HOW COME? When identical twins are not the same

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Are identical twins really identical? asks Samantha Castellanos, of ManhassetIdentical twins may look alike, laugh alike and sometimes be dressed alike. But when it comes to twins, new research shows that "identical" doesn't mean "exactly alike." In fact, identical twins can have differences in their genes -- or in how those genes are expressed -- that predispose one to develop a disease like Type 1 diabetes or Parkinson's, while the other twin remains healthy.

Identical twins result when an egg, already fertilized by a sperm, splits in half. Each half contains the same gene lineup as the original embryo. Identical twins are the same sex, share the same blood type and often look almost exactly alike as children.

But just after conception, changes can occur. Scientists have known for many years that twins that start out "identical" develop differences due to the environment, and the first environment is the womb. For example, one twin may receive a better supply of nutrients from the placenta than the other.

And as they grow up, identical twins also diverge. Studies have shown that changes are most obvious in twins separated at birth and raised by different parents. But even among those who grew up together, everything from diet, exercise and sun exposure to life experiences and income levels have an effect. The result can be "identical" twins who look and act increasingly less alike over their lifetimes.

Now, research has shown that identical twins may be much less similar from the outset than scientists had believed, perhaps accounting for differences in developing illnesses from cancer to schizophrenia.

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Even before birth, epigenetic factors -- chemical markers attached to a gene that increase, slow or shut off its activity -- can vary between twins. And epigenetic changes increase over a lifetime, affected by diet, smoking and other factors. Then there are chance mutations in one embryo's DNA, as is thought to occur in some cases of cleft palate.

In 2008, researchers found that one of a twin pair sometimes had extra copies of DNA segments or were missing DNA coding letters. In one case a missing set of genes pointed to an increased risk of leukemia. That twin developed leukemia; his brother didn't.

The genome is not unchanging, scientists note, and dividing body cells lose or acquire DNA as they develop. In 2011, researchers found that about 12 percent of DNA can vary between identical twins. And last fall, researchers reported that the average twin pair has about 359 genetic differences that appear early in their development.

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