Nobody knows exactly what causes psoriasis, but the disease clearly has a genetic component. It is believed that a specific trigger, such as emotional stress, injury to the skin, certain infections (especially streptococcal infections) or a reaction to medication can trigger the skin condition in people who are genetically predisposed to the disease.
One way researchers have been able to learn about the role genes play is by looking at families who have the disease. About 2 percent of the population has psoriasis. But if you have a brother or sister with psoriasis, you have a 50 percent risk of eventually developing the disease if both parents have the disease, a 16 percent risk if one parent has the disease and an 8 percent risk if neither parent is affected.
An especially useful source of genetic information has come from studies involving twins with psoriasis. Fraternal twins have a 20 percent chance of also having the disease, while identical twins have a 70 percent chance. These family studies illustrate that the more genes you share with someone who has psoriasis, the greater the chances that you'll have it, too.
The studies also show that genes don't tell the whole story, which is why researchers are working to learn more about what triggers psoriasis. The genetic theory is complicated by the fact that up to a dozen genes may play a role in psoriasis, and people must inherit a combination of these to develop the disease.
Copyright 2009 NBC Health
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