Bible Verses About Birthright: Belonged to the firstborn

"If a man has two wives, the one loved and the other unloved, and both the loved and unloved wives have borne him children, and if the firstborn son is the child of the unloved wife;

then, when it comes time for him to pass his inheritance on to his sons, he may not give the inheritance due the firstborn to the son of the loved wife in place of the son of the unloved one, who is in fact the firstborn.