You already answered it in your question, genetic problem. All of us are born according to the Gods will, His unchangeable laws. The genetic distribution is random, but random according to the laws of the genetic distribution. It is not, 100% not , payback for any previous life or parents sins. Good news, in this trauma, is that Allah does not place a burden beyond the capacity. The another good news is that the capacity increases as you face the problems. The bearer of the genetic deficiency, if it can be termed so negatively, and the helpers get help beyond the imagination. It depends on the attitude. My wife had stroke and is disabled for last eight years. Both of brothers, who recently passed away, also had strokes and so did her sister and she is gone too. yes it is genetics at play, may be not as severe as you may be referring to. I am very lucky that she is very content and alway say that Allah ka shukr hai. She always says that it could have been worst.
One of my friend's son recently had cancer in one of the eye and it had to be removed. he is a teenager. His father was devastated and kept on saying what did he do wrong??? I tried all my best to calm him down and told him that it is not his fault, and it is certainly not any payback for a past life or present life's sins. He kept saying that he does 5 times Namaz, then why it happened to him.
simply stating, it just happens, and it has happened to others as well. May Allah give us strength to go through all of the challenges. We will be tried with wealth, health and all that makes the life.