I just read an article stating that the rate of genetic deafness in the US has doubled in the past 200 years, since the advent of schools for the deaf, which taught sign language and enabled deaf individuals to intermarry. Parents who each have the same recessive gene for any one particular form of deafness pass that deafness on to all of their children.

No shit, Sherlock. Ya think?

This happens anmong the visually impaired, as well. Very often they marry and tend to live near each other. The social interaction at schools for the blind contributes to this the same way it does to deafness.