Genetics and depression

It has long been known that depressive illnesses can run in families, but until fairly recently it was not fully known whether people inherited a susceptibility to these illnesses or if something else such as the environment was the true culprit. Those who research depression have been able to determine that to some extent depressive illnesses can be inherited. What appears to be inherited is a vulnerability to depression. This means that if we have close relatives who have clinical depression, we may inherit a tendency to develop the illness. It does not mean that we are destined to become depressed.

Genes that we inherit from our parents determine many things about us such as our gender and the color of our eyes and hair. Our genes also determine which illnesses we may be vulnerable to at some point in our lives. Every cell in the human body contains somewhere between 50,000 and 100,000 genes. They are all made up of something called deoxyribonucleic acid, or DNA. Genes are located on chromosomes within the nucleus of each cell. All of the cells in the body, except sex cells, contain 46 chromosomes, and genes are typically located in a specific place on a particular chromosome. Except for identical twins, no two people in the world have the exact same genetic makeup.

Research on the heredity of depression within families shows that some individuals are more likely to develop the illness than others. If you have a parent or sibling that has had major depression, you may be 1.5 to 3 times more likely to develop the condition than those who do not have a close relative with the condition. You would also have a higher chance of developing bipolar disorder. Because close relatives of those with clinical depression have such a vulnerability to developing the condition themselves strongly suggests that it can be an inherited illness.

Bipolar disorder has a strong genetic influence. Of those with bipolar disorder, approximately 50% of them have a parent with a history of clinical depression. When a mother or father has bipolar disorder, their child will have a 25% chance of developing some type of clinical depression. If both parents have bipolar disorder, the chance of their child also developing bipolar disorder is between 50% and 75%. Brothers and sisters of those with bipolar disorder may be 8 to 18 times more likely to develop bipolar disorder, and 2 to 10 times more likely to develop major depressive disorder than others with no such siblings.

Twin Studies
Much of what we know about the genetic influence of clinical depression is based upon research that has been done with identical twins. Identical twins are very helpful to researchers since they both have the exact same genetic code. It has been found that when one identical twin becomes depressed the other will also develop clinical depression approximately 76% of the time. When identical twins are raised apart from each other, they will both become depressed about 67% of the time. Because both twins become depressed at such a high rate, the implication is that there is a strong genetic influence. If it happened that when one twin becomes clinically depressed the other always develops depression, then clinical depression would likely be entirely genetic. However because the rate of both identical twins developing depression is not closer to 100% this tells us that there are other things that influence a person's vulnerability to depression. These may include environmental factors such as childhood experiences, current stressors, traumatic events, exposure to substances, medical illnesses, etc.

Research has also been done with fraternal twins. Unlike identical twins who have the same genetic code, these siblings share only about 50% of their genetic makeup and do not necessarily look alike. Studies have shown that when one fraternal twin becomes depressed, the other also develops depression about 19% of the time. This is still a higher rate of depression when compared to overall rates for the general public, again pointing towards a genetic influence in the development of clinical depression.

A Gene for Depression?
Research on the genetic causes of clinical depression has attempted to identify one or more specific genes that may lead to the development of a depressive illness. Although there have been a number of studies that appear to name a particular gene as the culprit there has been little consistency among their results. However, the outcome of some research has suggested that there may be specific genes that cause clinical depression to develop within certain families and not in others.

At this time there is much that we do not know about how genes may predispose a person to a depressive illness. Research has yet to identify a clear link between a specific gene and a vulnerability to depression in everyone. Rather than the possibility of only a single gene being responsible for the development of clinical depression, it appears to be more likely that a number of genes acting together may cause a person to become vulnerable to depression.
Just because a person inherits a gene that predisposes him or her to a depressive illness, it does not mean that he or she is destined to develop major depression or bipolar disorder. It is believed that a genetic influence is only partially responsible for causing depression. Other factors may also play a role.