The Mitzva of Piru U’Rivu requires you to have one son and one daughter. If someone’s son and/or daughter die in their lifetime r”l they are not yotzei the mitzva. However the gemara says that for this purpose, grandchildren are like children and can count in place of their parents.
However there are rules to this. For example if the son dies and he leaves both a son and a daughter of his own and the daughter also dies and does not leave any children, even though there is a grandson and granddaughter from the next generation, the grandfather is not yotzei the mitzva. He must have offspring from both the son and daughter for them to be counted after their own death. Additionally the gemara says that if both the children die and the son leaves behind a daughter and the daughter leaves behind a son, since there is a grandson and granddaughter from both a son and daughter he is yotzei his Chiyuv. (Yevamos 62b)
What happens if a son and daughter both die and they each leave one son and no daughters? According to the Rambam (Ishus 15:5) he is not yotzei because he needs a grandson and granddaughter and two boys aren’t sufficient. According to Tosfos (Yevamos 62b d”h V’Kol Shekein) if each one leaves a son he would be yotzei since he had a son and a daughter and they left behind sons. The Shiltei HaGiborim goes even further and says that even if the son and daughter each left behind a daughter, he is yotzei his Chiyuv of Piru U’Rivu.
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