Parsha Mitzvot: Acharei Mot: Mitzvah 193 – 195 – Concept 143 – 145
“The nakedness of your son’s daughter or your daughter’s daughter, you shall not uncover their nakedness; for they are your own shame (Vayikra 18:10) You may not have relations with your son’s daughter or your daughter’s daughter (Rambam, Hilchot Issurei Bi’ah – The Laws of Forbidden Relationships).
The case here is that the parent of this forbidden granddaughter, your son or your daughter, was born to you out of wedlock. The verse could not be speaking about a granddaughter born of a legitimate child, because that prohibition is found in verse 17, which states clearly that a man is forbidden to live with a woman and her daughter or any of her grandchildren. Such a granddaughter is forbidden to her grandfather (Rashi, based on Sanhedrin 76a).
This is a parent who is deliberately rejecting his relationship to his illegitimate child, “his shame.” A parent may never so reject a relationship with a child.