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| ©Leif Parsons |
THE FACTS: It is a basic tenet of human biology, taught in grade schools everywhere: Identical twins come from the same fertilized egg and, thus, share identical genetic profiles.
But according to new research, though identical twins share very similar genes, identical they are not. The discovery opens a new understanding of why two people who hail from the same embryo can differ in phenotype, as biologists refer to a person's physical manifestation.
The new findings appear in the March issue of The American Journal of Human Genetics. The scientists examined the genes of 10 pairs of monozygotic, or identical, twins, including nine pairs in which one twin showed signs of dementia or Parkinson's disease and the other did not.
It has long been known that identical twins develop differences that result from environment. And in recent years, it also has been shown that some of their differences can spring from unique changes in what are known as epigenetic factors, the chemical markers that attach to genes and affect how they are expressed. These epigenetic changes -- which accumulate over a lifetime and can arise from things like diet and tobacco smoke -- have been implicated in the development of cancer and behavioral traits like fearfulness and confidence. Epigenetic markers vary widely from one person to another, but identical twins were still considered genetically identical because epigenetics influence only the expression of a gene and not the underlying sequence of the gene.
"When we started this study, people were expecting that only epigenetics would differ greatly between twins," said Jan Dumanski, a professor of genetics at the University of Alabama at Birmingham and an author of the study. "But what we found are changes on the genetic level, the DNA sequence itself."
The specific changes that Dumanski and his colleagues identified are known as copy number variations, in which a gene exists in multiple copies, or a set of coding letters in DNA is missing. Not known, however, is whether these changes in identical twins occur at the embryonic level, as the twins age or both.
"Copy number variations were discovered only a few years ago, but they are immensely important," said Carl Bruder, another author of the study at the university. Certain copy variations have been shown in humans to confer protection against diseases like AIDS, while others are believed to contribute to autism, lupus and other conditions. By studying pairs of identical twins in which one sibling has a disease and the other does not, scientists should be able to identify more easily the genes involved in disease.
In the meantime, a lot of biology textbooks may need updating.
Dumanski pointed out, for example, that as his study was going to press, the following statement could be found on the Web site of the National Human Genome Research Institute, the group that financed the government project to decode the human genome: "Most of any one person's DNA, some 99.9 percent, is exactly the same as any other person's DNA. (Identical twins are the exception, with 100 percent similarity)."
That, we now know, no longer appears to be the case.
THE BOTTOM LINE: Identical twins apparently do not have identical DNA.
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