OUr genes determine, quite literally, who we are. Everything from our hair and eye color, to our temper, to our hand-eye coordination, to our height and weight and sexual preference is in our genes. Also, you'll find our propensity toward genetic diseases written there as well.
It's important to understand that genetic diseases doesn't just mean something simple like "My father had rheumatoid arthritis, so there's a good chance I'll develop it as well." Thought that is a part of it, as I know from experience. Both my father and my biological grandfather suffered from rheumatoid arthritis, and by the age of nine I was diagnosed with it as well. This is a clear case of genetic diseases being passed down from generation to generation, and it's a simple fact of life.
But there are other type of genetic diseases, more likely genetic predispositions, that have to be accounted for as well. My father, for example, suffered his first heart attack before he turned forty years old. So did his father. So did my uncle on my father's side. All of them, one way or another, developed heart disease at a young age. All of them were predisposed to it because of genetics. Two of them - my father and grandfather - died at age sixty five or young.
So when it comes to genetic diseases, that's the type of thing you have to be aware of. Your family history can clue you in as to what to expect of your own health and well-being over the years, and if you're more likely tol develop an inflammatory disease (heart disease, diabetes, arthritis) than not.
Cancer also plays a role in your family history of diseases. My paternal grandmother, for example, developed and eventually died from colon cancer. While the other stuff (heart disease) scares me, the prospect of developing colon-rectal cancer absolutely terrifies me. I doubt there's a worse way to die than that, and as a result of my predisposition toward these genetic diseases, I've adjusted my diet and exercise routines accordingly.
For example, I now eat at least five servings of fruit and vegetables per day. This has multiple benefits, from releasing anti-imflammatory enzymes into the bloodstream (therefore alleviating my arthritis) to cleansing out my digestive tract, making me much less likely to develop colo-rectal cancer. You need fresh fruits and vegetables, and tons of fiber when you're predisposed to these kinds of things.