Having a rough and a smooth I thought I'd just add in, a huge number of breeders in both do test for MDR and some breeders do have totally clear lines or carrier only, as with CEA. But with both breeds the gene pools aren't huge, particularly the smooths due to being so rare but roughs became so heavily overbred that they are a genetic bottleneck--or where before certain breeders imported stock resulting in 0 percent inbreeding COI and bringing in clear eye/MD genes. To not breed from affected dogs would impoverish the gene pool even further, both breeds (well variety, every other county they're bred together soon which probably helps the whole genetic diversity business too!) would result in even more of a bottleneck (I feel anyway) and i think many feel we can't afford to narrow the gene pool anymore.
They are both fanastic though, the modern fluffy type rough is a world away in temperament and personality to a classic type rough and to smooths. Smooths are a little more headstrong I find, and free thinking. Both can be quite independent and both are very very noisy but it's part of the charm, I don't just mean barking but they grown and chuff and make all sorts of Wookie type noises! Very expressive! Very chilled though, couldn't have anything else now.