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  #1  
Old 12th February 2018, 08:59 PM
DavidB DavidB is offline
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Chromosome browser questions

A few questions about the FTDNA chromosome browser:

1) Why are the chromosomes graphically depicted as getting shorter as they descend down the display? Also, why is the X chromosome depicted as longer than chromosome 22?

2) A parent and child have so many overlaps that each chromosome is solid with overlaps (except for the gray, unanalyzed areas). Didn't the child inherit only half the mother's DNA though, in which case the chromosomes should be only half filled?

3) I have read Roberta Estes and others on this but still struggle to understand it:

If two individuals have an overlap with me in the same place on a chromosome, how can they not match each other Doesn't the overlap with me in that place show that they also have an overlap with each other in the same place?

Thank you,

David
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  #2  
Old 12th February 2018, 10:30 PM
KATM KATM is offline
mtDNA: K1a3 | Y-DNA: R-L1308*
 
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Quote:
Originally Posted by DavidB View Post
A few questions about the FTDNA chromosome browser:
1) Why are the chromosomes graphically depicted as getting shorter as they descend down the display? Also, why is the X chromosome depicted as longer than chromosome 22?
The chromosomes are indeed physically different in size, and are depicted in the browser to reflect the size differences. The X chromosome is larger than Chr. 22, and many of the others. See the image on the Wikipedia page for "Human genome," which shows the chromosomes.

Quote:
2) A parent and child have so many overlaps that each chromosome is solid with overlaps (except for the gray, unanalyzed areas). Didn't the child inherit only half the mother's DNA though, in which case the chromosomes should be only half filled?
You get one set of autosomal chromosomes from each parent. Thus, you get two each of chromosomes 1-22, so 50% of your DNA from each. The DNA testing cannot separate which of each chromosome came from which parent, and the Chromosome Browser display therefore cannot represent the two. So, when you see each chromosome filled when you compare a parent, it is showing that you got one of each chromosome from that parent. I imagine if you had both parents tested, and compared them to see how they matched you in the Chromosome Browser, you would have two solid lines shown, each in a different color. I only have one parent who tested, so can't check that.

Quote:
3) I have read Roberta Estes and others on this but still struggle to understand it:
If two individuals have an overlap with me in the same place on a chromosome, how can they not match each other Doesn't the overlap with me in that place show that they also have an overlap with each other in the same place?
No, because one may match you on your paternal chromosome, and the other in the same place, but on your maternal chromosome. (see "Nine Autosomal Tools at Family Tree DNA," scroll down to #5, Chromosome Browser, and read the "Cautions" section). You would need to contact the people who match you on the same segment, to see if they match each other. That is called "triangulation."

Last edited by KATM; 12th February 2018 at 10:34 PM.
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  #3  
Old 13th February 2018, 02:23 AM
Fern Fern is offline
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Also take a look at this post from Roberta Estes: https://dna-explained.com/2013/12/15...nd-the-matrix/ It's a vey clear explanation of how each chromosome has two sides (maternal and paternal) and that they need to be "unzipped". It starts with a discussion of the Matrix and ICW tools, but after the first few paras it becomes clear where Roberta is going. Do read the first few paras, too
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  #4  
Old 13th February 2018, 04:15 AM
prairielad prairielad is offline
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2) We have 46 single chromosomes, 23 pairs
23 single chromosomes from father, each a random mixture(recombination) of each his pairs, except chromosome 23 which he passes y to sons and X to daughters.
23 single chromosomes from mother, each a random mixture of each of her pairs

Thus it is 50% of each parent.

Chromosomes browsers merge the two single chromosomes into one due to nature of testing can not distinguish the individual values of each
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  #5  
Old 13th February 2018, 06:55 AM
Biblioteque Biblioteque is offline
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DavidB, In addition to the aforementioned, e.g., if you have 5 people in your "triangulation" (yes, I know tri means 3), you now need to have all of your matches upload to Gedmatch and do a one-to-one comparison to separate/prove the segment.

Everyone in your triangulation has to match each other person. A=B=C=A.... This proves that All of these come from the same parent. All of the leftovers come from the other parent.

Proving segments can only be done at Gedmatch.

The experts say we only need 3 to triangulate. I prefer to have more.
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