Identical Twins Are Not So Identical, Study Suggests
An anonymous reader quotes a report from The Guardian: Genetic differences between identical twins can begin very early in embryonic development, according to a study that researchers say has implications for examining the effects of nature versus nurture. Identical -- or monozygotic -- twins come from a single fertilized egg that splits in two. They are important research subjects because they are thought to have minimal genetic differences. This means that when physical or behavioral differences emerge, environmental factors are presumed to be the likely cause. But the new research, published on Thursday in the journal Nature Genetics, suggests the role of genetic factors in shaping these differences has been underestimated. [Kari Stefansson, the co-author of the paper and head of Iceland's deCODE genetics] and his team sequenced the genomes of 387 pairs of identical twins and their parents, spouses and children in order to track genetic mutations. They measured mutations that occurred during embryonic growth and found that identical twins differed by an average of 5.2 early developmental mutations. In 15% of twins, the number of diverging mutations was higher. When a mutation happened in the first few weeks of embryonic development, it would be expected to be widespread both in an individual's cells and in those of their offspring. In one of the pairs of twins studied, for example, a mutation was present in all cells in one sibling's body -- meaning it is likely to have happened very early in development -- but not at all in the other twin. Stefansson said that out of the initial mass that would go on to form the individuals, "one of the twins is made out of the descendants of the cell where the mutation took place and nothing else," while the other was not. "These mutations are interesting because they allow you to begin to explore the way in which twinning happens."
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