The Talmud in Bava Kama (38a) states that Hashem spared the nations of Amon and Moav from certain destruction because of the two very special righteous women, Rus the Moabitea and Naama the Amonite who would descend from them. The Michtav M’eliyahu illustrates the concept of zechus avos (merits of ancestry) with the analogy of one who has a gold watch attached to a chain which is in turn attached to his garment. The chain, because it maintains the watch in its proper place, benefits its owner greatly. However, in its own right, it really has very little significance. Despite the apparent minimal importance afforded to the chain, if only one link of this chain breaks, the whole watch is jeopardized. This is very similar to the concept of ancestry. Hashem promised the Patriarchs that ultimately the whole purpose of Creation would be fulfilled via their progeny. The culmination would be realized with the advent of the Moshiach and the sanctification of Hashem’s Name at that time. However, in the chain of generations from the time of Avraham until Moshiach there will be many links who perhaps will achieve their purpose only by virtue of their ancestors or progeny. This is considered one of the lowest levels of zechus avos. This is to be noted from the above Talmud. These two nations were spared only because of their being a link in the chain of lineage which would produce these two righteous women.