The Shulhan Aruch, toward the beginning of the Eben Ha’ezer section, rules that one fulfills the Misva of Peru U’rbu (procreation) by begetting at least one boy and at least one girl. This ruling follows the view of Bet Hillel, as recorded by the Talmud in Masechet Yebamot (61b). Bet Shammai disagrees, and maintains that one fulfills the Misva by begetting two sons.

Interestingly, the Rash (Rabbi Shimshon of Sens, France, 12th century), in the Yerushalmi Yebamot, advances a different understanding of Bet Hillel’s view. He contended that if Bet Hillel considers one to have fulfilled the obligation of Peru U’r’bu by begetting a boy and a girl, then certainly he considers the Misva to be fulfilled by fathering two boys. The only argument between Bet Hillel and Bet Shammai, according to the Rash, is whether one fulfills the obligation by fathering a boy and a girl, but all views agree that one fulfills the Misva by fathering two boys.

This is not, however, the accepted understanding of Bet Hillel’s view. The Shulhan Aruch explicitly states that in order to fulfill the Misva of Peru U’r’bu, one must have at least one boy and at least one girl.

Furthermore, in order to fulfill the Misva, one must have at least one son and one daughter who are capable of reproducing. If an only son or an only daughter is physically incapable of begetting children of his or her own, then the father does not fulfill the Misva. If a person produces a Mamzer or Mamzeret (child born from certain forms of illicit relationships), that child counts toward the fulfillment of the Misva. A Mamzer is allowed to marry a Mamzeret, and such children are therefore considered capable of reproducing. Thus, with regard to fulfilling the Misva of Peru U’r’bu, a Mamzer or Mamzeret is no different than an ordinary child. This applies as well to a child who is deaf or mentally impaired. Since these children are capable of reproducing, they count toward the father’s fulfillment of the Misva of Peru U’r’bu.

If a person fathered a son and a daughter, but they passed away during his lifetime, has he fulfilled his Misva of Peru U’r’bu?

The answer depends on whether or not the children had themselves begotten children before their deaths. If one of the children (either the son or the daughter) had produced a son and the other a daughter, and then they died, the grandfather has fulfilled his Misva. If, however, the son or the daughter died without having produced any children, then even though the surviving child has produced a son and a daughter, the grandfather has not fulfilled the Misva of Peru U’r’bu.

Summary: In order to fulfill the Misva of Peru U’r’bu (procreation), a man must father at least one boy and at least one girl, both of whom are physically capable of reproducing. If one begot a boy and a girl and they died during his lifetime, he has not fulfilled the Misva, unless one of them left behind at least one son and the other left behind at least one daughter.