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  • Extra chromosomes in tests

    I'm just curious but how do extra chromosomes effect DNA testing? If a person is XYY would the y-dna test know it? How about for autosomal testing? Will you have 3 letters in your raw DNA instead of 2?

  • #2
    No you would not know XYY in the FTDNA test. As to the question about two or three letters, the autosomal will only show two alleles; regardless, the Family Finder test doesn't use X or Y anyway.

    Since FTDNA only tests one Y chromosome at a time, if you have two you will need to pay double for the test.




    (just kidding about the double $$ cost)


    Matt.
    Last edited by mkdexter; 28 August 2012, 11:36 PM.

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    • #3
      I would assume that any polychromosome issues might be detectable in various tests tho? Not that it would be looked for, but that it would cause all sorts of strange results until someone found out why?

      (P'haps not if it was a y-chr test on someone XXY? I dunno...)

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      • #4
        double Y

        Originally posted by Osiris View Post
        I'm just curious but how do extra chromosomes effect DNA testing? If a person is XYY would the y-dna test know it? How about for autosomal testing? Will you have 3 letters in your raw DNA instead of 2?
        As to your second question: My initial response is that the two Y chromosomes are genetically identical and came from the same direct paternal ancestral line anyway, so the haplogroup and subclade would be identical.

        I heard of this happening once: Many years ago, a grad student was taking basic genetics with our undergrad class. We were karyotyping (correct my spelling) and he turned out to have an extra Y chromosome.

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        • #5
          Originally posted by mkdexter View Post
          the Family Finder test doesn't use X or Y anyway.
          For the Family Finder I was wondering about Down's Syndrome which is XXX on chromosome 21. I have no idea how they capture and identify each chromosome in our DNA. I know it's on a chip with micro-beads but what effect on the test would having the extra chromosome have on it.

          I was thinking for the extra Y in the Y-DNA test it wouldn't show because you normally have many Y in the teeny tiny drop they use for testing. So having twice as much would just make it easier for them to isolate the Y. I'm 6'5" and the extra Y happens in something like 1 out of every 1000 men I read so it's not too far fetched I could have an extra Y. Very unlikely but possible.

          I think you're right though, mixedkid, about the two Y being the same haplogroup.

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          • #6
            Originally posted by Osiris View Post
            For the Family Finder I was wondering about Down's Syndrome which is XXX on chromosome 21. I have no idea how they capture and identify each chromosome in our DNA. I know it's on a chip with micro-beads but what effect on the test would having the extra chromosome have on it.
            No effect. The test will still report two alleles, not three. Now will those alleles be from just two of the three trisomes? No, they'll be a reading obtained from all three but displayed as if they were from two chromosomes.

            Originally posted by Osiris View Post
            I was thinking for the extra Y in the Y-DNA test it wouldn't show because you normally have many Y in the teeny tiny drop they use for testing. So having twice as much would just make it easier for them to isolate the Y. I'm 6'5" and the extra Y happens in something like 1 out of every 1000 men I read so it's not too far fetched I could have an extra Y. Very unlikely but possible.

            I think you're right though, mixedkid, about the two Y being the same haplogroup.
            Right, the YY will look like the Y; just two copies of the same one. Not sure what the point is about the Y though. The tiny drop you mention is amplified in the PCR process so it doesn't matter if there is one Y or two Y in the tiny drop..

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            • #7
              Originally posted by Osiris View Post
              For the Family Finder I was wondering about Down's Syndrome which is XXX on chromosome 21. I have no idea how they capture and identify each chromosome in our DNA. I know it's on a chip with micro-beads but what effect on the test would having the extra chromosome have on it.
              The base calling software is tuned to the expectation of two alleles and three possible genotypes, e.g. AA or AG or GG. In the case of a heterozygous result, the strength of the signal for an A and a G would be similar, but it would be imbalanced if the alleles were AAG or AGG. I had occasion to look at the raw data for trisomy 21, and there were a huge number of no-calls for that chromosome.

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