Jim, Perhaps the link below to Wikipedia will help with your question. http://en.wikipedia.org/wiki/Centimorgan  I wasn't able to quickly find two matches I have at 7.7 cM, but I found two matches at 8.03 cM.  Hopefully, these matches will make the point.  One is on chr 20 and the other is on chr 2.  The position numbers are shown, followed by the number of cM and the number of SNPs in the segment.  Finally, the number of bases is shown.  You can see that the number of bases on chr 2 is twice the number on chr 20 even though the cM is the same.  The number of SNPs per million bases drives the cM calculation.  So the chr 20 segment has less diversity than the chr 2 segment (which is what the cM captures). 20 38171398 43337995 8.03 1500      5,166,597 2 56186100 66550469 8.03 2400    10,364,369 Regards, Larry ________________________________ From: Jim Bartlett <[email protected]> To: Larry Vick <[email protected]>; "[email protected]" <[email protected]> Sent: Friday, January 6, 2012 10:00 PM Subject: Re: [AUTOSOMAL-DNA] Subject: How do you work a 5 way FF match with few clues ? Larry Your explanation is excellent, and the light bulb came on - thanks for your brevity and clarity. Now - that explains one location. How many ACTGs are there in 7.7cM? I thought the point was that with a long segment we could rely on the fact that it was an exact match. I guess the point is, in a way, it is an exact match. So what's the probability of another segment of exactly the same long length being an exact match, just with a scrambled up/different arrangement of ACTGs? If in fact groups of 3, 4 or 5 matches turn out to not be related, then the odds must be high. Although I understand how the matching is done, and how it can create many different combinations from the same segment, it still seems ... unusual? ... that these will match several others from the relatively small community of folks who have taken this test. Jim - Sent from my iPhone - FaceTime! On Jan 6, 2012, at 5:14 PM, Larry Vick <[email protected]> wrote: > Jim, > > If you have AG at a location and you have two matches one of whom has AA and the other has GG, you will  match both, but they will not match each other.  FF has no way to know which of your parents you inherited the A from and which you inherited the G from. > > Regards, > > Larry

One correction -- the number of SNPs is not driving the cM calculation. The cM unit is empirically derived by looking at cross-over points in a large dataset of father/mother/child/extended families. It's not a fixed quantity -- it varies somewhat depending on the dataset (e.g. Iceland vs CEU), and there is a big difference between males and females (so sex-averaged values are used). It also varies over the genome, as your example of a 5 and 10 Mb segment shows. Ann Turner On Sat, Jan 7, 2012 at 5:49 AM, Larry Vick <[email protected]> wrote: > Jim, > > Perhaps the link below to Wikipedia will help with your question. > > http://en.wikipedia.org/wiki/Centimorgan > > > I wasn't able to quickly find two matches I have at 7.7 cM, but I found > two matches at 8.03 cM. Hopefully, these matches will make the point. One > is on chr 20 and the other is on chr 2. The position numbers are shown, > followed by the number of cM and the number of SNPs in the segment. > Finally, the number of bases is shown. You can see that the number of > bases on chr 2 is twice the number on chr 20 even though the cM is the > same. The number of SNPs per million bases drives the cM calculation. So > the chr 20 segment has less diversity than the chr 2 segment (which is what > the cM captures). > > 20 38171398 43337995 8.03 1500 5,166,597 > 2 56186100 66550469 8.03 2400 10,364,369 > > Regards, > > Larry >

Ann, Thanks for the correction.  I have tried to learn from your postings and those of others like Tim JANZEN, but I still have more to learn.  I am still thinking about Tim's revelation (at least for me) that there is more significance to have a child share a match with me than I had previously thought. If only I had appreciated my high school biology class then the way I would today I might have done better than the "D" I got.  I only got a "D" because I was allowed to do extra credit. Regards, Larry ________________________________ From: Ann Turner <[email protected]> To: [email protected] Sent: Saturday, January 7, 2012 10:16 AM Subject: Re: [AUTOSOMAL-DNA] Subject: How do you work a 5 way FF match with few clues ? One correction -- the number of SNPs is not driving the cM calculation. The cM unit is empirically derived by looking at cross-over points in a large dataset of father/mother/child/extended families. It's not a fixed quantity -- it varies somewhat depending on the dataset (e.g. Iceland vs CEU), and there is a big difference between males and females (so sex-averaged values are used). It also varies over the genome, as your example of a 5 and 10 Mb segment shows. Ann Turner