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Medicine Science

Identical Twins Not Identical After All 159

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.""
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Identical Twins Not Identical After All

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  • by Daimanta ( 1140543 ) on Thursday February 21, 2008 @05:03AM (#22500022) Journal
    You think they are exactly the same, but they are always slighly different.
  • by MosesJones ( 55544 ) on Thursday February 21, 2008 @05:04AM (#22500030) Homepage
    I'm sure we've all had that embarrassing occasion of going out with one twin, getting drunk and waking up with the other. It used to be okay to claim you were drunk and couldn't tell them apart, you'd still get a slap but not a knee in the groin... now they can be told apart using simple genetic testing there really is no excuse.

    In other news the part of the movie industry targeted with making crap movies aimed at teenage boys was shut down as plot lines had become "medically unsupportable".
  • So.... (Score:3, Funny)

    by Anonymous Coward on Thursday February 21, 2008 @05:05AM (#22500032)
    What gene difference made those twins so different (Danny de Vito and Arnold Schwarzenegger)
  • by Anonymous Coward on Thursday February 21, 2008 @05:09AM (#22500046)
    ...decades of experiments that assumed twins have identical DNA. One twin may not be such a good control after all...
    • by mrbluze ( 1034940 ) on Thursday February 21, 2008 @05:25AM (#22500134) Journal

      ...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).

      • *AHEM* (Score:5, Funny)

        by AlgorithMan ( 937244 ) on Thursday February 21, 2008 @06:40AM (#22500424) Homepage
        http://xkcd.com/263/ [xkcd.com]
      • Re: (Score:2, Interesting)

        by WickedScorp ( 973777 )
        It is also important to note mosaic trisomy 21 is reported to be a vast minority in the literature. Certainly there seems to be a correlation between % trisomic cells and clinical phenotype / degree of mental retardation. However, perhaps even more interestingly is the huge variation in associated phenotypes complete trisomies. The number of associated phenotypes is huge, and two individuals with Down syndrome likely only share a small subset.

        More importantly, this idea of genetic heterogeneity should be ex
    • by JeffL ( 5070 ) on Thursday February 21, 2008 @06:37AM (#22500408) Homepage

      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).

      • Re: (Score:3, Funny)

        by XavidX ( 1117783 )
        you mean eating your veggies does not make you grow big and strong?
      • Re: (Score:3, Informative)

        by mrami ( 664567 )

        I'll tell you what I first thought when I read this:

        I read in, I think it was a Steven Pinker book, about studies done on identical twins using the big five personality traits [wikipedia.org]. What he said was that on the big five, identical twins raised together were roughly 50% similar, and identical twins raised apart were... roughly 50% similar. So when it comes to nature/nurture on the big five, you get 50% genetics, maybe 1-2% environment, the rest comes from ?????.

        As you say, those numbers are probably based o

        • Re: (Score:3, Funny)

          by zappepcs ( 820751 )
          The rest comes from the fact that you'd want to fucking be different too if they called you a "discordant monozygotic twin"
    • by Oswald ( 235719 )
      Thank you for anonymously bringing this out, as it was the first thing I thought of when I read TFA. I guess if we hadn't already figured out that those experiments were fucked by shared prenatal nutrition (minor oversight, that), we could now stop loudly announcing that intelligence is 80% heritable.
      • by Eivind ( 15695 ) <eivindorama@gmail.com> on Thursday February 21, 2008 @07:33AM (#22500674) Homepage
        Actually, scientists aren't (universally) that dumb.

        If identical twins are much MORE similar in intelligence compared to non-identical twins, we can conclude that there is a high likelihood that the difference is genetic.

        Identical twins should not normally have more similar nutrition (in pregnancy or thereafter) than nonidentical ones.
        • Re: (Score:3, Funny)

          by somersault ( 912633 )

          Actually, scientists aren't (universally) that dumb.
          No, a scientist is smart. Scientists are dumb, panicky, dangerous animals and you know it.
    • by skjolber ( 933754 ) on Thursday February 21, 2008 @09:13AM (#22501148)

      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.


      Two for the price of one

  • News Flash! (Score:3, Funny)

    by Strange Ranger ( 454494 ) on Thursday February 21, 2008 @05:09AM (#22500050)
    After the sperm penetrates the ovum [wikipedia.org], a zygote [wikipedia.org] is formed. After which, chaos [wikipedia.org] ensues.

    Growth and development of one copy != growth and development of the other.
    • Re:News Flash! (Score:5, Interesting)

      by repapetilto ( 1219852 ) on Thursday February 21, 2008 @05:32AM (#22500156)
      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.
    • well its not chaos, but it is pretty obvious that changes are going to occur as soon as cells divide, especially when stuff goes wrong.
    • After the sperm penetrates the ovum, a zygote is formed. After which, chaos ensues.
      Try telling THAT, to your children.
  • by reprint ( 1162711 ) on Thursday February 21, 2008 @05:13AM (#22500072)
    This is certainly another tool for those hunting genes associated with disease. These are complex diseases however with multiple genetic alterations so the identification of a single gene may not provide the whole story. Also a gene identified this way may not apply to the larger afflicted populaton since this is a correlation seen in a small study group. It should be relatively easy to check though.
  • by Anonymous Coward on Thursday February 21, 2008 @05:35AM (#22500164)
    Well, duh. [wikipedia.org]
  • by brit74 ( 831798 ) on Thursday February 21, 2008 @05:38AM (#22500182)
    I'm sure this will explain why the evil twin is evil. It's an amazing breakthrough in the field of soap opera science!
  • the article didn't even mention the Olsen twins!
  • Whenever I read scientific articles like this I think "I could have sworn scientists already knew this..." I'm probably wrong but scientists seem to just be researching two things, new ways to kill or blindingly obvious.
    • Re: (Score:2, Interesting)

      well you still have to research the obvious, because sometimes intuition is wrong. Ever hear the thing about how many times you would have to fold a normal piece of paper in half in order for it to reach the moon? Really guess. The answer is 42. And I'm not just saying that because its the answer to everything.
      • well you still have to research the obvious, because sometimes intuition is wrong.
        And you still have to research new ways of killing people, because sometimes the ethics committees catch on to what you're currently up to.
    • Re:Err..... (Score:4, Informative)

      by dltaylor ( 7510 ) on Thursday February 21, 2008 @06:13AM (#22500312)
      Blindingly obvious? It requires a tremendous ability to detect genetic differences when the difference is a nothing more than the number of copies of a few base pairs in an entire genome (~3,000,000,000 base pairs). We still don't know whether there are any epigenetic differences, or what those might be, because that is even more difficult to measure.

      While it may seem obvious to the uneducated that twins are "different", there is a lot of research that shows high correlation, even when the twins are raised apart, so identifying the cause(s) of the differences and whether those are "nature" or "nurture" is still of value. Even within a family, the differences may be simply something like feeding order, where the earlier fed may get different (not necessarily better or worse) nutrition or bonding experience than the later fed, rather than, necessarily, a genetic difference.

      When it is copy numbers, or very small polymorphisms, and there is some somatic variation, we can use the data to more closely identify which genetic values are associated with the variation.
      • I heard about 4 studied cases of homozygotic twins which had different genders (a boy and a girl).

        So, yes, in a way, it could seem "Blindingly obvious".

    • The fools could have figured it out ages ago by simply using linux: taliesin@charis> diff genomeA genomeB | less
  • Could this be why mothers can tell apart their twin offspring?
  • by bogd ( 912084 ) on Thursday February 21, 2008 @06:27AM (#22500362)
    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"...
    • by Ihlosi ( 895663 )
      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.

      Well, quite probably not even all the cells of your body have 100% identical DNA.
    • Re: (Score:3, Informative)

      by Rhabarber ( 1020311 )
      Hmm, you mean just like my DNA is 99% identical [psu.edu] to that of every chimp ;)

      Also remember that many of your cells carry DNA of all those viruses you got exposed to without even noticing. And while we talk about infections, the immune system comes in mind, with all those crazy DNA recombinations taking place during its development. Not to mention spontaneous mutations which are not that insignificant tumorigenesis [wikipedia.org].

      Nobody actually ever believed that twins are 100% identical. They just want to make up their
    • Anyone who knows identical twins very well at all could have told you this.

      I married one. From the day I met them, I have had no trouble telling them apart, even the one time her sister tried to fool me as a practical joke.

      I suppose it's good to know the details; knowledge is generally good. But the announcement that identical twins aren't is right up there with "Politician caught lying!" My immediate thought is "Wow! Really! Who knew?"
  • Maybe some of those small differences are reflected, for example, in whether one twin makes more efficient use of a particular protein than the other which in turn leads to tiny differences in how they make out on an identical diet, or with exposure to the same environment.

    I wonder if this might in turn be reflected in small physiological differences that explain why people who know twins well can usually tell them apart when they're together.

  • by tsa ( 15680 ) on Thursday February 21, 2008 @07:17AM (#22500596) Homepage
    When 3 billion basepairs are copied when a cell is divided, it seems logical to me that errors occur. So why isn't the fact that 'identical' twins are not truly identical a no-brainer to the experts? I know this remark sounds like a troll, but I'm genuinely surprised.
    • Re: (Score:3, Insightful)

      by mrbluze ( 1034940 )
      Scientific research is only as good as the people conducting it and what motivates them. From what I've seen, only a small number of people are genuinely excited by the stuff they are researching. The rest of them are chasing a PhD or some other claim to fame. But also coming up with a good study and designing it well to make it valid is not easy. It takes creative imagination and intelligence. Whilst there are many people with these qualities, it's frighteningly hard to find them at scientific meetings.
    • are not truly identical a no-brainer to the experts

      Maybe it's because they know the rate of mutation for the human race (which seems like it should predict how likely errors that can cause disorders are to occur)?
      Maybe because they know a lot about the mechanisms to keep errors from occurring?
      Maybe this particular effect doesn't happen as much for non-twins?
  • by mlwmohawk ( 801821 ) on Thursday February 21, 2008 @07:36AM (#22500690)
    I wonder, maybe the egg always splits when minor genetic errors happen as a method to protect the embryo. The vast majority of the time, the part that splits dies and is reabsorbed. On odd occasions, the genetic mutation is viable and becomes a twin.

    That would imply that the second twin always has some sort of mutation from the first.
  • Allergies (Score:2, Interesting)

    I've wondered about this. I'm going to get married to a twin in a couple months and my fiance is has many medical allergies - penicillin and similar drugs (like her mother). Her identical twin sister on the other hand has allergies such as pollen or other outdoor allergies.

    I was never sure how much genes would play a role into this.
    • slightly OT, but very small numbers of people are actually allergic to Penicillin. Around 20-30 years ago doctors thought that if you had some kind of reaction going on the medication, you were allergic to it. However, the kinds of illnesses treated with penicillin tend to 'put up a fight', as it were and cause a reaction as they are treated. Not to mention that most of these people had an adverse reaction ONCE, when they were 4 or something... Most 'allergic' people have no reaction to penicillin alone.
    • Re: (Score:3, Interesting)

      by Peter Mork ( 951443 )
      (Caveat: IANAMB) As I understand it, immunoglobins are encoded by genes in so-called hypervariable regions. Basically, there are regions of the genome in which variability is beneficial because it allows the body to more easily generate immunities to various antigens. However, allergies turn the immune system against harmless antigens. So, if there's any genetic variation between identical twins, I would expect it to be in the hypervariable regions and therefore exhbited as differing allergies.
  • by marzipanic ( 1147531 ) on Thursday February 21, 2008 @08:39AM (#22500966) Journal
    As a proud mum of identical twin boys I had noticed that mine seem distant as opposed to those who feel empathy for each other.

    They are part of a twin study, which basically involved some of the placenta, blood tests and for a few months DNA swabs from inside the cheek. The study never got back to us with the results yet as I believe it is still ongoing. But yes it was confirmed they are definitely identical.

    As babies I used to "colour code" them so I knew who was who, now they are teenagers, totally different but I put it down to personality and obviously different tastes in dress. Silly me....

    I will be following this with interest though! Esp. as one of my sons has autism and they are saying that could be a genetic thing, well I was told by a specialist if one had autism the other would have too? However my GP said that is crap. I have no idea but they are like chalk and cheese except for their voice.
    • Esp. as one of my sons has autism and they are saying that could be a genetic thing, well I was told by a specialist if one had autism the other would have too? However my GP said that is crap.

      I recently heard a lecture on the genetics of autism - to summarize, there's only a 90% influence of genetics - there's still a 10% unaccounted-for 'trigger' (or something not understood). These results were based on twin studies. Basically, your non-autistic boy only had a 10% chance of not being autistic, but the
  • I can think of a scenario off the top of my head that'd be one plausible explanation for this sort of thing:

    1. Egg gets fertilized
    2. Egg splits into two fertilized eggs
    3. Both eggs start dividing
    4. After a few cell divisions, an extremely well-aimed cosmic ray strikes egg #2, shearing off a few base pairs from one of its copies of chromosome 3. Egg #1 is unaffected.


    5. Egg #2 grows up to be the evil twin.
  • Do I have to go through all nine pages to find out what I want?

    The American Journal of Human Genetics is a pathetic wallpaper for not enforcing the proper paper structure (Introduction, Methods, Results, Discussion). /rant
  • by Mandovert ( 1140887 ) on Thursday February 21, 2008 @11:11AM (#22502426)
    ...do MD5 match?
  • Epigenetics (Score:3, Interesting)

    by ecbpro ( 919207 ) on Thursday February 21, 2008 @11:31AM (#22502782)
    Epigenetics is an important factor that should not be neglegted!
    Depending on the life-style of each twin (and other factors) twins could be identical genetically but very different epigenetically. That means that even though they have the same set of genes they can be completely differently regulated, thus resulting in different susceptibility to diseases.
    (For those interested: One important epigenetic mark is the methylation of DNA at Cytosines thus resulting in the shut-off of genes.)
  • and let my twin do the time. Does that mean they can tell us apart now?
  • by abes ( 82351 ) on Thursday February 21, 2008 @12:47PM (#22503898) Homepage
    Unfortunately the article does not seem to address the fact that there are other possible causes. Many environmental factors can affect DNA over time. For example, genes commonly have two attributes associated with them: penetrance and expression. Just because you have a gene doesn't mean it will necessarily be transcribed (i.e. does it get expressed?). Both internal and external cues can help determine this event occurring. Even when the gene does get expressed, how many copies get made can vary (penetrance).

    These are the two of the most classic examples of differences between genes, but there are other mechanisms that exist. For retrograde viruses can insert themselves into genes.

    Not to say their theory of CNV is wrong, just that other mechanisms have already been known.

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