Narnian -
Here is a link to an article describing how "Mountain Majesty Hybrid" was bred. It gets very complicated. In this case, about a dozen genes were combined by crossing, selecting for desired traits, making another cross, etc.
http://tomatoes.ces.ncsu.edu/wp-cont...011.pdf?fwd=no
For more information on tomato genes and breeding, try googling "Randy Gardner tomato", "TGRC tomato", or "Frogsleap Farm".
Fred