This is a common and difficult question I get asked. Like everything in psychiatry, the answer is not clear.
When people think about genetic disorders, they tend to think about classic genetic diseases. Some examples would be sickle cell anemia or cystic fibrosis. There is a clear pattern of inheritance with a single gene involved in these diseases
The human genome project set out to sequence the entire human genome. While it accomplished the goal it did not offer the personalized medicine and targeted interventions initially promised. What it did reveal was a more complicated interplay of genetics and environmental factors. Depression is a multifactorial disease and does not have a single gene involved in the disorder.
Let’s look at some the evidence supporting the genetic influence on the development of depression.
What Can Family studies tell us ?
The first place to look for a genetic link is family studies. This is one reason we obtain a family history in a psychiatric interview.
MDD is common in families. It’s found 2 to 3 times more often in first-degree biological relatives (e.g. mother or father) of individuals with the disorder than the general population. It’s important to note that the influence of genetics on the development of depression depends on the percent of the genome shared by the individuals. For example, first-degree relatives who share 50% of their genome will have a much greater influence than a second-degree relative who shares 25% of the genome.
What can twin studies tell us ?
The second area of evidence that supports the influence of genetics on depression comes from twin studies.
From the data we know for monozygotic twins (identical twins), there is a 50% chance that one twin will develop the trait (e.g. depression) if the other twin has depression. This number decreases to 20% for fraternal twins who only share 50% of their genome. One flaw in many of these studies is the twins were often raised together in the same environment. There is clearly something to be said for the influence of environment. Some researchers believe twins will influence each other’s behavior when raised together. Identical twins have been known to be treated more similar by their parents than fraternal twins. Taken at face value, when a twin with 100% of the same genetics (identical twins) develops depression the other twin is more likely to also develop depression. Keep in mind, they do not always develop depression even if they share 100% of the genome.
What do adoption studies add?
Adoption studies make an attempt to differentiate the influence of genetics from environmental factors. These studies examine differences in rates of illness among biological relatives as opposed to adoptive relatives. The studies show higher rates of illness among biological parents rather than adoptive parents. This provides some additional evidence to support a genetic influence.
There is clearly a genetic component to depression. However, it’s a complicated process that involves multiple genes interacting with the environment. This makes identifying a single causal gene difficult and likely impossible. There are people biologically predisposed to developing depression, but not everyone with biological predisposition will go on to develop depression.
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