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Archiver > GENEALOGY-DNA > 2004-01 > 1075041866


From:
Subject: Re: [DNA] Charlemagne's DNA?
Date: Sun, 25 Jan 2004 09:44:26 EST


In a message dated 01/23/04 11:56:50 AM Pacific Standard Time,
writes:

> However, when you look at the chromosome passed on to the next generation,
> it is neither pure "F" nor pure "M", but rather a mosaic of seamlessly
> re-stitched pieces of both, as in
>
> FFFFFFFFFFFFFFFFFFFFFFFFFMMMMMMMMMFFFFFFFFFFFFFMMMMMMMMMMMMM
>
> There do seem to be limitations on exactly where the crossing over
> can occur, but I haven't heard of anything that would prevent it from
> occurring in the middle of a gene. This is actually a hot topic of
> research at the moment, so the process could be a whole lot better
> understood in the future.


I did some browsing on this topic. Yes, crossing over can occur within gene
boundaries -- the buzz words for more Google searches seem to be "intragenic
recombination."

The crossover points are random in the sense that they are unpredictable, but
they aren't randomly distributed across the whole chromosome. There are
hotspots and coldspots (or jungles and deserts, as some authors describe it), so
that some "haplotype blocks" (another set of buzz words) may stay intact longer
than you would predict. The length of haplotype blocks is longer in Europeans
than Africans, which is the basis for deducing a population bottleneck (a
thread earlier this month, which included this URL).

http://www.broad.mit.edu/media/archive/pr_01_drlinkage.html

We are used to the word haplotype in the context of the Y and mtDNA, but the
word is actually borrowed from autosomal chromosomes (non-sex nuclear
chromosomes). It refers to markers which are linked together on one chromosome (and
therefore inherited from one parent). The NRY (non-recombining Y) and mtDNA can
only be inherited from one parent, so markers are automatically linked.

Getting back to Charlemagne, I was musing about the proper units to use when
calculating whether one has inherited ANY DNA from him. The probability of
inheriting a specific marker is very low. Charlemagne's son (or daughter, since
we're thinking about autosomal DNA) has 50% of his DNA from Charlemagne and 50%
from his mother. Which version does he/she pass on to the next generation?
There's only a 50% chance that it will be Charlemagne's. If Charlemagne's DNA
survives that transfer, there's only a 50% chance that it will be preserved in
the following generation. After 40 generations, the chance of inheriting that
specific marker drops to .5^40 (.5 raised to the 40th power, or .5 multiplied
by itself 40 times), about .0000000000009 (if I've counted the zeros
correctly!).

But I'm not sure what units should be used to calculate whether one might
have inherited SOME DNA from Charlemagne. It seems to me that haplotype blocks
might be more appropriate than chromosomes (23) or genes (20 to 30 thousand) or
SNPs (several million known) or individual bases (3 billion). But since
haplotype blocks can be chopped up, they aren't a fixed unit.

Ann Turner - GENEALOGY-DNA List Administrator
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