The case of Muhammad Allahdad Khan v. Muhammad Ismail Khan is a landmark Privy Council decision that authoritatively deals with the doctrine of "Acknowledgement of Paternity" (Iqrar-ul-nasab) in Muslim law. The court held that if a man acknowledges a child as his own, it establishes not only the child's paternity but also presumes a valid marriage between the man and the child's mother, provided there's no legal impossibility for them to have been married. This doctrine is used to establish parentage in cases of uncertainty, not to legitimize a child known to be illegitimate.