Is it permitted to have a child which you know might have a genetic defect that although not life threatening would affect his life (ex. hearing loss)?
It is permitted to have such a child, for a number of reasons. Firstly, we have a commandment to have children and populate the world. The condition of the child is Hashem’s decision. Chizkiyahu didn’t want to have children because he foresaw with Ruach Hakodesh that in the future they would sin. He got sick and was about to die. Yeshayahu came to him and told him to prepare himself to die. He asks why is this? He answered “because you didn’t get married and have children”. He answered him, “it was because I foresee that they will be sinners”. To this Yeshayahu answered him, “The hidden things of heaven are not your business”, you are commanded to fulfill what you are commanded, and Hashem will do what He feels is correct. It is the same with us.
Another point, just because the child will have some difficulty, what right do we have to deny him all of the other advantages of life? On a physical level we are denying him the pleasure, of simply living, of enjoying food, fresh air, and all the pleasures of this world. But on a deeper level, by denying this child the right to live, we are denying him the opportunity to do mitzvos and earn a portion in the world to come.
Chazal tell us that many times a soul will come to this world, with a certain defect, and it is specifically this defect that helps the child’s soul attain perfection, get cleansed, and then the soul can return to its proper place in the world to come.
Hashem should help you, that all your children should be healthy, in all ways.