The sons of Abraham. - In 1Ch 1:28 only Isaac and Ishmael are so called; Isaac first, as the son of the promise. Then, in 1Ch 1:29-31, follow the posterity of Ishmael, with the remark that Ishmael was the first-born; in 1Ch 1:32 and 1Ch 1:33, the sons of Keturah; and finally in 1Ch 1:34, the two sons of Isaac.
The two sons of Isaac. Isaac has been already mentioned as a son of Abram, along with Ishmael, in 1Ch 1:28. But here the continuation of the genealogy of Abraham is prefaced by the remark that Abraham begat Isaac, just as in Gen 25:19, where the begetting of Isaac the son of Abraham is introduced with the same remark. Hence the supposition that the registers of the posterity of Abraham by Hagar and Keturah (1Ch 1:28-33) have been derived from Gen 25, already in itself so probable, becomes a certainty.