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July 5, 2005

Explaining Differences in Identical Twins

For years we have wondered why identical twins, who have the same genetic composition, can sometimes end up so differently. Why is it that sometimes one twin will develop schizophrenia, bipolar disorder, or cancer, and the other will remain perfectly healthy? Part of the reason is just due to unexplainable environmental factors, but there is more to it.

As Wade (2005) puts it, "a whole new level of explanation has been opened up by a genetic survey showing that identical twins, as they grow older, differ increasingly in what is known as their epigenome. The term refers to natural chemical modifications that occur in a person's genome shortly after conception and that act on a gene like a gas pedal or a brake, marking it for higher or lower activity."

Identical twins are born with the same epigenetic marks, but as they grow older those marks begin to change. Why they change has two different explanations. One is that personal experiences and the differing environments may result in altered epigenetic marks. The other is that since epigenetic marks are "lost" as one grows older, they would obviously manifest themselves differently in each of the twins. More likely it is a combination of these two factors. The article showed a picture of Chromosome 1 of a pair of 3-year-old twins next to a picture of Chromosome 1 in a pair of 50-year old twins. It was obvious that Chromosome 1 was much more similar between the 3-year-olds than between the 50-year-olds. The more time spent in different environments will lead the epigenome of twins to differ to a greater degree.

There is currently a discussion to start an international human epigenome project which would possibly be even more complex than the Human Genome Project; this is due to the fact that the human genome is the same in every cell of an individual's body, whereas the epigenome is different for the "250 or so human cell types."

"Among the most important components of the epigenome are small chemical handles known as methyl groups, which are added directly to the chemical units of DNA. A wave of demethylation occurs in a sperm's genome shortly after an egg is fertilized, followed by the extensive readdition of methyl groups in early embryonic development" (Wade, 2005).

Having a fuller knowledge of the epigenome could help scientists discover what role the epigenome has in many diseases and afflictions, such as schizophrenia, bipolar disorder, and the many forms of cancer.

The source of this article was The New York Times.

To learn more about epigenetics go to these 2 sites:


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