The back of the iris is supposed to be clear.if it start to yellow what is the cause of the condition?

CloudsInYourCoffee??: Technically, you really can't see the "back" of the iris because it's hidden. I'll assume you mean what you see through the pupil that lies behind the iris. That's the lens- and if it becomes cloudy or yellow or anything but clear, that's called a "cataract" and will affect your vision. It can be caused by a lot of things, but most commonly it is part of the aging process. See an opthalmologist.

Jaundice: The white area that surrounds the iris is called the sclera. If it is yellow the condition is called jaundice. Jaundice occurs when the level of bilirubin rises due to liver disease. There are many causes. If you have this you need to see your doctor. Here is more info; http://www.medicinenet.com/jaundice_in_adults/article.htm