Blind parents have a blind son?

So tonight at work there was a blind man and his wife, later his parents showed up and they too were obviously visually impaired, I didn't want to stare but it looked like they were each missing one eyeball. For both parents to be blind and appear to be missing and eye, and for their son to also be blind makes me think it's some hereditary eyeball degenerating disease or something. Does anybody have any idea what the story could be behind something like this? Just curious