Or HaChaim on Genesis 46:12/אור החיים על בראשית מ״ו:י״ב

1 ובני יהודה ער ואונן. And the sons of Yehudah, Er and Onan, etc. Why were people who had died already listed as part of the family which travelled to Egypt? Besides, why did the Torah repeat here again that these sons of Yehudah had died in the land of Caanan, something that we are all familiar with? Furthermore, why does the Torah write: ויהיו בני פרץ חצרון וחמול, instead of the customary ובני פרץ חצרון וחמול? We find all the other grand-children introduced as: ובני…וגו. Perhaps it is all connected with the concept of the levirate marriage. יבום is aimed at re-establishing a presence on earth for the brother who died without leaving children. In our instance Yehudah performed this commandment instead of his son Shelah. It was Yehudah's duty to bring Er and Onan's offspring to Egypt. The Torah tells us by means of an unusual syntax that Yehudah performed this duty by bringing Chetzron and Chamul (his grandchildren) to Egypt. These two were to be considered the replacements of Er and Onan respectively. [If I understand the author correctly, he argues that because Yehudah belonged to a generation prior to that of Shelah the surviving brother of Er and Onan who should have performed the levirate marriage, only the second generation, i.e. Peretz's children, were the reincarnates of Er and Onan. Had Shelah himself performed the rites of יבום, his first two children would have been considered as the reincarnates of Er and Onan. Ed.] All this is hinted at by the word ויהיו.

2Looking at the plain meaning of the verse, the Torah may have explained here that the only reason that Er and Onan committed the sins that caused G'd to kill them was that they lived in the land of Canaan, a country in which sexual perversity had taken root. They had been corrupted by their environment. Apart from this one sin they were not basically bad. G'd killed them so that one should not copy their example, much as the Torah warns the Jewish people not to copy the sexual perversions practiced both in Egypt and the land of Canaan in Leviticus 18,3.

3The expression ויהיו tells us that these two were a phenomenon by itself (הויה), unlike any observed elsewhere. Were this not so they could hardly be the members of the family from which the Messiah will devolve, someone who will serve as a shining example of morality to all of mankind.