It wasn't until the end of the 1980s that researchers found the genetic roots of a mental illness -- bipolar disorder. After a decade-long study, they found that people affected by the illness shared a genetic mutation on chromosome 11 [source: Maugh]. For the first time, researchers had a way of focusing in on specific, distinctive genetic material that could help diagnose a mental illness rather than relying solely on a patient's symptoms, which may (and often do) overlap among several disorders. Bipolar disorder and schizophrenia both, for instance, share certain psychotic symptoms as well as depressive episodes, which can make diagnoses difficult in some patients.
Fast-forward across a few more decades of research, and growing evidence suggests that indeed a handful of psychiatric illnesses appear to be hereditary. In 2013, results of the largest genetic study of mental illness to date found that five common mental health conditions may share among them more than just symptoms; they -- including bipolar disorder and schizophrenia -- may share a variation in their DNA.
Bipolar disorder, schizophrenia, autism, attention deficit hyperactivity disorder (ADHD) and major depression may all seem very different from each other, but researchers studying inheritance patterns of mental illness found that people with these five disorders all share a common genetic variation. The variation appears to occur in four regions of human DNA. Two of those regions, two calcium channels known as CACNA1C and CACNB2, are involved in how well calcium gets to and from the brain cells, and helps those brain cells communicate. CACNA1C, for example, is known to be associated with emotions, memory, attention and thinking. (And in previous studies, CACNA1C was linked to bipolar disorder, major depressive disorder and schizophrenia.) Genetic variants linked to these five mental illnesses were also found on chromosomes 3 and 10; continuing research will need to be done to find out more about the impact of the variation in all four DNA regions, as well as the potential role of calcium channel blockers in treating these mental illnesses [source: Cross-Disorder Group of the Psychiatric Genomics Consortium].
Scientists also know that a gene variant may run within a family but may not be expressed the same way from person to person, which explains why your grandmother, your sister and your mother may share the same abnormal genetic variant as part of their unique genetic makeup but have three different diagnoses. Your grandmother could be affected with major depression, your sister with ADHD and your mother with ... nothing. It happens because individual bodies read genetic code in different ways -- the abnormal genetic pattern may exist in all three women, but it may not be activated or interpreted in the same way (or at all) each time. Your set of genes is unique and so is the way you express them.