Adam Clarke on Genesis 46:12

The sons of Pharez were Hezron and Hamul - It is not likely that Pharez was more than ten years of age when he came into Egypt, and if so he could not have had children; therefore it is necessary to consider Hezron and Hamul as being born during the seventeen years that Jacob sojourned in Egypt, See Clarke on Gen 46:8 (note): and it appears necessary, for several reasons, to take these seventeen years into the account, as it is very probable that what is called the going down into Egypt includes the seventeen years which Jacob spent there.