1) THE DAUGHTER OF A YISRAEL MARRIED TO A "CHALALAH"
QUESTION: The Mishnah states that the daughter of a Yisrael who married a Chalalah (a woman who is prohibited from marrying a Kohen) is permitted to marry a Kohen. The Gemara quotes Rebbi Yochanan in the name of Rebbi Shimon (or Rebbi Yishmael) who says that the source for this law is a Gezeirah Shavah of "v'Lo Yechalel Zar'o b'Amav" (Vayikra 21:15) and "Lo Yitama Ba'al b'Amav" (Vayikra 21:4).
The Gemara earlier (end of the previous Perek) teaches various guidelines for establishing one's lineage. The Mishnah there (66b) states, "Wherever [a marriage is performed in which] Kidushin takes effect and no transgression is involved, the status of the offspring follows the status of the father." Since the case of the Mishnah here (a Jewish man marries a Chalalah) involves no transgression, it is obvious that the daughter has the status of the father (and is a full-fledged Yisraelis and not a Chalalah). Why is a special source necessary to teach this Halachah?
(a) In his first answer, the PNEI YEHOSHUA explains that "v'Lo Yechalel Zar'o b'Amav" is the verse that teaches that a child from a union which involves an Isur Kehunah (a Kohen married a woman prohibited to him) is a Chalal. The Torah states "Zar'o" ("his offspring") with no conditions, implying that all of his offspring without exception -- even future generations that will be born from this Chalal -- are disqualified from the Kehunah. Therefore, a special source is needed to teach that his grandchildren are not necessarily disqualified from the Kehunah.
(b) In his second answer, the Pnei Yehoshua explains that the source for the principle of the Mishnah earlier (66b) is the Gemara here. One might have limited the rule that "the status of the offspring follows the status of the father" to a case in which both parents are of completely pure lineage, such as where the father is a Kohen and the mother a Yisraelis, or where the father is a Yisrael and the mother is a Kohenes. Perhaps only in such a case does the child have the status of the father, based on the verse, "l'Mishpechosam l'Veis Avosam" (Bamidbar 4:2; Rashi to 66a, DH Kohenes). Therefore, the Gemara here needs to prove that even when only one side is disqualified, the child's status follows that of the father, as long as the marriage involves no Isur.
2) "KOL SHE'HU B'YIKACH, HAREI HU B'LO YIKACH"
QUESTION: The teaching of a Tana was related to Rav Sheshes. The Tana taught that "Kol she'Hu b'Yikach, Harei Hu b'Lo Yikach..." -- when the Torah prohibits the Kohen Gadol from marrying certain types of women (an Almanah, Gerushah, Chalalah, and Zonah), it includes only women who otherwise would have been permitted to him (had she not been an Almanah, Gerushah, Chalalah, or Zonah). Consequently, a Kohen Gadol who lives with his sister who is an Almanah does not receive Malkus for transgressing the Isur against marrying an Almanah. The Gemara concludes that the reason why these prohibitions do not apply when the Kohen Gadol is already prohibited from marrying the woman is the principle, "Ein Isur Chal Al Isur" -- an Isur cannot take effect on a pre-existing Isur.
The Gemara later quotes Rav Ashi who states that a Kohen who has relations with his unmarried sister renders her a Zonah but not a Chalalah. Consequently, if another Kohen marries her he receives only one set of Malkus (for the transgression of marrying a Zonah). If, however, the Kohen has relations with his sister a second time (when she is already a Zonah), he renders her a Chalalah, and another Kohen who marries her receives two sets of Malkus (for marrying a Zonah and a Chalalah).
Why does a Kohen who has relations with his sister who is a Zonah render her a Chalalah? A woman becomes a Chalalah only through transgressing a prohibition of Isurei Kehunah but not through any other act of forbidden relations. The prohibition of having relations with a Zonah does not apply to this Kohen since she is already prohibited to him as his sister, as the Tana has established. Accordingly, he does not transgress an Isur Kehunah and his act of relations with his sister should not render her a Chalalah.
ANSWER: RASHI (DH Prat l'Kohen Gadol) writes that the Kohen "does not receive Malkus for [transgressing] an Isur Kehunah, but only for [transgressing] the Isur of having relations with his sister." Rashi's words imply that the principle of "Kol she'Hu b'Yikach..." applies only with regard to Malkus but not with regard to whether or not there exists an Isur altogether. There certainly is an Isur; that Isur merely is not considered to be in the category of an ordinary Lav to make the Kohen liable for Malkus if he transgresses it.
Since the status of "Chalalah" depends on the presence of an Isur and not on the Kohen's liability for Malkus, the Kohen who has relations with his sister who is a Zonah indeed renders her a Chalalah even though he does not receive Malkus for having relations with a Zonah.
3) TWO PROHIBITIONS WHICH TAKE EFFECT CONSECUTIVELY
QUESTION: The Gemara earlier teaches that the principle of "Ein Isur Chal Al Isur" -- an Isur cannot take effect on a pre-existing Isur -- does not apply when the second Isur is more severe than the first ("Isur Mosif"). The Gemara states that according to this rule, a woman who was first a Chalalah can acquire the additional (and more severe) Isur of Zonah, but a woman who was first a Zonah cannot acquire the additional Isur of Chalalah.
Accordingly, a Kohen who has relations with his sister who is a Zonah should not render her a Chalalah. Why, then, does the Gemara state that the Kohen's sister who is a Zonah becomes a Chalalah when the Kohen has relations with her a second time?
ANSWER: The SHA'AR HA'MELECH (17:8) quotes the MAHARIT who presents a novel understanding of the principle, "Ein Isur Chal Al Isur." The common understanding of this principle is that a second Isur cannot take effect on the object or person when another Isur already exists. The Maharit proposes that both Isurim take effect. The principle of "Ein Isur Chal Al Isur" teaches that one does not receive punishment for a second Isur when there is a pre-existing Isur. Both Isurim, however, are in force. Consequently, if witnesses would give Hasra'ah and warn the person not to transgress the second Isur, Malkus could be given for that Isur.
This explains why the woman is considered a Chalalah. The Isur of Chalalah takes effect even though one does not receive Malkus for that Isur (unless the witnesses specifically give Hasra'ah for that Isur). (See RASHASH for a different approach. See also Insights to Sotah 27:1.)