According to the Stanford School of Medicine, no one simply "inherits" depression thanks to the genes of their parents. Rather, each person inherits a unique combination of genes, which in certain combinations can predispose her to a particular illness. While the exact combinations involved with depression are unclear, what is certain is that some combination of genetics, psychological and physical factors can come together to bring about the onset of this disease.
Quanitifying the Genetic Risks
Family history and genetics play an undisputed role in depression disorders, according to a 2008 article appearing on CNN.com titled "Are you at risk for depression? A quick family history could tell." The article summarizes the research findings of Dr. Terrie E. Moffitt, a professor of psychology and neuroscience at Duke University's Institute for Genome Sciences & Policy. Her study, published in the "Archives of General Psychiatry," concluded that "the more relatives a person had with major depression, anxiety disorder, or alcohol or drug dependence, the more likely he or she was to have that condition too." In addition, the study concluded that the stronger the family history, the greater the risk of developing recurrent bouts of the condition.
Twins Studies and Gender Risks
Much of what is known today about depression and genetics has been learned from studies completed on identical twins. According to the National Twin Registry website, twin studies are considered the "gold standard for research when investigating the relative importance of genetics and environmental influences to the development of complex traits and diseases in human populations." Furthermore, the use of twins can significantly enhance the statistical power of a particular genetic study by limiting the amount of genetic and/or environmental variability.
To that end, a 2006 study article titled " A Swedish National Twin Study of Lifetime Major Depression," which appeared in the January 2006 issue of the "American Journal of Psychiatry," indicated that inheriting a predisposition or liability toward major depression was much higher in women than in men. In fact, the study concluded that 46 percent of women with a family history of depression developed the condition, while only 29 percent of men experienced the same thing. These numbers suggest that some genetic risk factors for developing depression may be sex-specific.
Isolating a Depression Gene
While some rare diseases like Huntington's disease or certain types of cystic fibrosis are caused by a single defective gene, this does not appear to be the case with depression. Instead, Stanford's School of Medicine argues that depression appears to be the result of combinations of genetic changes that predispose some people to become ill. While the exact number of genes involved in depression are unknown, it is doubtful that one specific gene is responsible for the varying types of depression experienced by millions of people.
