The issue of heredity is mirrored in the Almighty’s words: “From what substance did He create him? (18) From a drop of fluid He created him and proportioned him (19)” (Sūrat ʿAbasa). The act of proportioning occurs within that very drop, for every hereditary attribute is borne by the male sperm-cell, passed down from fathers and forefathers, and likewise by the female gamete—the ovum—coming from the mother’s own paternal line.

Hence the Prophet’s ﷺ saying, “Perhaps a lineage has asserted itself,” sets forth the mechanism by which recessive hereditary traits are transmitted. Although such traits do not appear outwardly in either parent, both may carry them, and they can surface in some of the children. Allah knows best.