Study Guide Kiddushin 68 The derivation that there is no betrothal in a case of a forbidden relationship from the case of a man who marries his wife’s sister as the last verse compared all cases of forbidden relationships. But why would we not derive it from a man who betroths a woman who is a nidda where the betrothal would be valid as she is also one of the forbidden relationships? Even though the Gemara brings an answer to the question, Rav Acha Bar Yaakov offers a different source – derived from a yevama. Why then, wouldn’t we learn from there to all cases of negative prohibitions? There is a different verse from which we derive that a marriage that is forbidden by a negative prohibition is a valid marriage. Only Rabbi Akiva holds it is not – what does he do with that verse?  From where do we that betrothal to a Canaanite slave or gentile woman is not valid and that the child follows the mother?