okay, here's the thing about genetic diseases. We all have two variants of the same gene, right (remember high school biology?). Genes are translated into proteins which do almost all the work of the body. Some genes are inherited dominantly-meaning that only one variant of the gene has to be mutated in order for the disease to occur (this would be like Huntington's-a genetic disease that you always get it you have it from one parent). Crohn's is a more complex case. Research tells us that multiple genes are involved in the disease, and that it may not be the same genes for every person. Furthermore, many of these genes may not be expressed dominantly. For example, say you are born with a few mutated variants of a disease that predispose you to crohn's. Because you still have one functioning variant of the protein, your cells are still able to behave normally. However, our cells make mistakes and are dividing every day. Each cell division is another chance for a new mutation. Probability says that there is a chance (over our lives) that the non-mutated variants will mutate. When you have a genetic predisposition (one mutated variant passed from one of your parents), and this happens, there is no longer a functioning protein in the cell and it can no longer behave normally. In somebody with crohn's, it is a mixture of this sort of chance event, mixed with more dominantly inherited genetic variations, and then environmental factors which can speed up the rate of mutation in your cells. See, it is all a genetic lottery. That is not to say that the disease is entirely genetic, but some part of it is definitely due to this.
Sorry-I am a bio-sciences girl. There are a lot of misconceptions about genetics out there. Hope I cleared up some things for those of you who aren't so biology-crazy.
