Genetics and Schizophrenia - A Twin Study


One would assume that identical twins are, well, identical. But a new study from The University of Western Ontario shows that there are DNA difference among every two people--even if those two people are monozygotic twins.

Shiva Singh, a molecular geneticist, and psychiatrist Dr. Richard O’Reilly have been working to determine the genetic sequencing involved in the development of schizophrenia. Because schizophrenia tends to run in families, with the risk of developing the disorder much greater for people with a sibling or parent with schizophrenia, it is clear that there is a genetic component at work. Singh explains, “We started with the belief that monozygotic twins are genetically identical, so if one member of identical twins has schizophrenia, then the risk for the other twin should be 100 percent, if it's all due to genes. However, studies over the years have shown that the risk of the disease in both twins is only 50 percent." This finding means that either schizophrenia has non-genetic causes or the twins are not completely identical.

To learn more, Singh examined around one million markers of identical twins and their parents where only one of the twins had schizophrenia. What he found is that about 12% of DNA can vary across individuals, meaning that monozygotic twins are not genetically identical. As Singh explains, “Cells are dividing as we develop and differentiate. More importantly, these cells may lose or acquire additional DNA. The genome is not static." He points out that “if schizophrenia is in the genes, then the difference in the genetic makeup of monozygotic twins, with only one disease twin, must have something to do with the disease."

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