"And they bear him sons": Where the sons are his, excluding these who are not his sons.
Variantly: "and they bear him sons": Sons come under this ordinance (of bechor) and not daughters. Because we find that daughters divide the inheritance where there are no brothers, (viz. Bamidbar 27:8), we might think that this ordinance obtains with them (as well as with sons); it is, therefore, written "and they bear him sons" — Sons come under this ordinance and not daughters.
"and the first-born son be the hated one's": and not a tumtum (one whose sex is concealed) or a hermaphrodite.