Do genes exist outside of persons?
He was referring to the children of promise, Jewish believers in Christ in the passage, with Isaac as the model. (Romans 9:7)
The true significance of God's choice of Isaac had nothing to do with genetics. Rather, Isaac was the "child of promise", a product of the faith and obedience of Abraham and Sarah. Faith, because both Abraham and Sarah believed God's promise that Abraham would sire a son even though both of them were well beyond the point of physical capability. (
Hebrews 11:11-12) Obedience, because Abraham was willing to obey God's instructions to sacrifice Isaac, in apparent violation of His own promise. Yet in further faith he obeyed to the point where God's intervention was elicited. (
Hebrews 11:17-19). God's response was to honor all three of them by choosing Isaac's lineage to be that through which Messiah would come.
Hebrews 11
11 Through faith also Sara herself received strength to conceive seed, and was delivered of a child when she was past age, because she judged him faithful who had promised.
12 Therefore sprang there even of one, and him as good as dead, so many as the stars of the sky in multitude, and as the sand which is by the sea shore innumerable.
17 By faith Abraham, when he was tried, offered up Isaac: and he that had received the promises offered up his only begotten son,
18 Of whom it was said, That in Isaac shall thy seed be called:
19 Accounting that God was able to raise him up, even from the dead; from whence also he received him in a figure.
We then further see Isaac, the "child of promise", continue his parents' spiritual legacy to also become a spiritual progenitor of those of faith and obedience in and to Christ -- His Church -- His "children of promise" heirs:
Romans 9:7-8
7 Neither, because they are the seed of Abraham, are they all children: but, In Isaac shall thy seed be called.
8 That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
Galatians 4:28
Now we, brethren, as Isaac was, are the children of promise.
Galatians 3:29
And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
Romans 8:16-17
16 The Spirit itself beareth witness with our spirit, that we are the children of God:
17 And if children, then heirs; heirs of God, and joint-heirs with Christ.
Thus Isaac, in the full tradition of his parents Abraham and Sarah, continued their legacies of faith and obedience, to the benefit of us all who follow in their spiritual footsteps.