The Basics of Judaism: Basic Values and Morality: Inheritance by the First Born:
Do the laws of inheritance by the first born apply only to males?
The word BECHOR is used only for males in Tanach. The sources don't
explicitly say that a boy is called a BECHOR even if he was preceded
by girls, but from the word "sons" in Deut.21:15 the Midrash Sifrei
concludes that the laws of firstborn inheritance apply only to a son
who is his father's first born. A boy who was preceded by girls doesn't have to be redeemed, but that's because he wasn't the first to come out of his mother's womb (PETER RECHEM).
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