Kiddushin 4:3-4

Kiddushin 4:3

All those whose statuses preclude them from entering the general marriage pool are permitted to marry one another, though Rabbi Yehuda prohibits (certain unions). Rabbi Eliezer says that the people who are definitely in one of these categories may marry others who are definitely in one of these categories but definite with doubtful, doubtful with definite, and doubtful with doubtful are prohibited. The one who are doubtful are the shtukei, the asufei and Samaritans.

Kiddushin 4:4

If a man marries a woman from a kohein family, he must investigate (claims of damaged lineage) for four mothers, who are eight, namely: her mother, her maternal grandmother, her maternal grandfather’s mother and her mother, her father’s mother and her mother, her paternal grandfather’s mother and her mother. If one marries a woman from a Levi or Yisroel family, they examine one additional mother.