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Subject: [DNA] Add your sibling Relative Finder crossovers (was FamilyFinder Test)
Date: Wed, 31 Mar 2010 10:19:16 -0400


This is a quick poor man’s method of figuring crossovers per chromosome
through visual inspection, but since there was an interest in the
subject here, I wanted to mention that anyone can contribute their
number of transitions between siblings at:

http://docs.google.com/Doc?docid=0AR3qUyFYAhKuZGQ2M2NrcDZfMGM4NXc4a2hz&hl=en

This Google Doc is displayed very primitively compared to what some
families are doing with their 23andMe data, but I simply am interested
in comparing the very rough estimate of crossover transition point
numbers in many families.

The blocks can be found in your Family Inheritance Section at 23andMe
where you can see the blue (half-identical), the dark (completely
identical) and the white (not identical) blocks. We are counting each
transition point. Measured Gbs can be found under Estimates.

In order to report the half IBD number of segments as is measured by
23andMe you have to look in the Advanced Labs section, Family
Inheritance: Advanced. Here you will see that the estimated half IBD
includes both the half matching and the identical blocks together
without distinguishing between the two. So it really does not measure
crossovers.

Here are further instructions that I gave our study group at DNA-Forums
which admittedly is not as good as looking at raw data in a full
genome, but it gives some guide lines for comparison purposes:
Please enter your siblings' crossover points between haplotype blocks.
This means every time there is a junction between two colors, it will
count as one.
If a no-match becomes a full-match, count one; then if the full-match
becomes a half-match count another and so on.
Please do NOT count obvious centromeres, i.e. any place there is a
pinching of the chromosome.
Do NOT count the grey zones. For example, this means on chromosomes 13,
14, 15 do not start where the grey becomes solid. Cut off the grey
completely. Start with the next transition to count recombination
points.
If there is grey between two blues, count just as one blue. Pretend the
grey is not there.
Use magnification to see those very tiny colored blocks.
Each family group will have a number. Let us know if male (m) or female
(f).

Each chromosome pictured under Family Inheritance actually shows the
transitions in four chromosomes because each sibling has two
chromosomes. To estimate some kind of crossover number per chromosome,
you need to divide by 4. The main pitfall here is if two chromosomes
have exactly the same crossover point, it will only count as one. It
also would not count the crossovers in the grey zones. But for a quick
method to gather lots of information, it is not bad, especially when I
don't have to do all the work.

Kathy J.







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