Crohn's is known to be herditary (my grandfather has/had two brothers with it and my aunt has it). Gall bladder disease is also herditary (just about everyone in my family has had some problem with it, including me). I can't see why those two things would be herditary and IBS wouldn't be.
Of course, IBS can be caused by a number of different things, some of which won't be herditary. For instance, if you have a bacterial overgrowth that casues IBS, then that's probably not herditary but is most likely caused by diet and/or medication (although some people may inherit a weak immune system which makes them more likely to catch and retain bad bacteria). If you have IBS because of gall bladder problems--or the lack of one--then that's inherited in so far as bad gall bladders are herditary. If you have IBS because your intestines just don't function properly, then I think that could definitely be herditary.
And there's certainly a connection between different digestive tract ills. I was reading just the other day that ulcers are almost--if not always--caused by bacteria. So your mother has ulcers and you have IBS. Think maybe you also have a problem with bacteria, only it's in the intestines rather than the stomach? Your mother has acid reflux, you have bile diarrhea. Think maybe her bile goes up yours goes down? All of that stuff is related, even if it manifests itself in different locations. In the same way that your mother can have arthritis in her hands and you can have it in your back.
I think there has been a backlash against blaming genes for everything because people don't want to think that they are helpless in the face of DNA. For instance, fat. Fat is genetic. This has been proven, even allowing for outside influences like eating habits. You have your dad's eyes, your mother's hair, your grandmother's nose, why wouldn't you have the body frame of someone in your family? We expect tall people to have tall children and white people to have white children; why shouldn't fat people have fat children? People say that fat, unlike all the other body traits, is something that's controllable and that only weak people are fat and that people are using genes as an excuse. I think that some doctors have this same mind set about IBS. It's only been recently that doctors have started to realize that it's not a disease beloning solely to hypocondriacs or that it's not caused by stress (although stress will make it worse). But for some there's still that mindset that IBS is your fault--that you worried yourself into it and that if you quit worrying it will go away (ulcers were blamed on stress for ages too). They completely ignore the fact that everyone in your family has had digestive disorders of some sort. It makes us and doctors feel helpless to say, "Well, it's this way just because of genes." In some ways it's easier to blame ourselves because then we feel like we might can change things. If we never ate again, maybe we'd be a size 0; if we never worried again, maybe we'd have normal bowel movements. It creates hope, but really it's false hope. We just have to say that God/natural selection created us with IBS and rather than blaming ourselves for it, we need to find ways to combat it and to cope with it. People are born with and develop diseases all the time. You and everyone else here is no more to blame for their IBS than a person is to blame for being born with diabetes.