For the source text click/tap here: Yevamot 70
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According to our daf, the source for the halakha that an arel cannot eat terumah is the word parallels between terumah and the Passover sacrifice where it is clearly stated that an arel cannot participate (see Shmot 12:48).
Rashi explains that the arel being discussed by the Mishnah is someone who does not undergo circumcision because he had brothers who died after they had a brit milah.
In such a case we rule that the person should not be circumcised lest he suffer the same fate as his brothers.
Rabbeinu Tam argues that such a person is, essentially, free from the obligation of circumcision, and therefore cannot be considered an arel.
We explore this early reference to bleeding disorders, the Halacha of circumcision and the cultural history of hemophilia (especially the Royal family).