You have probably been told that identical twins have identical DNA. For example, if you ask for answers from Yahoo, you will learn,

identical twins, formed when one fertilized egg splits, are the only people in the world with identical DNA.

You might even read that in a textbook.

It makes sense - identical twins look alike. Sometimes even late in life, they can be hard to tell apart.

However, when we look at the question critically, it is not nearly so simple.

First, identical twins can also look quite different for reasons that have nothing to do with DNA.

If one twin has a job that requires heavy lifting and the other watches a lot of daytime TV, we would not be surprised to see some differences. Environment can trump heredity.

Epigenetic factors: Why identical twins do not have identical DNA

But another key reason, recently revealed by geneticists, is epigenetic factors.

These are changes to the chemicals that control how your genes are expressed. Because of these changes, probably triggered by environment factors, your genes might be expressed more often or less often. Or a gene might be shut down altogether by such changes.

So just having a gene is not enough. The gene must be expressed to make any difference to you. 

Scientists have learned in recent years that identical twins do not have identical DNA. Epigenetic factors are considered to be the key reason.

A 2005 European study, for example, looked at 40 twin sets, aged three through 74, in Europe, analyzing their blood for epigenetic differences. And what did they find?

The youngest set of twins had the most identical genomes. Genetically, the oldest twins were the least alike. (Science Daily)

So their genomes had actually changed over time! Why might they have changed? According to the Times Higher Education Supplement,

... a finding that scientists said was particularly groundbreaking, the epigenetic profiles of twins who had been raised apart or had especially different life experiences - including nutritional habits, history of illness, physical activity, and use of tobacco, alcohol and drugs - differed more than those who had lived together longer or shared similar environments and experiences.

Researchers say the results support the theory that environmental factors, including smoking, diet, and physical activity may affect a person's gene activity and explain some of the differences in disease risk found among identical twins.

More recently, a 2008 study looked at 10 older pairs of twins. One member of each of nine pairs had dementia or Parkinson's disease - but the other twin did not. These researchers found something very interesting. Genetics professor Jan Dumanski of the University of Alabama at Birmingham told The New York Times,

... what we found are changes on the genetic level, the DNA sequence itself.

Do genes rule?

Such research challenges the suggestion we sometimes hear that a person is simply the helpless expression of his or her genome, which does not change through life.

Thus, we hear about the "fat gene" or the " infidelity gene" or even a "God gene" that supposedly causes people to be religious.

Resistance to "Genes rule!" provided the inspiration for the 1997 movie Gattaca: There Is No Gene for the Human Spirit, in which

Gattaca Corp. is an aerospace firm in the future. During this time society analyzes your DNA and determines where you belong in life. Ethan Hawke's character was born with a congenital heart condition which would cast him out of getting a chance to travel in space. So in turn he assumes the identity of an athlete who has genes that would allow him to achieve his dream of space travel.

A single dropped eyelash could betray the hero of Gattaca's tale ... and now what do you think happens?

However, even without the recent research that shows that our genomes can and do change - and thus cannot be treated as Fate - there are other reasons for treating claims that genes rule us with caution.

Many such claims are based on research on twins that were separated at birth and then reunited. While such research can yield interesting results, many accounts should be treated with caution for several reasons.

First, we may notice similarities in twins because we are looking for similarities. But some similarities are due to chance. For example, twins separated at birth may both end up playing softball. But many people play softball. If they were not twins but merely two acquaintances, we might hardly notice the fact that they both play softball. But if they are separated twins, we say, "See, it is because they are twins! That shows how similar they are!" Does it?

Some similarities are due to similar environment. You might be impressed to learn that a pair of separated twins each ended up working in a lumber mill. But what if you learned that they both happen to live in towns where the lumber mill is the only decent paying job in town? Is it still as surprising?

Natalie Angier noted in The New York Times that a documentary is far more likely to be made about separated twins who are very similar than about separated twins who are not very similar. In a similar way, studies that show a high level of similarity might get more attention that those that don't.

Also, when twins are raised together, which is the usual situation, it is common for one to be the dominant twin and the other the less dominant one. The resulting behaviour changes may produce many other changes throughout their lives.

Obviously, our genes are an important part of our makeup. But the rulers of Gattaca are - as the movie impassionedly insists - wrong in thinking that genes rule or that all you need to know about a person is in his or her early genome. The design of life is far more complex than that.

Notes:

See also: The Scientist 7-7-2005 'How epigenetics affects twins in genetically identical siblings, DNA methylation and histone acetylation correlate with age and lifestyle' University Of Alabama (2008, February 20).

Identical Twins Not As Identical As Believed. ScienceDaily. Retrieved April 2, 2008, from http://www.sciencedaily.com /releases/2008/02/080215121214.htm

Also, here is Roger Ebert's review of Gattaca.