How do we know that the Bris Milah must be performed on the male organ?
Rashi and Ramban #1: From the words "Arel Zachar," which connote the part of the body that distinguishes between a male and a female. 1
Ramban #2 (citing Ibn Ezra): "Orlaso" (without specifying which) refers to the location of the Ervah. 2
Ramban #3: The words "Besar Orlaschem" (17:11) imply that one cuts off the flesh that covers a limb, and the only location on the body that fits that description is the flesh that covers the "crown" (of the male organ).
Seforno: The word "bi'Vesarchem" (in the previous Pasuk 17:13) is a refined word for the male organ. 3
Gur Aryeh: We already leaned above (17:10) that this Mitzvah only applies to males. The repetition of the word "Zachar" in this verse teaches the place that the Milah is performed.
Orlos of other locations are always specific -such as Orlas Lev and Orlas Sefasayim.
Why does the Torah designate specifically the male organ for the Mitzvah of Bris Milah?
Seforno (to 17:13): Because it is the limb that is responsible for eternity, which Bris denotes, and because it is the limb that creates the children, whom the Bris includes. 1
When the Pasuk sentences someone who fails to perform the Bris Milah to Kareis, to whom is it referring?
Rashi: It is referring to the child himself, whose father did not perform the Mitzvah, and who now becomes Bar-Mitzvah, and fails to perform it. 1
A father who fails to perform the Mitzvah transgresses an Aseh but is not subject to Kareis.
What is the definition of Kareis?
Rashi: It means that the sinner will die childless, before his allotted time. 1
See Sifsei Chachamim.
QUESTIONS ON RASHI
Rashi writes: "'That soul will be cut off' (Kareis) - He will be "Ariri," and die before his time." What does "Ariri" mean, and how it is it implied in the Pasuk?
Gur Aryeh: "Ariri" means "childless;" either he will not father children, or his children will die young. 1 This is part of "Kareis," which means to be "cut off" at the source. One's children are comparable to the young branches of a tree; they too stem from the tree's roots.