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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 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).

  • 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.
  • Re:Err..... (Score:2, Interesting)

    by repapetilto ( 1219852 ) on Thursday February 21, 2008 @05:56AM (#22500244)
    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.
  • 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)

    by BenjiTheGreat98 ( 707903 ) on Thursday February 21, 2008 @08:30AM (#22500920)
    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.
  • 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.
  • 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

  • by theaveng ( 1243528 ) on Thursday February 21, 2008 @09:24AM (#22501218)
    Well then.

    That explains why Playboy's "Dalmer Triplets" have differing breast sizes. I thought I was just seeing things, but apparently there really IS a difference.

    (ducking and running)
  • by sexybomber ( 740588 ) <> on Thursday February 21, 2008 @10:20AM (#22501732)
    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.
  • Re:Allergies (Score:3, Interesting)

    by Peter Mork ( 951443 ) <> on Thursday February 21, 2008 @10:41AM (#22502010) Homepage
    (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.
  • 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.)
  • by Roadkills-R-Us ( 122219 ) on Thursday February 21, 2008 @06:23PM (#22508690) Homepage
    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?"
  • by WickedScorp ( 973777 ) on Thursday February 21, 2008 @09:57PM (#22510576)
    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 expanded to all diseases. Not just in the way of copy number variants, but also causative alleles. No two individuals have exactly the same course of any genetic disease. Furthermore, there are cell to cell differences throughout the body that differ in mutation content and copy number variation. You might have a cell in your thumb deleted for a colon cancer tumor suppressor, but if that gene doesn't perform that function in your thumb it doesn't matter.

In seeking the unattainable, simplicity only gets in the way. -- Epigrams in Programming, ACM SIGPLAN Sept. 1982