If myopia is hereditary how come my parents don't have it?

There is a small heritary factor in myopia. if your parents are myopic you are more likely to be myopic but its quite common for children to be myopic and their parents not. Reading gles are soemthign one develops a need for with age. If you are myopic when you get into your 50's you then need bifocals/varifocals to deal with this problem.