Identical Twins Not Identical After All

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.""

Disease gene hunting

[reprint]

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.

Re:Wonder how this affects...

[JeffL]

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).

Copying introduces errors

[tsa]

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:Wonder how this affects...

[Eivind]

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:Copying introduces errors

[mrbluze]

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.

Re:I cannot believe it

[0100010001010011]

No most of their adult fanbase lost interest when they got scrawny and anorexic. The same with numerous other (what I would consider attractive) young female stars. Olsen Twins, Lohan, Spears, I'm sure Duff and Cyrus will follow suit. They were all decently attractive women until around 18 then they decided that being 95 lbs was more attractive than 130 (I'm sure drugs had nothing to do with it either). Other than Spears, she just went batshit crazy and needs some good drugs.

(Note, 18 is only the age of consent in the USA, they would have been perfectly legal earlier in other countries)

Dear women of slashdot (ha) quit trying to be a size -1.

possibly many causes

[abes]

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.

Re:Cloning

[Matt Edd]

Clones are more different than identical twins. They start with th same DNA (just like a twin) but are subject to the same errors during development. In addition to that, however, they are growing in a different environment unlike twins who share at least the same environment for the first 9 months if talking about humans. Hence, clones are less similar than twins.

gene expression a product of epigentics, duh

[Anonymous Coward]

I won't even read the article. Slapstick on the mod's part. Epigenetic markers determine gene expression. In yourger twins, one will see a congruency in genotype. It is exposure to the environment that alters the genotype of one twin, effectively distinguishing that individual from its identical sibling. GOOGLE: EPIGENETICS NOVA for the layman.