The descent to Egypt took place close to the time of the birth of Peretz and Zerach, but before Chetzron and Chamul were born! This would explain the compression of events within the twenty-two years. What remains to be explained is the inclusion of Chetzron and Chamul among those who descended to Egypt, although they had not yet been born.
The Chumash states, “The sons of Yehuda are Er, Onan, Shela, Peretz, and Zerach. Er and Onan perished in the land of Canaan, and the sons of Peretz were (Va-yehiyu) Chetzron and Chamul” (46:12).
This latter verse implies that Chetzron and Chamul come to round out the number of Yehuda's descendants to five, after the death of Er and Onan. Although Er and Onan perished before the descent to Egypt, their place among the seventy is 'held for them' until the subsequent birth of Chetzron and Chamul who replaced them. Thus, Chetzron and Chamul are also counted in the book of Bamidbar among those who are slated to receive a share in the Land, and there as well Er and Onan are mentioned.
