'Heir' in the Bible

Now I say, That the heir, as long as he is a child, differeth nothing from a servant, though he be lord of all;

Wherefore thou art no more a servant, but a son; and if a son, then an heir of God through Christ.

Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.