I'm a genealogist and I've done their tree. I've found no common ancestors between Scott and Gwen within the past 6 generations (if you want me to go farther you'll have to gibs me dat), although Gwen's tree seems to have a few instances where they kept it in the family...Basic genealogy is as such:
Cal and the Spuds are supposed to have two biological parents, 4 biological grandparents, 8 great-grandparents, 16 great-grandparents, 32 great great grandparents, 64 great-great-great grandparents.
If most of the 16 of their great-great-grandparents b. 1890-1910, and subsequently 8 great-grandparents (b. 1910-1930) all emerge from Midwestern farm communities, and continue to settle in midwestern suburban communities, giving birth to their 4 grandparents in the '40s and '50s, I would say that the 8 grandparents may have convergent biological lines.
We are talking about them coming from 64 individuals, in small rural farming communities in the midwest, originating from farming communities in Europe, in the 19th century.
NOT that I am an authority on the matter, but in some places and times the gene pool is rather narrow and folks could be breeding with their relatives. So if Gwen and Scot share a common great great grandparent, and there are other cousin marriages in their line, the freaky potato gene can find its match.
This is a posibility if this odd trait passes recessively from one generation to the next.