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Subject: Re: [DNA] Add your sibling Relative Finder crossovers (was FamilyFinder Test)
Date: Wed, 31 Mar 2010 23:36:03 -0400


For those of you still wondering how these crossovers can be counted,
here is a practice exercise demonstrating the full and half matches
between siblings.

If you only look at a three generation pedigree, and either you or your
sibling represent 1st generation, your father is number 2, your mother
is number 3, your paternal grandfather is number 4, your paternal
grandmother is number 5, your maternal grandfather is number 6 and your
maternal grandmother is number 7 as in a standard chart, the following
might be what you (then your sib) inherited from the four grandparents
for a particular autosome pair from left to right:

#1 You get the following from persons #2 and #3:
#2 4444444455555555544444444 has two crossovers
#3 6666666777766666666666777 has three crossovers

#1 Your sib gets the following from persons #2 and #3:
#2 4444444444444555555555555 has one crossover
#3 6667777777666677777777766 has four crossovers

Use Courier New font to line up the numbers. Each block of numbers
matches a sequence from that grandparent.

The #2 line comes from your father where the recombination of 4 and 5
takes place, the #3 line from your mother where the recombination of 6
and 7 occurs. Let us just say each position above represents about 10
million bases on chromosome 1.

Now colorize every four numbers at each position from top to bottom
with blue or black ink.
If there is an exact match e.g. 4,6 + 4,6 color it (draw line through
the four numbers) black,
If there is a half match such as 4,6 + 4,7 or 5,7 + 4,7 color it blue.
If no match e.g. 5,7 + 4,6 leave white. Then color in the blocks.

You should end up with 10 transitions creating 11 blocks.
Counting from left to right at each transition:
Black 1 Blue 2 Black 3 Blue 4 White 5 Blue 6 Black 7 Blue 8 White 9
Blue 10 White End.

10 divided by 4 = average of 2.5 crossovers in this chromosome for
these two sibs.

If your grandparents and parents are deceased, there would be no way of
knowing which blocks came from which grandparent without more
information, so we are stuck with just looking at the sibs' results and
calculating the crossovers from the color transitions at 23andMe.
Another way would be to actually look at the raw data.

Kathy J.



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