“Rav Yehuda said that Rav Asi said: With regard to a gentile who betrothed
a Jewish woman nowadays, we are concerned that the betrothal might be
valid, despite the fact that a betrothal of a gentile is meaningless, lest he
be from the ten tribes of Israel who intermingled with the gentiles. The
Gemara raises an objection: But there is an important principle in halakha
that any item separated, i.e., not fixed in its place, is presumed to have
been separated from the majority. In this case, it can be assumed that any
individual singled out from the gentiles belongs to the majority of gentiles
and has no Jewish [i.e., Israelite] roots at all.
The Gemara responds: Rav Yehuda means that there is a concern only
with regard to those who came from the permanent dwelling places of the
ten tribes. As Rabbi Abba bar Kahana said that the verse states about
those exiled from Samaria: 'And he put them in Halah, and in Habor, on
the river of Gozan, and in the cities of the Medes' (I Kings 18:11). Rabbi
Abba bar Kahana proceeded to identify these places. Halah; this is the
place called Ḥalzon. And Habor; this is Ḥadyab. The river of Gozan; this
is Ginzak. And the cities of the Medes; this is Ḥamadan and its
surroundings. And some say: This is Nihar and its surroundings. The
Gemara asks: Which are its surroundings? Shmuel said: Kerakh,
Mushkhei, Ḥidkei, and Domakya are the surroundings of Ḥamadan. Rabbi
Yoḥanan said: And all of them are for disqualification. In other words, if
someone from one of these places wishes to convert, there is concern
that he might be a descendant of a Jew [i.e., Israelite] and therefore a
mamzer. Consequently, they all are disqualified. ...
The Gemara asks: Aren’t there Jewish [i.e., Israelite] girls who were
captured by gentiles, whose children are considered to be Jews? And
Ravina said: Learn from this that the son of your daughter from a gentile
is called your son. If so, the descendants of Jewish [i.e., Israelite] women
captured by gentiles would indeed be Jews [i.e., Israelites]. The Gemara
answers: This is no concern, as it is learned as a tradition that the girls
from the ten tribes of that generation became barren and did not give birth
to any offspring, whereas some of the exiled men of the ten tribes married
gentile women. Consequently, all of the children born there were gentiles.
There are those who say that Rav Yehuda actually related the following:
When I said this halakha before Shmuel, he said to me: They did not move
from there, the place where they deliberated on this matter, until they
rendered all of them, including those who intermingled with the ten tribes
in different locations, full-fledged gentiles. Consequently, there is no
concern that their betrothals might be of any effect, as it is stated: 'They
have dealt treacherously against the Lord, for they have begotten strange
children' (Hosea 5:7).” (The Babylonian Talmud, Yevamoth 16a-17a,
https://www.chabad.org/torah-texts/5448697/The-