How thin is too thin?

by Tammy
(Vancouver, WA)

I recently inherited (literally)my grandfather's Jack Russell. He is four years old, and his coat is so thin, I can see his skin beneath it? Is this normal? I have recently learned that he could have been fed the wrong kind of dog food. I have been told that Jacks have a tendency to be allergic to the corn in some dog foods. Could this cause his coat to be too thin?