Depression runs in families. There has been some debate whether this is the result of genetics (a depression gene is inherited from a depressed parent) or whether it is more environmental (the negative effects of living with a depressed parent). It turns out that both play a substantial role.

Bipolar disorder, in which a person usually has episodes of both mania and depression, best illustrates the genetic connection. This disorder affects about 1 percent of the general population. But a person with a parent, sibling or child who has been manic has a 15% chance of developing this disorder during his or her lifetime. If both parents have bipolar disorder, there is a 75% chance that one or more of their children will have it. If one identical twin has the disorder, there is a 60% to 80% chance that the other will too.

This last statistic is interesting, because it shows that genetic inheritance is not all there is to it. Identical twins have identical genes, so you would expect 100% matching among twins. However, because the risk is less than 100%, environmental influences (including life experiences) must have some effect on whether or how bipolar disorder occurs.

The chance of having major depression is also higher for people who have relatives with the disorder, although the link is not as strong as it is for bipolar disorder. In milder forms of depression, genes may be less influential. Scientists believe there are many genes controlling mood and influencing depression.

Genes supply the instructions for how the brain develops from conception through death and for everything the brain does in between. There are huge numbers of instructions. Genetics controls an almost infinite variety of processes. Chemicals relay messages inside cells and between cells. There is complicated circuitry along which millions of signals travel. The circuits group into major pathways with specific jobs, such as regulating mood or hanging on to memories.

It is still unknown where depression "lives" in the brain, but advances in genetics and in new technology (specifically the ability to take pictures of the brain at work) are likely to give detailed answers in the coming years.