Concubinage: Laws Concerning: Children of, not Heirs
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Genesis 15:4
Then the word of the LORD came to Abram, saying, “This one will not be your heir, but one who comes from your own body will be your heir.”
Genesis 21:10
and she said to Abraham, “Expel the slave woman and her son, for the slave woman’s son will never share in the inheritance with my son Isaac!”