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Archiver > GENEALOGY-DNA > 2014-01 > 1390023058


From: Jim Bartlett <>
Subject: Re: [DNA] Help with autosomal dna, please
Date: Sat, 18 Jan 2014 00:30:58 -0500
References: <CAJWb4BR4XSF3dw8uzuu8MA2tijX0Qss6=QreFrk7ztumb6pOPA@mail.gmail.com><1056059348.576988.1390019029232.JavaMail.root@sz0101a.emeryville.ca.mail.comcast.net><00a601cf1408$947911d0$bd6b3570$@com.au>
In-Reply-To: <00a601cf1408$947911d0$bd6b3570$@com.au>


Actually the theory is correct - exact in fact. You do get exactly half of your atDNA from each parent. And each parent does this by giving you exactly half of their own atDNA. Each parent has two sets of chromosomes 1-22; they each give a child one set of chromosomes 1-22; and then the child has two sets of 1-22 - just like every human. The issue is that each chromosome is made up of random segments from their ancestors - at many levels. And each chromosome that is passed to a child is made up of big parts from the two chromosomes the parent got from their parents. This process is recombination and at each generation a few segments from the preceding generation are subdivided (usually from 0 to 3 subdivisions for each chromosome for each generation) Since there are relatively few subdivisions (crossover points) in each generation, most of the smaller segments (but still over 7cM) are not subdivided. So many pass from parent to child for a generation (or few) about the same !
size.

I used to call these sticky segments, but I now realize they are the result of a natural process and are actually to be expected.

Hope this helps in understanding why generation estimates based on segment length cannot be accurate.

Jim - Sent from my iPhone - FaceTime!

> On Jan 17, 2014, at 11:48 PM, "Belinda" <> wrote:
>
> It is very dangerous to accept the "predicted" degree of relationship exactly. This is partly because the theory says that a person is most likely to inherit exactly half the DNA from each parent at each generation. But it can just as easily be 100 percent, zero percent, or some fraction in between. The range is a better indication, but even that can be wrong. Inheritance is far more random in practice than the average expectation. Don't accept the "more reliable" predictor, unless it agrees with other independent evidence.
>
> Belinda
>
> -----Original Message-----
> From: [mailto:] On Behalf Of Vance Wiley
> Sent: Saturday, 18 January 2014 3:24 PM
> To:
> Subject: [DNA] Help with autosomal dna, please
>
> Hello, all.
>
> I need some guidance in defining a common ancestor suggested thru autosomal testing.
>
> Two cousins and I have data on Gedmatch. We share a common surname. Two of the kits are mother/daughter.
>
> Gedmatch reports 3.9 generations to a common ancestor for me and the mother, and 4.1 generations t o a common ancestor for me and the daughter. C ouldn't it just have been one digit different and make it obvious?
>
> I understand that dna in the 22 chromo somes is not passed on exactly equal but which of the reported generations are more reliable for focusing on a common ancestor?
>
> I f I round off the 3.9 to 4, the common ancestor is - one to my father, two to my grandfather, three to my great grandfather (identified), and four to my great great grandfather (not identified).
>
> 4 generations from the mother's 'viewpoint' is her great great grandfather also.
>
> If I round off the daughter's results from 4.1 to 4, my great grandfather (3 generations from me) becomes the common ancestor from the daughter's 'viewpoint' and he is the daughter's great great grandfather - and only her mother's great grandfather( 3 generations from the mother) , NOT her mother's great great grandfather. Say what? Is it safe to round UP to 5 generations for the daughter to make this work ? Am I caught in some kind of a 'once removed' thing? Maybe a time warp?
>
> Some comments would be really appreciated.
>
> Thank you,
>
> Vance Wiley
>
>
>
>
>
>
>
> Kit numbers are F295448, A574837, and A277626 on Gedmatch.
>
>
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