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Thread: Big Difference in cMs Reported

  1. #1
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    Question Big Difference in cMs Reported

    I recently recruited an AncestryDNA match of mine for y-dna testing with FTDNA, but he also ordered Family Finder. At Ancestry the man matches me on only 9.2cMs across 1 segment. On Family Finder, however, he matches me on 31 cMs, with the longest block being 16 cMs.

    That's quite a difference.

    What gives?

    The man also matches my dad, my youngest son, my youngest daughter, and a 3rd cousin of mine at Family Finder, which is a good sign that he is related to me the way I believe he is (on the y-dna line), but the difference in reports from Ancestry to FTDNA is kind of startling.

    His y-dna test results are still pending.
     


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    Y-DNA: R1b-FGC36981 (L21> DF13> Z39589> CTS2501> Z43690> Y8426> BY160> FGC36974>FGC36982 >FGC36981)

    Additional Data:
    Lactase Persistent:
    rs4988235 AA (13910 TT)
    rs182549 TT (22018 AA)

    Red Hair Carrier:
    Arg160Trp+ (rs1805008 T) aka R160W

    Dad's mtDNA: K1a1

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  3. #2
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    Quote Originally Posted by rms2 View Post
    I recently recruited an AncestryDNA match of mine for y-dna testing with FTDNA, but he also ordered Family Finder. At Ancestry the man matches me on only 9.2cMs across 1 segment. On Family Finder, however, he matches me on 31 cMs, with the longest block being 16 cMs.

    That's quite a difference.

    What gives?

    The man also matches my dad, my youngest son, my youngest daughter, and a 3rd cousin of mine at Family Finder, which is a good sign that he is related to me the way I believe he is (on the y-dna line), but the difference in reports from Ancestry to FTDNA is kind of startling.

    His y-dna test results are still pending.
    I've also noticed differences between tests that are significant, but not of that magnitude.
    Living DNA Cautious mode:
    South Wales Border-related ancestry: 86.8%
    Cornwall: 8%
    Cumbria-related ancestry: 5.2%
    Y line: Peak District, England. Big Y match: Scania, Sweden; TMRCA 1,280 ybp (YFull);
    mtDNA: traces to Glamorgan, Wales, 18th century. Mother's Y line (Wales): R-L21 L371

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  5. #3
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    I'm currently keeping track of differences and was planning a post in my thread in the Genealogy section about it, but that's a huge difference. IME, Ancestry tends to have lower matches (which I assume is due to their supposedly correcting for IBS/IBC through Timber), but that one is a real miss.

    I haven't read this in any detail, but Ancestry: https://www.ancestry.com/corporate/s...hite-Paper.pdf

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  7. #4
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    Quote Originally Posted by rms2 View Post
    I recently recruited an AncestryDNA match of mine for y-dna testing with FTDNA, but he also ordered Family Finder. At Ancestry the man matches me on only 9.2cMs across 1 segment. On Family Finder, however, he matches me on 31 cMs, with the longest block being 16 cMs.

    That's quite a difference.

    What gives?

    The man also matches my dad, my youngest son, my youngest daughter, and a 3rd cousin of mine at Family Finder, which is a good sign that he is related to me the way I believe he is (on the y-dna line), but the difference in reports from Ancestry to FTDNA is kind of startling.

    His y-dna test results are still pending.
    I've seen this sort of difference multiple times, but one quick example I can give is for a 3rd cousin/3rd cousin once removed. By "3rd cousin/3rd cousin once removed", I actually don't mean to say that he's one or the other of these possibilities. Rather, he's both at once. I'm related to him through both of his parents.

    Anyway, Ancestry reports that we share 48 cM across 5 DNA segments. 23andMe reports 54 cM in 5 segments, with a longest segment of 18.84 cM. FTDNA says the total sharing is 74 cM, and the longest block is 19 cM. So "longest block/segment" is pretty much the same for both 23andMe and FTDNA.

    Here's the thing, though. I only get to FTDNA's 74 cM if I count all reported blocks -- some of which are very small. (The smallest is 1.10 cM, which is really too small to take seriously.) If I only add up blocks of 5 cM or more, then I only get to about 58 cM -- which is reasonably close to 23andMe's number.

    I really hate Timber with a passion. It eliminates quite a bit of genuine matching, in my opinion. Plus it's presumptuous. It represents Ancestry's decision that they know better than the customer. Sometimes maybe they do, but you can't possibly evaluate what you aren't even permitted to see. Show me all the matching, Ancestry -- certainly at least the sum of all segments of at least 7 cM -- and let me decide how "real" they might be.

    The fact is, with matches that Timber may find "suspicious" because of how much matching there is, it might simply be that certain lines really were in fact that prolific. For example, I'm in a DNA Circle with over 400 members. The head of the circle had at least 20 children by 4 different women, so of course there may be many customers who share some of his DNA with each other. But it's genuine sharing, even if there's what some people might consider an "excessive" amount of it.
    Besides British-German-Catalan, ancestry includes smaller amounts of French, Irish, Swiss, Choctaw & possibly Catawba. Avatar picture is: my father, his father, & his father's father; baby is my eldest brother.

    GB

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  9. #5
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    Yes, I've seen this happen to me before. I have a match who is on FTDNA, 23andme and Ancestry. On FTDNA and 23andme, I match the person at 95 and 104 cM respectively. On Ancestry, I match this same person at 13 cM. Very strange!

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  11. #6
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    Quote Originally Posted by msmarjoribanks View Post
    I haven't read this in any detail, but Ancestry: https://www.ancestry.com/corporate/s...hite-Paper.pdf
    Thanks for the link. That paper is completely insane. It reinforces my reasons for not purchasing AncestryDNA kits.

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    Quote Originally Posted by C J Wyatt III View Post
    Thanks for the link. That paper is completely insane. It reinforces my reasons for not purchasing AncestryDNA kits.
    I wouldn't write off Ancestry though. I did it recently after completing the other big vendors first, and it's provided some of my best matches. I've even found a second cousin once removed who is about to mail me letters from my grandfather and information on distant lines that I didn't know existed. I think the size of their database makes it well worth the price of the test.
    Living DNA Cautious mode:
    South Wales Border-related ancestry: 86.8%
    Cornwall: 8%
    Cumbria-related ancestry: 5.2%
    Y line: Peak District, England. Big Y match: Scania, Sweden; TMRCA 1,280 ybp (YFull);
    mtDNA: traces to Glamorgan, Wales, 18th century. Mother's Y line (Wales): R-L21 L371

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    I agree with JonikW, all the usable matches I have I got from Ancestry due to their overwhelming superiority in database size and so far I've got nowt useful from 23andMe. The FF db is way too small to be of use to me.
    YSEQ:#37; YFull: YF01405 (Y Elite 2013)
    GEDMatch: A828783 (autosomal DNA), 9427684 (GEDCOM) for segment matching DO NOT POST ADMIXTURE REPORTS USING MY KIT
    WGS (Full Genomes Nov 2015, YSEQ Feb 2019, Dante Mar 2019) - further WGS tests pending ;-)
    Ancestry GCs: Scots in central Scotland & Ulster, Ireland; English in Yorkshire & Pennines
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    I have big differences between FTDNA and Ancestry as well. One of my matches on my mother's side matches me 42cm - largest segment 10cm on FTDNA and he only shows as 6.9cm on Ancestry. He's related to us through a very prolific side of the family, through a 3x great grandfather who had 23 children with 4 different wives. Same is the case for our shared matches, massive difference between Ancestry and FTDNA.
    3/4 European, 1/4 Mauritian Creole. Genealogy enthusiast and Wow nerd.

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  19. #10
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    Quote Originally Posted by JonikW View Post
    I wouldn't write off Ancestry though. I did it recently after completing the other big vendors first, and it's provided some of my best matches. I've even found a second cousin once removed who is about to mail me letters from my grandfather and information on distant lines that I didn't know existed. I think the size of their database makes it well worth the price of the test.
    That's good advice, and their web site and its tools are much easier to navigate than what FTDNA has set up for Family Finder, with the exception of the Chromosome Browser FTDNA has, which Ancestry completely lacks.

    I would not have known about this man, who shares my surname and is likely a 5th cousin on that line, without Ancestry. All the same, Ancestry just barely snagged him, and I'm glad I got him over to FTDNA for a more fulsome look. Things would be easier, too, if Ancestry did y-chromosome testing.

    Thank God for this find, however, and the fact that he was willing to buy a 111-marker y-dna test and Family Finder from FTDNA.
    Last edited by rms2; 12-21-2018 at 12:19 PM.
     


    Hidden Content


    Y-DNA: R1b-FGC36981 (L21> DF13> Z39589> CTS2501> Z43690> Y8426> BY160> FGC36974>FGC36982 >FGC36981)

    Additional Data:
    Lactase Persistent:
    rs4988235 AA (13910 TT)
    rs182549 TT (22018 AA)

    Red Hair Carrier:
    Arg160Trp+ (rs1805008 T) aka R160W

    Dad's mtDNA: K1a1

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