Sandra and Sara are identical twins. When Sara turned 20, she began experiencing auditory hallucinations (hearing voices). She was diagnosed as having schizophrenia. Sandra became concerned because the psychiatrist told her that this disorder tends to run in families and that she had a very good chance of developing it. However, 20 years later, Sandra lives a mentally healthy life with no symptoms of schizophrenia. Even though Sandra and Sara have identical genes, Sandra did not develop the disorder. This example serves as evidence of the importance of:

In gene-environment interactions, the field of epigenetics looks beyond the genotype itself and studies how the same genotype can be expressed in different ways. Scientists studied how the same genotype can turn out to be very different phenotypes. Gene expression is most of the time influenced by the environment in ways that are not always obvious. As we see in this case, identical twins share the same genetic information, their genetic material is exactly the same, but even with identical genes, there can be an incredible amount of difference in how gene expression can turn out throughout each twin’s life, one with schizophrenia and one completely healthy mentally.