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Interpreting match -- can brothers be off by one number on one marker?

Interpreting match -- can brothers be off by one number on one marker?

Posted: 14 Mar 2013 4:40PM GMT
Classification: Query
Edited: 14 Mar 2013 8:14PM GMT
I have a match between my uncle and another man who was born 2 years prior to my uncle and put up for adoption. I believe they are half brothers, sharing the same father. They match on all markers except one: DYS 456 17 (my uncle) DYS 456 16 (possible half bro). Does this one number difference on one marker mean that they can't be brothers? Do they share grandfathers instead?

I'm attaching the comparison chart of the two DNA tests.

Thanks!
Attachments:

Re: Interpreting match -- can brothers be off by one number on one marker?

Posted: 14 Mar 2013 5:54PM GMT
Classification: Query
Mutations have to happen sometime, and when they do, brothers will be different. Very occasionally, brothers will even differ on more than one marker.

The best way to validate the relationship is with an autosomal test. Half-brothers share about 25% of their DNA on the average. 23andMe would be a good choice for this test, since you can see exactly where the brothers match.

Re: Interpreting match -- can brothers be off by one number on one marker?

Posted: 14 Mar 2013 6:29PM GMT
Classification: Query
thanks for the info ... one of the brothers was adopted without his knowledge. I don't know if we can get him to provide another DNA test. Based on the results that we do have, are there other explanations for the one marker difference? I mean, is it possible for two men to have that much of a match and NOT be brothers? No matter what, they share a male ancestor, correct? So if not the same father, probably the same grandfather?

Sorry I'm so lame on this ... I don't understand enough about how the DNA tests work. Thanks for your help.

Re: Interpreting match -- can brothers be off by one number on one marker?

Posted: 14 Mar 2013 6:54PM GMT
Classification: Query
If you tested with FTDNA, they can often run an autosomal ("Family Finder") test off the same sample. (I know they prefer that the autosomal test is done first, but it doesn't always have to be.)

Re: Interpreting match -- can brothers be off by one number on one marker?

Posted: 14 Mar 2013 9:38PM GMT
Classification: Query
Mutations are random events, so you can't predict when they will happen. The Y-DNA by itself can't tell you how many generations have passed since the common ancestor lived -- it could be one, or it could be a dozen.

But you weren't testing two people at random -- you had other reasons to suspect the two men were half-brothers, so you can say the results are consistent with that possibility.

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