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This requires internet research
I would guess a defect in a specific gene or faulty protein. But @pottersheep is correct. You should research this on the internet.
The only things I can think of that can cause blindness are:
1) A virus/bacterial/parasitical/amoebic infection in the eye that caused the nerves to be damaged or deteriorated, or the eye itself could have been damaged in some way due to these things.
2) Same as #1, only that the part of the brain that controls sight could be affected
3) Physical damage to the eye, such as seriously cutting it, or getting some sort of chemical or debris in the eye.
4) Birth defect of some sort (too many birth defect possibilities, but the person would most likely be BORN blind). *Being born blind is technically a birth defect since they are born "abnormal." In this case, they are born without sight.*
5) Cancer of the eyes (which kinda goes wth #1, except that cancer is cancer... not a virus, bacteria, etc etc)
6) Staring at the sun... not trying to joke here, because this really CAN make you go blind by basically frying the optic nerves, or something like that.
7) Psychological condition. There are some true instances of people losing their sight psychologically, where their eye is not damaged in any way, but for some reason or another, they literally "think" they are blind. This one gets complicated, but basically, they cant SEE, but their eye WORKS.
8) While the person was developing in the womb, their mother may have been introduced to some kind of harmful substance; whether it was intentional or not. Examples would be drugs (the bad kinds, like crack cocaine and stuff), medications ("good" drugs can sometimes cause birth defects if the mother is pregnant while taking them), cigarettes, alcohol, and even in some cases, pollution. ...
*9)* Some people are just born blind. Basically, something went "wrong" while they were developing in their mother's womb, and nature/God simply decided that they would be born blind. This may be just one of those mysteries in our world that humanity simply cannot explain with a definite answer.
That is all that comes to mind at this moment.