It is often asked whether a man who carries a serious genetic disease is obligated to fulfill the mitzva of procreation in light of the real risk of having a child who suffers from that disease. Poskim disagree about this; some say that he still has an obligation to have a son and a daughter, for even if they are sick, he has contributed to populating the world. Moreover, there is a good chance that the child will be completely healthy. Others say that the Torah does not command us to procreate when there is a significant risk of having a child whose life will be one of constant suffering. In practice, in cases like these, a wise person should be consulted.
Nowadays, however, by God’s grace, there is a solution for most genetic illnesses, namely, it is often possible to do IVF and then, a few days later, to check the embryos for disease (pre-implantation genetic diagnosis, or PGD), and only implant disease-free embryos in the woman’s uterus. Where this option is available, the couple has an obligation to fulfill the mitzva in this way, so as not to have sick children.
According to R. Waldenberg, even a carrier of a genetic disease is obligated to fulfill the mitzva of procreation. However, if a fetus is shown to have a disease that will leave it physically and mentally disabled, an abortion may be performed up until the seventh month (Tzitz Eliezer 15:43). According to Devar Yehoshua 3, EH 1, a carrier may have children, but is not obligated to. R. Auerbach concludes that “more in-depth investigation is necessary” to determine whether one is obligated to fulfill the mitzva of procreation in such a situation, adding that if his fear of having sick children is so great that he would willingly spend twenty percent of his assets on preventing it, there may be grounds to exempt him, since we know (Rema 656:1) that one need not spend more than twenty percent of one’s money on a positive mitzva (Minḥat Shlomo 3:103:1). According to R. Naḥum Rabinovitch, when there is a risk that a child will suffer greatly and will not live long, it is forbidden to have children (Si’aḥ Naḥum §96).