>Should genetic screening be mandatory in certain cases?
You find a "bad" gene, you tell the subject about it and inform them that their children might/will have it too/will have a certain condition.
If at all required, mandatory screening and advisory would be the extreme limits of what should be allowed(But what conditions would require mandatory screening and how are you going to screen two people before they have sex? Think about the logistics and the invasion of privacy!). Even in those cases, the government shouldn't have the power to sterilize the subject.
You can run educational campaigns. Teach people this stuff with sex-education or have married couples take a course(you will still miss out on a lot of people).
Eugenics takes away a basic right of a life form.
Update:
I understand that you may be speaking about people who have been diagnosed as having a certain genetic condition. If a disease has the capacity to cause public harm, there are exceptions made to the professional secrecy that a doctor is supposed to maintain. This involves informing relatives/a potential spouse about a condition that the patient may transmit. In cases of infectious diseases, quarantine and isolation procedures are well established. But most are for infectious conditions which can harm the public. So if the off-springs of individuals with bad genes can infect large number of people with some contagious condition, isolation protocols may apply. But that is still hypothetical.
If we dream up an horrendously bad genetic condition which can disrupt the human race, someone is going to come with horrendous regulations to prevent that. But the truth is most people suffering from "bad" gene induced conditions do not lead a normal life(do not have kids). If the sufferer of a genetic condition can easily have kids, then that condition may not be that "bad". Carriers of such genes could do with some pre conceptional counseling.