Genetic diseases are diseases that are passed through DNA. These aren't diseases you can catch but are received through parents. Through gene mapping it is possible to identify these sequences and determine the chances offspring will have the disease. According to this article, this is done through genetic markers that identify the specific sequences that cause the disease. Even this isn't perfect due to recessive alleles and different combinations of genes having different outcomes. Again the field of genetics is constantly being explored and having new break throughs daily. If I find anything else i will post it to this blog. http://www.sciencemag.org/content/235/4796/1616.short