Yevamos 37

The case: the brother did Yibum too early. The woman afterwards gave birth. We have a Safaik if the child born was a nine-monther from the original husband or a seven-monther from his brother, the child is definitely Kosher regardless if the Yibum was valid or not. If they're Kohain, the child is eligable to be the Kohain gadol. Tosfos asks: another Gemara says that if there was a room of ten Kohanim and one seperated himselfs to have relations with a woman. The child from the relations cannot do Avodah. We need the Kohain to be able to trace his linage. So, here too, the child can't know his linage, since he doesn't know which brother he comes from. Tosfos answers: The Gemara later asks the question. The Gemara answers that it's only a K'nass (a penalty) and they only penalize by Z'nus. here, the father was legally married, so there is no penalty. Rabbi Chaim Smulowitz If you ever though that Tosfos was beyond you. Think again. A few more free copies of Gemara and Tosfos: Sukka 2a-11a is still available at https://www.tosfos.ecwid.com