Go Back   Family Tree DNA Forums > Universal Lineage Testing (Autosomal DNA) > Family Finder Basics

Reply
 
Thread Tools Display Modes
  #1  
Old 19th March 2017, 05:53 PM
FionaCal FionaCal is offline
FTDNA Customer
 
Join Date: Jan 2017
Posts: 6
Would we all have the same common ancestor?

Hi all,

I've noticed on my list of matches that there are around 30 people, most of them in the first couple of pages out of my 600 matches, who not only all share the same segment of the same chromosome as me but they also all appear "in common with" each other.

Does this mean that all of us from that group share the same common ancestor?

Then what does it mean if some of the people in that group are an x-match and others aren't?

Intrigued!

Thank you!
Fiona
Reply With Quote
  #2  
Old 22nd March 2017, 09:09 PM
Germanica Germanica is offline
FTDNA Customer
 
Join Date: Apr 2014
Location: Colorado (PA at heart)
Posts: 268
Quote:
Originally Posted by FionaCal View Post
Hi all,

I've noticed on my list of matches that there are around 30 people, most of them in the first couple of pages out of my 600 matches, who not only all share the same segment of the same chromosome as me but they also all appear "in common with" each other.

Does this mean that all of us from that group share the same common ancestor?
Probably, but be careful the segment you all share doesn't cross the centromere. My understanding is shared segments that cross the centromere, even if they are long enough to normally register as IBD, have a higher chance of being IBS.

Quote:
Then what does it mean if some of the people in that group are an x-match and others aren't?
Just that some inherited X-DNA and some didn't. Maybe for some, it passed through more than one male in a row, meaning X-DNA couldn't have been inherited, but the other autosomal DNA was.
Reply With Quote
  #3  
Old 5th April 2017, 05:50 PM
FionaCal FionaCal is offline
FTDNA Customer
 
Join Date: Jan 2017
Posts: 6
Quote:
Originally Posted by Germanica View Post
My understanding is shared segments that cross the centromere, even if they are long enough to normally register as IBD, have a higher chance of being IBS.
Thanks for your reply. Sorry, what are IBS and IBD?
Reply With Quote
  #4  
Old 5th April 2017, 05:55 PM
keigh keigh is offline
FTDNA Customer
 
Join Date: Sep 2013
Posts: 450
Identical by descent (DNA that shares an ancestor) or identical by state (the DNA just looks similar with no ancestor involved) is how I've had IBD and IBS explained to me.
Reply With Quote
  #5  
Old 6th April 2017, 09:44 AM
John McCoy John McCoy is online now
FTDNA Customer
 
Join Date: Nov 2013
Posts: 460
What is the source of the opinion expressed earlier in this thread that matching segments that cross a centromere are more likely to be spurious? I don't see the logic in that statement.
Reply With Quote
  #6  
Old 17th April 2017, 11:07 PM
mabrams mabrams is offline
FTDNA Customer
 
Join Date: Mar 2006
Posts: 167
There is a Matrix command that you can use, and verify that the people are actually related to each other. You can only try 10 people at a time, but you can see if the idea that everyone is related is a reasonable idea. It doesnt verify that they are all related on the same chromosome region but it would seem plausible.

Another concern is that these people are what is called by some, such as AncestryDNA, a pile-up. FTDNA does not check for pile-ups. AncestryDNA removes these segments, some of which are probably valid, and then often removes the match designation.

A lot (most) of X matches are very tiny and you shouldnt pay much attention to them. I have matches with tiny Xs to my mother and larger autosomal matches to my father.
Reply With Quote
  #7  
Old 18th April 2017, 05:31 PM
Germanica Germanica is offline
FTDNA Customer
 
Join Date: Apr 2014
Location: Colorado (PA at heart)
Posts: 268
Quote:
Originally Posted by John McCoy View Post
What is the source of the opinion expressed earlier in this thread that matching segments that cross a centromere are more likely to be spurious? I don't see the logic in that statement.
I believe it's from Genome Mate, I recall reading that is why Genome Mate has an orange alert whenever a shared segment crosses the centromere, to alert you to the fact that you should be cautious regarding it.

This isn't from the creator but:

http://genealogypuzzlesdna.blogspot....ation-and.html

"When a segment crosses a centromere, there is a greater chance the segment is a mismatch or IBS (identical by sequence) rather than IBD (identical by descent). When this occurs, the ICW group on the graph will be an orange color to alert to the possibility."
Reply With Quote
  #8  
Old 19th April 2017, 10:14 AM
John McCoy John McCoy is online now
FTDNA Customer
 
Join Date: Nov 2013
Posts: 460
Interesting! But still not citing any evidence! I have a feeling there is a hole in the argument someplace. Yes, there is less recombination (per millions of base pairs) around the centromere, and fewer SNP's to work with, but the odds of multiple crossovers occurring within such a region seem way too small to present a significant confounding factor for the matching algorithms.
Reply With Quote
  #9  
Old 20th April 2017, 05:23 PM
Germanica Germanica is offline
FTDNA Customer
 
Join Date: Apr 2014
Location: Colorado (PA at heart)
Posts: 268
Quote:
Originally Posted by John McCoy View Post
Interesting! But still not citing any evidence!
If advanced genetics software like Genome Mate isn't evidence, I don't know what is.

Download the User Guide for Genome Mate: http://getgmp.com/Download/UserGuide.zip

Found on page 127, it says:

"Note: Genome Mate Pro considers a segment that crosses the centromere to be a “Potential False Positive”. This is noted on the Chromosome page with an orange question mark in the “Side” column."

Quote:
I have a feeling there is a hole in the argument someplace. Yes, there is less recombination (per millions of base pairs) around the centromere, and fewer SNP's to work with, but the odds of multiple crossovers occurring within such a region seem way too small to present a significant confounding factor for the matching algorithms.
Perhaps multiple matches does make it more legit, but I haven't seen any comments about that anywhere. I am just relaying what I've read. Note it says "potential false positive", not definite.

I'm not seeing any evidence from you to the contrary?

Last edited by Germanica; 20th April 2017 at 05:25 PM.
Reply With Quote
  #10  
Old 20th April 2017, 05:30 PM
Germanica Germanica is offline
FTDNA Customer
 
Join Date: Apr 2014
Location: Colorado (PA at heart)
Posts: 268
Here's more evidence, from FTDNA itself:

https://www.familytreedna.com/learn/...nt-centromere/

"How does the Family Finder program account for the centromere?

Our bioinformatics team treats centromeres as SNP Poor regions. We use a hard stop for matching across each centromere. This assures that DNA segments (blocks) are truly Identical by Descent (IBD) and not Identical by State (IBS). If you seem to have an IBD segment that crosses the centromere, we evaluate it as two segments, one on each side of the centromere."
Reply With Quote
Reply

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Estimating Common Ancestor Dave123 Y-DNA Haplogroups & SNPs Basics 2 4th May 2015 06:06 AM
Distance to common ancestor kaithu Family Finder Basics 3 20th January 2014 01:43 PM
Common ancestor without shared dna? marietta Family Finder Advanced Topics 5 8th April 2013 10:08 PM
Do we have a common ancestor? HapJ1 DNA and Genealogy for Beginners 15 27th November 2008 11:04 AM
closest common ancestor D3273 DNA and Genealogy for Beginners 3 22nd April 2003 02:28 PM


All times are GMT -5. The time now is 07:20 PM.


Family Tree DNA - World Headquarters

1445 North Loop West, Suite 820
Houston, Texas 77008, USA

Phone: (713) 868-1438 | Fax: (832) 201-7147
Copyright 2001-2010 Genealogy by Genetics, Ltd.
Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.