[ti:Study: Identical Twins Are Not Genetically Exact Copies]
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[00:00.04]A new study suggests that identical twins
[00:04.56]are not exactly the same genetically.
[00:09.88]Identical twins are two babies
[00:13.12]that come from the same fertilized egg.
[00:18.00]Scientists in Iceland examined DNA from 387 pairs
[00:25.32]of identical twins, their parents, children, husbands or wives.
[00:32.80]The examinations led the team to find
[00:36.64]"early mutations that separate identical twins,"
[00:41.60]lead researcher and geneticist Kari Stefansson said.
[00:47.72]He is a professor at the University of Iceland
[00:52.08]and founder of the company deCODE genetics.
[00:58.08]Mutations are small changes in DNA that can happen
[01:03.44]when a cell divides in an attempt to copy itself.
[01:09.04]These small changes can influence a person's physical appearance
[01:14.76]or control a person's ability to fight a disease.
[01:21.40]The newly-discovered mutations show that identical twins
[01:26.80]do have genetic differences, the researchers said.
[01:32.48]The results were recently published in Nature Genetics.
[01:39.00]On average, identical twins have 5.2
[01:44.28]of these early genetic differences, the researchers found.
[01:49.80]But about 15 percent of identical twin pairs have more than that.
[01:56.80]Some might have as many as 100 genetic differences, Stefansson said.
[02:04.88]These differences represent a small part of each twin's genetic material.
[02:12.76]But they could influence why one twin is taller
[02:17.12]or why one is at greater risk for some cancers than the other.
[02:24.32]In the past, many researchers believed physical differences
[02:29.84]seen in identical twins were related mostly to environmental influences,
[02:37.16]such as nutrition or lifestyle behaviors.
[02:43.00]Jan Dumanski is a geneticist at Sweden's Uppsala University.
[02:49.36]He was not involved in the study.
[02:53.72]He praised the findings as "a clear
[02:57.00]and important contribution" to medical research.
[03:02.24]"The implication is that we have to be very careful
[03:07.00]when we are using twins as a model" for examining
[03:11.44]the influences of genetics or the environment, Dumanski said.
[03:18.76]A 2008 paper in The American Journal of Human Genetics
[03:24.24]found some genetic differences between identical twins.
[03:30.12]The new study, however, goes beyond earlier work
[03:34.60]by including the DNA of parents, children,
[03:39.00]husbands and wives of identical twins.
[03:43.96]Studying family members permitted the researchers to examine
[03:49.00]when genetic mutations happened in two different kinds of cells:
[03:54.20]those present in only one individual
[03:58.00]and those passed on to the person's children.
[04:03.04]Stefansson said his team found twins
[04:06.48]where a mutation is present in all cells of the body of one twin,
[04:12.20]but not in the other twin at all.
[04:16.12]However, "sometimes the second twin may show the mutation
[04:21.36]in some cells, but not all cells," he added.
[04:27.00]The researchers said they also found mutations
[04:31.44]that came about before the developing embryo split in two.
[04:37.80]Nancy Segal is a psychologist who studies twins
[04:42.20]at California State University, Fullerton.
[04:46.88]She was not involved in the study.
[04:49.92]But she called the results "heroic and really significant."
[04:56.88]Segal added that the research is likely to persuade more scientists
[05:03.00]to rethink the influences of genetics and environment on twins.
[05:10.52]"Twins are very alike, but it is not a perfect similarity," she said.
[05:18.64]I'm Bryan Lynn. 更多听力请访问21VOA.COM