosome there is a crossover point > (technically between two SNPs; in practice, the algorithms look at > blocks, so the actual crossover point is a little fuzzy). Actually, as > discussed before, on Chr 1, there are usually 3 such points (on Chr 20 > usually 1) that separate your grandparents on that Chr. So the much > smaller segments that are adjacent at that point are from much deeper > ancestors from your grand parents. Similarly all crossover points from > many generations will create segments from different ancestors. > Now if you have, say, a 20cM segment that was passed along several > generations, you my find Matches from the first part of that segment > (say 12cM) with a different Common Ancestor than Matches from the > other part (8cM). These two ancestors would be closer than usual, with > one ancestral to the husband and the other to the wife who provided > the 20cM segment. > In reality, of course, the DNA comes down from many distant ancestors. > But a way to look at it in the last paragraph, is like a branching > tree, starting with a main trunk from your father, and splitting into > say 4 large large branches (2 for each of your father's two > grandparents); and then in the next few generations each of those > branches splitting into two more branches. In practice each branch is > not split in half at each generation going back; instead, on Chr 1 for > example there will be an average of about 3 branchings each > generation. I need to make a picture of this with Kitty's mapper (for > her to show in a blog) to show that each segment is not divided at > each generation.... > Jim - Sent from my iPhone - FaceTime! > > On Oct 9, 2013, at 3:50 AM, Barton Lewis wrote: > >> Hi list, >> >> My aunt matches her fifth cousin on chr 17 as follows: >> >> 25354798 30027010 7.84 1249 33868089 47167537 12.67 2900 >> Is it likely these 2 sections are from the same ancestor? How close >> do >> segments have to be to make it likely they are from the same >> ancestor? What >> accounts for a section being "broken up" in this way? What's a good >> layman's way of understanding "how big is big" when talking about >> these >> kinds of break up on a chromosome? >> >> Thank you. >> >> Barton >> > > > > ______________________________ > For answers to Frequently Asked Questions about mailing lists, please > see: > http://dgmweb.net/MailingListFAQs.html > > > ------------------------------- > To unsubscribe from the list, please send an email to > AUTOSOMAL-DNA-request@rootsweb.com with the word 'unsubscribe' without > the quotes in the subject and the body of the message >