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Archiver > GENEALOGY-DNA > 2000-11 > 0975349013


From: Terry Carmichael <>
Subject: Re: [DNA] The X chromosome
Date: Mon, 27 Nov 2000 10:16:53 -0800
References: <c0.c3d5c02.2753fb7e@aol.com>


It would not affect the DNA testing because both X chromosomes are present, and
the process of denaturing and fragmenting the DNA will disrupt the Barr Body
(inactive X chromosome).

However, you will not be able to determine which X is maternal and which is
paternal unless you had the mother or the father available for testing.

Another note, crossing over does happen between the Y-chromosome and the
X-chromosome! Just to add another twist for discussion. Don't worry, though. The
region on the Y that are being compared for genealogical purposes are conserved,
and don't cross over with the X.



wrote:

> In a message dated 11/26/00 9:16:30 AM Pacific Standard Time,
> writes:
>
> > I read somewhere that actually the Xchromosome from the male dominates the
> > character of the female child just as his Ychromosome would have made the
> > child a male. Certain characteristics come from the male through the X or
> Y
> > chromosome and the contribution from the female is depressed. I'll have to
> > look for the source of this idea.
>
> My genetics textbook (Concepts of Genetics by William S. Klug and Michael R.
> Cummings, Prentice Hall 1997) talks about "dosage compensation." That may be
> what you're thinking of. Females have two X and males have one, so females
> have a double dose of some genes. That could have adverse effects (Down
> syndrome is caused by an extra chromosome 21).
>
> But there is a solution. Quite early in development, one of the X chromsomes
> is inactivated. This is a random choice, and some cells will inactivate the X
> from the father and other cells will inactivate the X from the mother. The
> "descendants" of those cells inactivate the same X. This leads to a "mosaic"
> as illustrated by female calico cats, where coat color is an X-linked trait.
>
> I'm not sure how this would affect DNA testing of X markers for genealogical
> purposes.Would blood samples or buccal swabs pick up examples of both kinds
> of cells and pool them? Or can markers be read from inactivated chromosomes?
> If so, we would still be able to detect maternal and paternal contributions.
>
> ==============================
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--
Terry Carmichael

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