The Genetics Of Mental Illness

Have you ever noticed that many things run in families? Whether it's heart disease, arthritis or red hair, it is often fairly simple to trace the lines of inheritance throughout the generations. But sometimes, clarity is hard to come by. In some families, several different hereditary disorders are present, but seemingly with no logical pattern.

Take autoimmune disorders, for example. It is not uncommon to see a family in which one person has multiple sclerosis, another has lupus and still another has myasthenia gravis or diabetes. These are all autoimmune disorders, but considerably different conditions. So why does this happen? Autoimmune diseases are not passed down by one single gene, but likely a combination of several genes along with other factors, such as the environment, that trigger the disease to develop.

It is much the same with the inheritance of psychiatric disorders. It is not uncommon for a family to have individuals with schizophrenia, bipolar disorder, clinical depression, eating disorders, addiction, and/or other conditions. Typically, psychiatric disorders do not follow the simple rules of Mendel's laws of inheritance. And although the full picture is not yet known, it is clear that they arise as a result of both genetic and environmental factors.

Genetic counselors who provide information to people diagnosed with or at risk for psychiatric disorders often use a "jar model" to explain risk. The metaphor involves our mental health being represented by a jar that has large rocks, pebbles and sand within. The large rocks, for example, may represent our genetic predisposition toward psychiatric illness. The pebbles and sand may represent different types of environmental or other factors that add to the genetic background. If one accumulates too much to fit in the jar during the course of one's lifetime, the overflow results in the onset of psychiatric illness. But if one's rocks, pebbles and sand don't overflow the jar, the onset may never occur, even with a genetic predisposition.

Importantly, everyone has some genetic vulnerability to psychiatric illness, which cannot change. But we can add to the capacity of our mental illness jars by, in a sense, stacking rings on top of it. This creates extra space for environmental factors, so that the jar takes longer and requires more environmental impacts to fill. The rings on top may include protective factors, like sleep, nutrition, exercise, and good social support. So a genetic susceptibility to psychiatric illness increases the likelihood that someone will be affected by one (or more) of these disorders, but is certainly no guarantee, because so many other factors are necessary for their onset. Conversely, without a genetic predisposition, the individual will likely never be affected, regardless of the environmental factors.

Naturally, the jar model is just a tool. It is most effective and helpful in the hands of a genetic counselor or another health care professional familiar with the multi-factorial concepts being described. Genetic counselors can play a vital role in helping families with their feelings associated with mental and physical health conditions, as well as understanding how genetics contributes to conditions that run in the family. If you'd like to find a genetic counselor to discuss mental health or other healthcare questions, please use the Find a Genetic Counselor tool on the website of the National Society of Genetic Counselors (NSGC) at nsgc.org.

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