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1/2 DNA from Each Parent

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  • 1/2 DNA from Each Parent

    I inherit 1/2 of my DNA from each parent.

    What happens to the other "half"?

    1. Is it lost forever, and if so does that mean that if I had inherited the "other half" or portions in a different combination from my mother my mtDNA results could have been different?
    If so, does that mean that portions of heredity can be "bred out" over generations?

    2. If not, does that mean DNA is cumulative - all the genes are there all the way back except the sex specific ones?

    I apologize to those who understand all this better than I. I'm trying to figure out what I will really know about my family history from all these tests I have paid for - especially since it now costs another $300 something to have it explained (and I don't know if after that I would know any more than I did before I paid for it).

  • #2
    shirley:

    mtdna, X and Y chromosomes are particular. but for the rest of the DNA, it is as you say.

    You inherit half of your DNA from your father and half from your mother. More specifically, chromosomes come in pairs, so for instance, you have two chromosomes number 1, two chromosomes number 2, etc. One of the chromosomes in each pair comes from the mother, the other from the father. In turn, for instance, the chromosome 1 you get from your mother is a random mixture of your mother's two chromosomes 1. And so on.

    So you get half of your DNA from your mother. The half of your mother's DNA that you don't get is "lost" in your lineage. You haven't got it, so that's it. If you have a sibling, then this sibling will have taken a different part of your mother's DNA. so this sibling will have some of the mother's DNA that you don't have. But in your lineage, it's lost.

    As you say, the fraction of DNA then gets halved in each generation. You share 1/2 of the DNA with your mother, but only 1/4 with each of your maternal grandparents. And so on. If you go back far many generations, then it is possible that there are some ancestors from which you actually don't have any DNA at all - simply it's lost.

    The mtdna is different, because it doesn't mix and match. You only get your mtdna from your mother, as is. So, apart from random rare mutations, you have exactly the mtdna of your mother, which is in turn exactly the mtdna of your maternal grandmother, and so on. The same happens for the Y chromosome in males.

    cacio

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    • #3
      That is a great question and I want to know too. I am surprised no experts have replied. I saw this thread hours ago. I have heard both trains of thought. It would be nice to know if all the DNA from all ancestors are retained in our cells. I hope and feel that it is. Maybe it is hidden and in our so-called"junk dna" and the scientists don't know it is there and can't read it yet. I think scientists say we get only half of what the mother has and only half of what the father has and the rest is gone forever. But maybe there are scientists that feel we inherit everything, it just needs to be read properly with the right technology or program. Right now I think it is kind of like trying to read some webpages without JavaScipt. Someday they will see it, I hope. Just my opinion. I am an amateur too.

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      • #4
        I see Cacio answered while I slowly texted with one thumb.

        Had chocolate in other hand.

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        • #5
          Re: what happened to the other half?

          It has to do with cell division: "Mitosis" and "Meiosis". I'm afraid to try to explain it off the top of my head. But any basic biology book or probably Google could explain it.

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          • #6
            Originally posted by rainbow View Post
            That is a great question and I want to know too. I am surprised no experts have replied. I saw this thread hours ago. I have heard both trains of thought. It would be nice to know if all the DNA from all ancestors are retained in our cells. I hope and feel that it is. Maybe it is hidden and in our so-called"junk dna" and the scientists don't know it is there and can't read it yet. I think scientists say we get only half of what the mother has and only half of what the father has and the rest is gone forever. But maybe there are scientists that feel we inherit everything, it just needs to be read properly with the right technology or program. Right now I think it is kind of like trying to read some webpages without JavaScipt. Someday they will see it, I hope. Just my opinion. I am an amateur too.
            Some ancestors, even quite recent, in theory are removed from the genome via recombination each generation. There are exceptions such as the X chromosome. Some ancestors (a fan chart reflecting the mathematical fibonacci series of numbers is available) will be strongly represented and others not at all. My uncle's father and ancestors are nowhere at all on the X, but an ancestor born in 1747 and only three meioses removed contributed (statistically) 12.5%.

            It is highly likely that in trying to find who is and who is not represented on the autosomes will be extraordinarily difficult because chance plays a huge part. Maybe this and maybe that. However, even if 23andMe cannot detect the presence of an ancestor's DNA it does not mean it does not exist in the genome. There could be 5 blocks of 1 million bases on separate chromosomes, all from one ancestor many generations ago. They will not be seen because 23andMe does not look below about 5 million bases so the rest is in a sense invisible. The reason is that the danger of false positives increases dramatically with smaller blocks which could for example be shared by both Europeans and East Asians. Tricky business.

            Still, although there are 3 billion base pairs in the genome, it is possible that a great grandparent's DNA did not come down to an ancestor but was lost to recombination. So maybe yes, maybe no.

            A good way to find these smaller pieces is to test many distant cousins and look for overlap. Unfortunately there is no browser at 23andMe to permit verification of overlapping identical blocks unless about 5 Mb when the new tool in testing, Relative Finder, will "see" chunks and automatically report matches. This is very very good news. It would be better news to be able to adjust the criteria somewhat but that would mean far too many "relatives" emerging who are not cousins at all, but just share rather common blocks from say one area of Germany.

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            • #7
              Is it possible to tell, in a pair of chromosomes, which is one came from father and which one from mother?

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              • #8
                JuanCarlos:

                I don't think you can, unless you also know the DNA of your parents, in which case of course you can compare.

                In any case, tests like 23andme don't tell you the two chromosomes separately, they just give you the two results, eg: SNP rsXXX = AG, without saying which chromosome has A and which one has G.

                cacio

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                • #9
                  Originally posted by JuanCarlos View Post
                  Is it possible to tell, in a pair of chromosomes, which is one came from father and which one from mother?
                  No, not directly, it would be necessary to test relatives and so infer which is which.

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                  • #10
                    Originally posted by shirleyblaylock View Post
                    I inherit 1/2 of my DNA from each parent.

                    What happens to the other "half"? ...
                    I once posed that question and was told those potions of DNA not re-integrated in recombination got recycled, broken down chemically, and that all potential information was lost. It would be nice if there was a DNA wastebasket that could be mined. No such luck.

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                    • #11
                      1/2 DNA from each parent

                      Thank you all for your responses!

                      Confirming what I think I know now:

                      I am truly unique in my genetic makeup. There are people my sisters, parents are related to or may be, that I am not related to because the genes were not passed along in the mix I received from my parents.

                      My mtDNA will come down directly from mitrochondrial Eve mother to daughter/son with only daughter being able to pass it down. Barring the rare occasion of male mtDNA recombining somehow at conception (how often does that happen and what disruption does it cause in the mtDNA inheritance?), I can rely on carrying the genes of these women. Somewhere along the way mutations occur in this DNA which is then inherited too right down the line from that woman.

                      Y DNA is passed down father to son from Y-Adam as well and that genetic relation is verified. Women do not inherit the Y.

                      The other genes are combined any which way with half coming from mother and half from father. No way to determine who the genes came from that you received unless everyone involved is tested. Since not very many people are willing to spend the $ to get tested or they're dead, the statistical probability of finding relatives through DNA at this point is probably less than winning the lottery. At best it can pinpoint the geographical location to research and rule out some people for some thousands of years.

                      Also, at some point - and someone has probably figured this out on paper somewhere (please share) - the paper genealogy I have been working on for about 30 years now literally becomes "family history" rather than "genealogy" because I am no longer related to most of these people in a biological sense and I probably won't be able to tell which ones I am still biologically related to.

                      Please correct any of the above, and I'll read some more about the X test and the Tribes tests. I'm a U5 and now waiting for the full mtDNA results.

                      I still prefer this to TV and other wastes of time. The mystery and history of it all is fascinating, but it is upsetting to know about the lost DNA.

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                      • #12
                        shirley:

                        regarding the mtdna, as far as I know, male transmission of mtdna has been detected only once, so this is an exceedingly rare event. Technically, the mtdna is not part of the nuclear DNA. They are separate organelles floating in the cell. They help the cell convert food into energy, and there are hundreds, if not thousands of them floating in each cell.

                        I can't think of any calculation regarding the probabilty of sharing any DNA with an ancestor living x generations ago. This is an intrinsically random event. Presumably, one can go back many generations before anybody is "left out" -with 22 pairs of chromosomes, plus the X and Y, the DNA is pretty big.

                        Ideally, one would want to track how each gene moves down through the generations. This is already done for particular variants of certain genes that confer a high disease risk.

                        cacio

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