Why is my vision so horrible?

I thought vision was hereditary, but my mother has 20/20 vision and my father only uses very thin gles for reading. When I was four or five it was discovered my vision is 20/200. I'm 17 now. With gles it is corrected to 20/20 but they are so thick that they make my eyes look huge and it makes me feel like I look like a freak, so I hate wearing them, but without them I am legally blind. Nobody, not even anyone in my extended family, has vision anywhere near this bad, so why me???