Gemara

2b: If any of these Arayos died, did Mi'un, was divorced, or was found to be an Ailonis (a girl that does not develop like a normal female), the Tzaros are permitted (i.e. obligated to do Yibum or Chalitzah with any brother).

12a - Rav Asi: Tzaras Ailonis is forbidden. "Asher Teled" excludes an Ailonis, since she is sterile.

Question (Rav Sheshes - Mishnah): Three brothers were married to three unrelated women. Reuven died; Shimon did Ma'amar to Reuven's widow and then he died. Shimon's widows do Chalitzah but not Yibum;

"And one of them died" - Yibum applies when she is widowed from one brother, not from two.

Rav Yosef: This is the case of Tzaras Eshes Ach that is forbidden because of the fall to Yibum. We do not find a case in the Torah like this.

Suggestion: Rav Yosef comes to exclude Tzaras Ailonis, who is permitted!

Answer: No, he excludes Tzaras Ailonis, who is forbidden.

Question: If so, why did he say this is the case?

Answer: In this case, the Tzarah needs Chalitzah. Tzaras Ailonis does not need Chalitzah.

Question: What is the reason?

Answer: The Isur to do Yibum in the Mishnah is only Rabbinic. An Ailonis is forbidden mid'Oraisa to do Yibum.

Question (Mishnah): In any of these cases, if she died ... or was found to be an Ailonis, the Tzaros are permitted.

Answer: In the Mishnah, the husband did not know that she was an Ailonis (so they were never truly married); Rav Asi discusses when he knew that she was an Ailonis.

Support: The Mishnah says that she was found to be an Ailonis. It does not say she was (already known to be) an Ailonis.

Rava: The Halachah is that Tzaras Ailonis is permitted, even if her husband knew that she was an Ailonis, and even if the Ailonis is the Yavam's daughter.

Question: But the Mishnah says if she was found (to be an Ailonis)!

Answer: It should say that if she was.

Ravin: Tzaras Mema'enes, Tzaras Ailonis and Tzaras Machazir Gerushaso are permitted.

Rishonim

Rif (2a): Tzaras Mema'enes, Tzaras Ailonis and Tzaras Machazir Gerushaso are permitted. The Tzarah is permitted, but these women themselves are forbidden. Ailonis does not do Yibum or Chalitzah. "Asher Teled" excludes an Ailonis, since she is sterile. Tzaras Ailonis is permitted even if her husband knew that she is an Ailonis, even if the Ailonis is the Yavam's daughter.

Ran (DH Boi): Since Yibum does not apply to an Ailonis, it is considered not b'Makom Mitzvah, therefore her Tzarah is permitted. Even though the Gemara said (Sof 12a) that the text of the Mishnah 'was found to be an Ailonis' supports Rav Asi, the conclusion is unlike this.

Rambam (Hilchos Yibum 6:20): If one of two widows was an Ailonis, her Tzarah does Chalitzah or Yibum.

Rambam (21): If an Ervah fell to Yibum and she is an Ailonis, her Tzarah does Chalitzah or Yibum.

Rosh (1:3): Tosfos says that even if she was found to be an Ailonis after her husband died, retroactively it is revealed that she was never truly married; it was mistaken Kidushin. This implies that an Ailonis does not need a Get even mid'Rabanan, for the Mishnah says that if an Ervah could have done Mi'un but didn't, the Tzarah may do Chalitzah but not Yibum. This shows that Kidushin mid'Rabanan forbids Yibum.

Korban Nesan'el (3): According to Rava, this is no proof. Even when her husband knew that she is an Ailonis and she was truly married, Yibum and Chalitzah does not apply to her, so her Tzarah is permitted! The Rosh learns from Rav Asi. If she needed a Get, Rav Asi would be unable to explain the Mishnah.

Question: A Mishnah (Kesuvos 100b) teaches that a Mema'enes, a Sheniyah or an Ailonis has no Kesuvah. This implies that she needs a Get. The Gemara (Kesuvos 73a) infers this regarding one who did Nisu'in Stam (without mentioning any condition) and found that the Kalah had vows!

Answer (Rosh, ibid.): There, the Gemara inferred that she needs a Get because the Reisha said that she is not Mekudeshes, and the Seifa said that she leaves without a Kesuvah. Also, the Mishnah (100b) could not have said that she does not need a Get because it also discusses a Sheniyah, who needs a Get. We do not infer from that Mishnah that a Mema'enes needs a Get (it is known that she does not); the same applies to Ailonis.

Question: The Mishnah (Gitin 46b) says that if one divorced his wife because she is an Ailonis, he may never remarry her. We are concerned lest she remarry and have children, and he will say 'I would not have divorced you had I known that you can have children. Your Get is invalid!'

Answer (Rosh, ibid.): There he divorced her because she was a Safek Ailonis, i.e. the signs of Ailonis were not clear. Surely this is so, for Tana'im argue about whether or not a Seris Chamah can be cured (Yevamos 79b), but all agree that an Ailonis cannot be cured! There is an opinion that if a man was Mekadesh a woman on condition that she has no blemishes, and did Nisu'in Stam, and she is found to have blemishes, she needs a Get. This refers to other blemishes, for people sometimes accept them. No one accepts Ailonis. Therefore, a Vadai Ailonis (based on the Simanim) does not need a Get.

Note: Tosfos (2b DH O she'Nimtza'u) adds that normally a man intends that even if she has a blemish, Bi'ah should be Kidushin, so it will not be Bi'as Zenus (extramarital relations). Ailonis is not common, so this does not cross his mind. We must say that he did not have relations with her from the time he suspected that she might be an Ailonis.

Poskim

Shulchan Aruch (EH 172:1): A Seris Chamah or Ailonis does not do Yibum or Chalitzah. Therefore if a Vadai Ailonis did Chalitzah she was not disqualified from Kehunah, and her Tzarah is permitted.

Shulchan Aruch (2): A Safek Ailonis does Chalitzah, but not Yibum.

Beis Shmuel (3): This is when Simanei Ailonis are not clear. Even if her husband did not know that she is a Safek Ailonis, she needs Chalitzah (we do not say that it was a Mekach Ta'os).

Shulchan Aruch (EH 173:9): If Ervah fell to Yibum and she is an Ailonis, even if her husband knew that she is an Ailonis, her Tzarah does Chalitzah or Yibum.

Beis Shmuel (8): If her husband knew that she is an Ailonis, if he died she may marry a brother only according to the Poskim that permit (a woman found to be) an Ailonis to remarry without a Get.