Identical twins are invaluable to research as scientists try to tease out the influence of genetics and the environment on behaviours and genetic disorders.
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Is it Environmental or Genetic?
Nature or nurture? It's probably one of the most frequently asked questions, especially when addressing anti-social behaviour, such as alcoholism or aggression, or trying to find out the root causes of debilitating disorders like depression or heart disease.
So why are identical twins used in genetics research and what can their genomes tell us about the causes of a condition under study?
First of all identical twins have identical genes, non-identical (fraternal) twins do not. So therefore any variations between identical twins are due to the environment.
If a researcher is trying to find out whether genetics or the environment play a greater role in a trait, the genes of identical twins, who share 100% of their genes are compared with the genes of fraternal twins who share 50% of their genes. If the frequency of the trait is greater in the identical twins, it suggests that genes are of importance.
According to the International Society for Mental Health Online, it's been shown that if one identical twin suffers from depression, the other sibling has a 76% chance of also being depressed. If they are raised apart they both become depressed about 67% of the time. In these studies of depression it was also shown that if one fraternal twin suffers from depression, the other sibling has 19% chance of being depressed.
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Identical Twins and Genetics
Now looking at those figures you might think, hang on a minute, if one identical twin suffers from depression, shouldn't the other one too, as their genes are identical? Instead of a 76% chance it should be 100%. It's a fair point but twin studies have never claimed to be full-proof and it also goes to show that with many conditions there is not a black and white answer. In the study above, genes may well be a significant factor but not the only one. Environmental factors and perception of the environment will also be important.
In fact the question should no longer be asked is it nature or nurture? More pertinent would be, how does nature interact with nurture to cause a condition?
It also turns out, according to research that was published in February 2008 that identical twins might not be so identical after all. American and European scientists studied the genomes of 19 pairs of identical twins and noticed tiny, subtle differences. The differences are in the copy number variation (CNV). This occurs when parts of the DNA are missing or when extra segments are produced. This may go to explain why one twin might suffer from a condition and the other stays healthy.
It won't be the end for the use of identical twins in genetics research, but it serves to further highlight the complex nature of studies looking at genetic and environmental influences.