The Sefer Ha’hinuch lists as the 195th Biblical command the prohibition against intimate relations with one’s daughter – a prohibition which, interestingly enough, is not explicitly introduced anywhere in the Torah.
The Gemara initially suggests deriving this prohibition through logical deduction (Kal Va’homer) – the Torah explicitly forbids relations with one’s granddaughter, so all the more so, it must forbid relations with one’s daughter, who is a closer relative. However, there is a rule that punishment is not administered to those who violate prohibitions which are deduced through a Kal Va’homer, and have no textual source. As there is a punishment for having relations with one’s daughter (as we will mention later), there must be some source for this prohibition.
The Gemara therefore concludes that the Torah alludes to this prohibition through a “Gezera Shava” – the use of a common word in two different contexts. The Torah says about “revealing the nakedness of” (meaning, intimate relations with) one’s granddaughter, “Ervatecha Hena” – “they are your nakedness.” The word “Hena” appears also in regard to the prohibition against engaging in relations with one’s wife’s daughter or granddaughter even from a different man. The Gemara thus established that just as the prohibition against relations with the offspring of a woman with whom he engaged in relations includes her daughter, likewise, the prohibition against relations with one’s granddaughter applies also to one’s own daughter.
This prohibition applies even to the product of rape; if a man forced himself on a woman, Heaven forbid, and she conceived and had a daughter, the man is forbidden from having relations with the daughter.
One who intentionally commits this transgression in the presence of witnesses after being warned is liable to Serefa (execution by burning). If witnesses did not see the act, then he is liable to Karet. If one violated this prohibition unintentionally, then he must bring a Hatat (sin-offering). If he had produced this daughter with a wife, then he is in violation of two prohibitions – the prohibition against relations with one’s daughter, and the prohibition against relations with one’s wife’s daughter.
If a man engaged in relations with a woman outside of marriage, then according to Torah law, he is permitted to marry that woman’s daughter from a different man. However, the Sages enacted that this man may not marry any of her close relatives – that is, her sister, daughter, mother, either grandmother, or granddaughter (from either her son or daughter). They enacted this prohibition out of the concern that if the man marries this woman’s close relative, and the woman visits their home, they may then have an adulterous relationship, since they are already comfortable with one another. Therefore, as a safeguard, the Sages forbade marrying a close relative of a woman with whom one had engaged in relations out of wedlock.