Abigail was the mother of Amasa, whose father was Jether the Ishmaelite.
Here is, I. The family of Jacob. His twelve sons are here named, that illustrious number so often celebrated almost throughout the whole Bible, from the first to the last book of it. At every turn we meet with the twelve tribes that descended from these twelve patriarchs.
Commenting on 1 Chronicles 2:1-17
And Segub begat Jair, who had twenty three cities in the land of Gilead. Which, according to Kimchi, he inherited in right of his wife, which, he says, he took out of the land of Gilead; but they seem to be rather what he took by force of arms from the former inhabitants; see Num 32:41. .
Jether the Ishmaelite--(compare Sa2 17:25). In that passage he is called Ithra an Israelite; and there seems no reason why, in the early days of David, anyone should be specially distinguished as an Israelite. The presumption is in favor of the reading followed by the Septuagint, which calls him "Jetra the Jezreelite." The circumstance of his settling in another tribe, or of a woman marrying...