One of the main reasons this site is around is to make basic genetic knowledge a casual background feature of the data bases of people who would otherwise not know much about this important science (another reason, at least for me, is to dump a lot of historical and non-science data out there to a scientifically literate audience so they can better form models which influence their view of public policy). So in that spirit, check out Using Karyotypes to Predict Genetic Disorders (it has some neat interactive movies, though if you know genetics, don't worry about it, but if you have been skipping the science posts for lack of a basic background, I advise you check out the link). Remember, to the first approximation it all starts out on basic Mendelian principles.