Maybe this link from the CCFC will help in your quest for understanding this DD...
http://www.ccfc.ca/English/research/todaysresearch.html
Many genes have been identified that are directly related to crohns disease which are different from UC, which makes sence since they are 2 seperate diseases with very similar symptoms...the thing is it's not just a matter of being predisposed to IBD it's also having it triggered and this is where researchers are having the hardest time...they have finally found that smoking (including second-hand smoke) is one known trigger for crohns, yet for UC the carbon monoxide in the nicotine can be quite a benefit in relieving symptoms, odd but true), stress has also been identified with UC (I'm sure it's linked to CD too but from everything I've read so far it hasn't been stated like it has been in regards to UC).
Also, according to Dr.Kevin Rioux, Genetic influences have long been suspected to play a role in IBD based on the following clues...
1) A significant % (~20%) of IBD patients have a relative that also suffers from IBD (either UC or CD).
2) Certain ethnic groups are at substantially higer risk for IBD than the general population.
3) There are extream cases of familial IBD where nearly every family member suffers from the condition.
4) In studies of identical twins, if one twin has crohns there is a 40-60% chance that the other twin will also have the disease (it's just a matter of it being triggered).
If you have IBD it has been estimated that your offspring will have a 10- to 15- fold increased risk of developing IBD compared with those who do not have a parent with IBD.
In the past few years, advanced genetic techniques have allowed scientists to discover at least 9 different gene clusters that contribute to the development of IBD and define the severity and behavior of the disease over time. Some of these genes encode factors involved in recognition and defense against bacteria. Mutations in immune response genes may adversely affect how the immune system reacts to harmless bacteria and this probably contributes to the development of IBD...
I guess that's why researchers are looking hard at probiotics and prebiotics and there benefits to IBD (IBS as well apparently)...it's known that IBDers are prone to bacterial overgrowth.
The good news is that they are learning more and more, the bad news is it's taking too darn long...I've had this thing non-stop for 16 yrs with no full remission...the only thing that keeps me going is the hopes they will soon figure it all out and cure us all. BTW, I have crohns since my early 20's and my mom has UC which wan't triggered (she didn't get sick with it) until she was in her mid 60's...so who's to say that many, if not all of us, have many family members that also have IBD but it just has not been triggered for them yet, our bodies are very complex and so is IBD, what triggers it for one may not necessarily trigger it for all (especially family members growing up on the same diet/lifestyle and such).