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  #1  
Old 18th July 2017, 07:05 PM
MoberlyDrake MoberlyDrake is offline
mtDNA: T2b5 | Y-DNA: J-M172
 
Join Date: May 2010
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Siblings - Matches In Common

My cousin has 594 matches. Of that number she shares only 134 with her full brother. Isn't that an uncommonly low ratio for shared matches between siblings? Her brother has a total of 534 matches.

The sister shares 105 matches with her aunt, and the brother shares 120.

With their uncle, the sister shares 44 matches, the brother shares 36.

With me (their double cousin), the sister shares 67 and the brother shares 95.

It just seems like the number of matches they share with each other is very low.
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  #2  
Old 19th July 2017, 10:37 AM
John McCoy John McCoy is offline
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Join Date: Nov 2013
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If the matches are weak (in the 4th cousin range, for example), your observation would be less significant than if the matches are stronger. However, the most meaningful comparison would be that of the two siblings with each other, particularly using a tool that can distinguish between full and half siblings, such as the One-to-One comparison tool on GEDmatch with the graphics option.
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Old 19th July 2017, 02:04 PM
MoberlyDrake MoberlyDrake is offline
mtDNA: T2b5 | Y-DNA: J-M172
 
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Oh, they are really full siblings! The amount of DNA they share with each other (2,575 cM) and with me (their double cousin), and with my parents (the aunt and uncle in my post)proves that without the slightest doubt.

I just think it must be rather unusual for siblings to have so few matches in common. To have 120 matches in common with your aunt and only 134 in common with your sister seems a bit strange.

Last edited by MoberlyDrake; 19th July 2017 at 02:09 PM.
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  #4  
Old 19th July 2017, 05:17 PM
ech124 ech124 is offline
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Join Date: Jul 2011
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Your stats made me curious. About 5 years ago, I studied several sets of siblings and the match patterns. There was some variation of course, but very generally the pattern was that siblings would share about two thirds of their FTDNA matches and one third was unique.

I just revisited this study and found the match pattern has flipped. Those same siblings now show one third in common and two thirds are unique.

Nothing has changed about the DNA of the siblings, but the FTDNA database has obviously changed significantly. My new theory is that there have been a massive influx of transfer kits. These transfers have altered the nature of the database, because a lot of these kits have match cutoffs at about the third cousin level. Because these transfer kits are not subject to complete matching (just partial matching), you cannot make apple to apple comparisons when you generalize about overall matching numbers. It is extremely easy to fall on the wrong side of the cutoff point at a third cousin level.

I am not overly concerned with the transfers because, in my opinion, having additional partial matching is better than not having the additional kits in the database.

Last edited by ech124; 19th July 2017 at 05:19 PM. Reason: grammar
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  #5  
Old 20th July 2017, 02:44 PM
jimbirk jimbirk is offline
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My full brother and I have both tested at FTDNA.

I have 917 matches, he has 704 matches and we share 254 matches (36%).

As for the difference in number of matches I have 3 pileup zones with over 120 matches and my brother has no pileup zones.

No other close relatives have tested.
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  #6  
Old 20th July 2017, 04:15 PM
MoberlyDrake MoberlyDrake is offline
mtDNA: T2b5 | Y-DNA: J-M172
 
Join Date: May 2010
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Pileup zones may be part of the problem. The brother has one with about 140 matches and the sister has one with about 183 matches, only in her case, it looks like a few may be matching on the chromosome she received from her Italian mother.
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