How does finding a genetic relationship to disease help with developing a medical treatment for it? Can you give easy to understand examples?
Hehe, Chalnoth understood your question one way, and I understood it in a completely different way. Here is my quick and woefully vague answer for the question as I understood it. I'm a biologist, but I'm not particularly interested in medicine, hence the lack of specific examples
Ideally, you want to treat diseases right at the cause. If you know that a gene or group of genes goes wrong in a given disease, you've got the foundation for that. I'm not sure I can think of any
successful (as in widely used in clinical practice) examples off hand, but a variety of approaches are possible (and indeed actively researched). For instance:
(1)
gene therapy: delivering the correct version of a disease gene (e.g. the chloride transporter mutated in cystic fibrosis) into the body so your cells can use it to make a functioning gene product.
(2)
antisense therapy (which I found while looking through the gene therapy article

): if the problem is a gene that does the wrong job, as opposed to not doing its job at all, you can try to turn it off. This exploits a natural mechanism of gene regulation, in which small RNAs (
such as these) bind to messenger RNA and prevent it from being translated into protein. If you know which gene you want switched off, you can synthesise a specific stretch of nucleic acid that'll target it.
(3)
drug design: if you know that a disease is caused by a malfunctioning protein, you can look for molecules that will specifically stop it from messing up. That's a lot easier than just trying random drugs until you get it right.