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Archiver > GENEALOGY-DNA > 2002-12 > 1040163725


From: "pwreed" <>
Subject: Re: [DNA] Ancestors are bred out?
Date: Tue, 17 Dec 2002 22:22:05 -0000
References: <20021217095806.1134.h017.c000.wm@mail.jarman.net.criticalpath.net>


----- Original Message -----
From: <>
> Although each parent contributes 50%, I don't think any
> biological rules will force that contribution to be of
> equal amounts from each grandparent. Inheritance can be
> heavily skewed towards one and the evidence of the
> other will appear in less than equal proportions.
> Eventually the genetic traces of an ancestor can be
> bred out entirely. Thus it seems these composit tests
> cannot be relied upon to tell us about ALL our
> ancestors.

I agree that there are no biological rules that force any equal
contribution from each grandparent, but because recombination must
occur on all chromosomes (and often occurs multiple times per
chromosome) when egg or sperm cells are formed, the egg or sperm cells
must contain a contribution from both parents for every chromosome
(although X and Y chromosomes in sperm are a special case). This means
that the contribution from each grandparent does not stray that much
from the average value of 25%. I can't quantify this precisely but I
would not agree that inheritance can be heavily skewed, not across the
entire genome. Do you know of examples where this has been found?

As far as genetic traces being bred out entirely, the link that Annie
provided gives calculations of this. there are a lot of assumptions
made in the calculations and the author gives various scenarios but
the sort of thing he comes up with is:

"In other words, more than 13 generations back, the prob. of
inheriting no DNA at all from one of your ancestors is more than 1/2.
Note that at 13 generations back (c. 1500s - 1600s) you have 8192
ancestors. "

http://computing.dcu.ie/~humphrys/FamTree/Royal/ca.genetic.html#dna.mr
ca

The assumptions may be wrong so he concludes "that within historical
times (3000 BC to 2000 AD) you have ancestors from whom you have
inherited no DNA"

So I think if you are concerned about ancestors dating back >500 years
then DNA (other than Y or mtDNA) may not be of 100% utility (may not
detect ALL ancestors). However you did inherit DNA from some ancestors
going back "forever" and even one ancestor out of thousands who
contributed Y or mtDNA DNA still fascinates most readers of this list,
so why not some (but not all) of your other ancestors? For periods
less than 500 years (and probably much further back) the potential for
DNA is great (potential to detect ALL ancestors). My guess is that
there are thousands if not tens of thousands of ancestry-specific
markers (like the AncestrybyDNA ones) that have yet to be well
characterised, so the AncestrybyDNA test is an incredibly "blunt"
tool. The main problem right now is the very low number of markers
analysed. The technology to analyse 1000's of markers for around $1
per marker has been around for a few years (I have personal
experience), its just that these tools have been developed for medical
research and the anthropological folks don't tend to have the same
funds or market to develop these "high-throughput" tools for human
ancestry as fast as the medical folks.

Peter Reed



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