Question: What about Abraham, Yitro, and Ruth? Their mothers were not Halachic Jews. Isnít it true that before "modern times", Jewish heritage was traced through the father? Answer: 1. Avraham - presumably the rules of matrilineal descent were initiated with the giving of the Torah. Nevertheless, the children of Sarah are Jews and not those of Hagar, indicating that Avraham considered the mother as the deciding factor. OTOHYitzchak had two children from the same wife, and only one, Yaakov, was a father of our people. It would seem that in the initial establishment of the Jewish nation the personal qualities of the Patriarchs were paramount.
2. Yitro - I assume you are referring to his children who were adopted into the community of Israel, apparently through conversion.As to Tzipora, Moshe's wife, either her conversion took place at Sinai, or Moshe converted her before marriage.
3. Rut is considered the paradygmatic convert, as her expression of loyalty to Naomi and her people (and her G-d) was considered by the Talmud as the model for future conversions.
If this is unclear I would be happy if you would refine the question and I will try to be more specific.