Psoriasis is a skin condition marked by a rapid buildup of rough, dry, dead skin cells that form thick scales. Arthritis causes pain and stiffness in your joints. Both are autoimmune problems disorders that occur when your body's immune system, which normally fights harmful organisms such as viruses and bacteria, begins to attack healthy cells and tissue. The abnormal immune response causes inflammation in your joints as well as the overproduction of skin cells.
It's not entirely clear why the immune system turns on itself, but it seems likely that both genetic and environmental factors play a role. Many people with psoriatic arthritis have a close relative, such as a parent or sibling, with the disease, and researchers have discovered certain gene mutations that appear to be associated with psoriatic arthritis.
Having a genetic mutation doesn't necessarily mean you'll develop psoriatic arthritis, but it does mean you have a greater tendency to do so than other people do. Physical trauma or something in the environment such as a viral or bacterial infection may eventually trigger psoriatic arthritis in people who have an inherited tendency.
The above information thankfully comes from the Mayo Clinic.com at the following link.