The only issue of Tum'ah for a Kohen is if he touched or sheltered a dead person, since he then becomes an Av ha'Tum'ah himself, which he is forbidden to become. However, a Kohen who was born from an Av ha'Tum'ah is a Rishon l'Tum'ah, and there is no problem with that. There are, however, many other issues involved, since there is a prohibition to derive benefit from a dead body, and the Acharonim dispute whether it is permitted for medical purposes or procreation to benefit from a dead body.

This issue has a number of facets: 1) Harming the sanctity of the dead, since the body remains after death a part of the soul which resided in it; 2) Disrespect for a dead body, derived from "v'Ahavta l'Re'acha Kamocha"; 3) Deriving benefit from the dead.

This issue is disputed by contemporary Poskim. Some permit using parts of a dead body to enable "Peru u'Revu," but most Poskim say that this is disrespectful and thus forbidden, and the widow can find another person with whom to procreate. Therefore, it would be best to remove the Zera before the husband is deceased. Rav Zalman Nehemia Goldberg is lenient if the husband expressed his wish explicitly before his death.

As for the issue of Bechor, it depends on whether the child is recognized as the Zera of his father. Rav Shaul Yisraeli proves that in such a case even those who ascribe such children to their fathers when the father is alive would agree that such a child has no Yichus to his father and therefore would not inherit him (Torah she'Ba'al Peh 33, 1992). (As for his mother, he is not entitled to a double portion of her property.)

Yoel Domb