Leviticus 18:10

“The nakedness of thy son’s daughter, or of thy daughter’s daughter, etc.] A man might not marry his granddaughter, whether a descendant of his son or of his daughter, nor any further off descending from him in a right line, not his great-granddaughter, and so on; and if he might not marry his granddaughter, much less his own daughter, as Jarchi observes, for the relation is still nearer; therefore that being prohibited, this in course must, though not mentioned: [even] their nakedness thou shalt not uncover; neither debauch nor marry such an one: for theirs [is] thine own nakedness; which sprung from his, being the descendants either of his son or daughter; the Targum of Jonathan is, “for…”

“The prohibition of marriage with a granddaughter, whether the daughter of a son or daughter, is explained in the words, "for they are thy nakedness," the meaning of which is, that as they were directly descended from the grandfather, carnal intercourse with them would be equivalent to dishonouring his own flesh and blood. Leviticus 18:11”