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Identical Twins Not Identical After All

Posted by samzenpus on from the one-of-these-things-is-not-like-the-other dept.

Hugh Pickens writes "Contrary to previous beliefs, identical twins are not genetically identical. Researchers studied 19 pairs of monozygotic, or identical, twins and found differences in copy number variation in DNA which occurs when a set of coding letters in DNA are missing, or when extra copies of segments of DNA are produced. In most cases, variation in the number of copies likely has no impact on health or development but in others, it may be one factor in the likelihood of developing a disease (pdf). "Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins....a pair of twins where one twin has a disorder and the other does not," says Carl Bruder, Ph.D. "If twin A develops Parkinson's and twin B does not, the region of their genome where they show differences is a target for further investigation to discover the basic genetic underpinnings of the disease.""

11 of 159 comments (clear)

  1. Wonder how this affects... by Anonymous Coward · · Score: 5, Interesting

    ...decades of experiments that assumed twins have identical DNA. One twin may not be such a good control after all...

    1. Re:Wonder how this affects... by mrbluze · · Score: 5, Interesting

      ...decades of experiments that assumed twins have identical DNA. One twin may not be such a good control after all...

      Well experiments can prove an association of two events, or causation of an event on another. Twin studies show associations and the experimenters usually jump to causation in their discussion to make the paper interesting to read. So it doesn't invalidate the experiments but shows that in all of science we can never assume we have excluded all confounding.

      Actually this finding isn't all that surprising. For example, Trisomy-21 (Down's Syndrome) has different severities depending on how far along the line the trisomy developed (how many cells existed when the trait was introduced). It shows that the genetic makeup within an individual is heterogenous, let alone between two 'identical' individuals. The genetic code in your left hand is likely to differ from that in your right hand by a (numerically) small degree.

      However, if the genetic change is an important one, then it follows that your left hand might be very different from your right (eg: more hair on one than the other, or one side more likely to develop cancer, etc).

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    2. Re:Wonder how this affects... by JeffL · · Score: 5, Insightful

      It doesn't have too much effect, really. MZ twins are similar on a trait because of genes that they share (traditionally, all of them) and environment they share (growing up in the same house, etc.) They are different on a trait due to environmental factors they don't share (such as going to different colleges) and error (measurement error in assessing the trait, random noise, etc.)

      DZ (fraternal) twins are similar on a trait due to the genes they share (on average, 50%, same as any other full siblings) and the environment they share. They are different on a trait due to the genes they don't share (on average 50%), environment they don't share, and error.

      These results say that the assumption that MZ twins share 100% of their genes is wrong. The real question is how wrong? Do MZ twins share 99.99% of their genes? Is that 0.01% difference right in the middle of some gene that has a large effect on the trait you're studying? For most of these new discoveries, it doesn't make any difference at all. Differences in silent mutations between twins isn't going to change scientists' conclusions that height is highly heritable (meaning: most of the difference in height between two people is due to the fact that they have different genes).

    3. Re:Wonder how this affects... by skjolber · · Score: 5, Interesting

      As an identical twin myself, I have been included in several national studies. These studies typically address the differences between identical and non-identical twins. It was actually determined through conventional means that it was > 99% certain that we were identical twins, even after living apart for several years. I.e. you would not need DNA testing for determining who's identical i most if not all cases.

      I really do not think this new information will affect previous studies much, because the loss in precition is probably much higher due to non-genetical, i.e. external, factors. However this new insight opens the door for new identical-only studies, where external factors also are kept to a bare minimum.

      I would also like to point out that to a twin, it is clear that identical twins are in fact not equal. I my case we looked very alike untill about 20 years, however now (at 29 years) it is clear to everyone to see that there are clear (visual) differences. This should, in terms, tell all you non-twins that your current physical apperance (++) is actually one within a possible range you could have become, for better or worse.

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  2. Re:Hell that makes it worse by RuBLed · · Score: 5, Funny

    I'm sure we've all had...


    Dude, you're still drunk... this is /. by the way.
    Look behind you, that's your bed... and there are no twins...
  3. Re:News Flash! by repapetilto · · Score: 5, Interesting

    Right, and it seems like their making a false dichotomy between environmental factors and DNA replication "errors," one can lead to the other (well replication errors can lead to susceptibility). Think about your skin cells, people who spend alot of time outside end up with their skin adapted. Its not just upregulation of whatever leads to more pigment, there is actually competition going on amongst the cell lines, those that survive better start to spread. Eventually even the whitest person you know will have permanently darker skin (if it doesn't kill them first that is). Same with every organ I imagine.

  4. Amazing breakthough! by brit74 · · Score: 5, Funny

    I'm sure this will explain why the evil twin is evil. It's an amazing breakthrough in the field of soap opera science!

  5. Re:Hell that makes it worse by cp.tar · · Score: 5, Funny

    Clearly this wholly explains all those evil twin plot lines.

    Yeah, I've always wondered why it is that the evil twin has a pointy beard.

    I'd never have thought that the cause was genetic.

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  6. That was known for quite a while by bogd · · Score: 5, Informative

    I remember that during medical school (maybe 7 or 8 years ago) we were told that while identical twins have extremely similar DNA to each other, that DNA is not 100% identical. Maybe 98 or 99% (more than any other two individuals on this planet), but not 100%. So while this is an interesting research, it hardly qualifies as "news"...

  7. Re:News Flash! by Anonymous Coward · · Score: 5, Funny

    well its not chaos...

    You clearly do not have a four-year-old. Now if you'll excuse me, I've got to go vacuum the bookshelf. Don't ask.

  8. *AHEM* by AlgorithMan · · Score: 5, Funny

    http://xkcd.com/263/
    ;-)

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