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  #1  
Old 25th June 2018, 03:06 PM
alice alice is offline
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Father and mother do not share children's match

I have a match that is clearly on my father's side based on other matches in common. Two of my three siblings share this match. Our mother does not match the person.

But, strangely, our father does not show a match with the person either. How is this possible? Could it be an error in FTDNA's display?

And I should add that we are definitely our parents' children based on the DNA test.
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  #2  
Old 25th June 2018, 06:32 PM
prairielad prairielad is offline
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What is the longest segments size?

FTDNA has a set criteria to be deemed a match.
One must share at least 20cM in total shared DNA.
This total also needs to have at least one longest segment of 7cM / 500 SNP or greater
Total includes all segments down to 1cM / 500 SNP
Note most segments under 7cM have a high rate of being false positive segments.

Failing to meet above criteria, if total shared DNA is under 20cM then if there is a longest segment of 9cM / 500 SNP or greater one will also be declared a match.

So if your longest segment is under 9cM, this could be the reason match is not showing on parents match list. Segment is likely present in parent but there is not enough smaller segments to give a total shared dna of 20cM or greater.

If you and match utilize gedmatch you will probably see same shared segment in parent.
Gedmatch only uses the matching criteria of 7cM / 500 SNP by default. One has the ability to lower that threshold if they so wish (not recommended unless looking at a specific location or segment. Cross referencing )

Last edited by prairielad; 25th June 2018 at 06:34 PM.
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  #3  
Old 26th June 2018, 11:02 AM
alice alice is offline
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Thanks for your reply, but I still don't get it.

The match is not on Gedmatch. The longest segment for each of the three siblings is 8 cM. Total matching is 39 cM for one sibling and 40 cM for the other two.
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  #4  
Old 26th June 2018, 11:21 AM
Frederator Frederator is offline
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Yeah, this only happens with very small segments. Eight (8) cM is right on the border of significance.

The short answer is that short segments are unreliable.

The long answer, which that other fellow was giving you, involves some math, to prove out exactly how close to the system's threshold of significance this match is.

For some people, the long answer isn't worth it to them. Knowing the precise math is interesting to a lot of people, but not so much to others.

In any event, it doesn't change the fundamental situation: small segments (i.e, around 8 cM) are not reliable.
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  #5  
Old 28th June 2018, 10:30 AM
JSW JSW is offline
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Just FYI the longest segment I have found to
be unreliable was 14.7 cM
and at 7.8 cM about 50% of the so called matches are
just not real.
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  #6  
Old 28th June 2018, 11:04 AM
Biblioteque Biblioteque is offline
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I have always agreed with the comments above about the unreliability of small segments, and like to work with segments over 12-15 cMs.

Although, yesterday at Ancestry, I had a tree match and we shared ggg grandparents. Her tree and my tree are well documented.
Of course, this is only a tree match and since she is not at Gedmatch, I cannot do segment matching to prove the match.

But, I only share 6.5 cMs with her! Just the randomness of DNA?

P.S. And, she only had 38 people in her tree.

Last edited by Biblioteque; 28th June 2018 at 11:16 AM.
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  #7  
Old 28th June 2018, 12:20 PM
John McCoy John McCoy is online now
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Yes, it is possible, or even probable, to encounter a 4th cousin with whom you share no significant segments (over 7-10 cM, depending on the vendor and the matching algorithm they use). The rule of thumb that is current in the genetic genealogy community says about 10% of 3rd cousins are not detected as matches using the normal default parameters, and about 50% of 4th cousins as well. I have examples of these among my own matches.

However, you should see that your 3rd and 4th cousins match at least SOME of your other relatives at the expected level, so that you know they really are your relatives! For me, I found that the 3rd cousins that didn't match me, still matched at the 3rd cousin level with all or most of the other known mutual 3rd cousins who had been tested. It is indeed the "luck of the draw" due to recombination and the random assortment of homologous chromosomes at each generation.

One consequence of this observation is that there are probably some real 4th or even 3rd cousins among my distant matches that I haven't noticed, because they share only a tiny amount of DNA, and because they have not posted their family trees. Sufficient reason to check occasionally for strong matches to my known 3rd cousins!
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  #8  
Old 28th June 2018, 12:50 PM
LHolt-Galvin LHolt-Galvin is offline
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Quote:
Originally Posted by alice View Post

But, strangely, our father does not show a match with the person either. How is this possible? Could it be an error in FTDNA's display?

And I should add that we are definitely our parents' children based on the DNA test.
I had a similar problem with my father's matches. I tried explaining it many times to people here, but everyone kept telling me that I didn't understand how DNA worked.

On my match list for FF, if I click on my father's name and then choose in common with, there is a person who shows up, but when I go to my dad's profile and look for that person, they do not show up.

I don't know what causes this, but the largest segment is 12.82cM and there is 3cM showing on the x chromosome. Perhaps the amount on the x is pushing the total up enough to show as a match to me.
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  #9  
Old 28th June 2018, 01:26 PM
Biblioteque Biblioteque is offline
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John McCoy: Thank you for your insight.

Also, at Ancestry I have another 115 dna matches with whom I share tree matches and with a range of 6.0-6.5 cMs.

They are 3rd, 4th and 5th cousins and a few beyond. I have not looked at their documents so do not know how reliable the matches are. I just saved them for whatever in the future......maybe a Chromo Browser at Ancestry?
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