Hello There are some diseases that are genetic If that family has any diseases that is hereditary then there are chances of their children getting that disease Having said that it is provisional diagnosis. Kindly consult local ophthalmologist and get proper eye examination done and discuss the issue with him

Only by observing in one generation you can't declare it genetic.its called congenital but when it any disease transmitted to at least three generations in a similar fashion then it's genetic. To decide and declare it confidently genetic counselling is necessary As per your query ,Yes if it's genetically transmitted then the marriages should  never be done in mutual relationships.

Hi, The reasons for blindness can be genetically transmitted. But that depends on the cause, consanguinity and other factors. A pedigree charting is a preliminary step to get an idea of the type of genetic transmission. Need to know the cause of blindness and the pedigree to know the chances of your relatives daughter offsprings having the disease.