(Mishnah): Both a son and a daughter receive an inheritance. A son can receive the double portion (if he is a firstborn) of his father's estate but not of his mother's estate. The daughters are sustained from their father's estate but not from their mother's estate.

(Gemara): It is written (Devarim 21:17), "(and give him a double portion) in all that he owns'' - how is this? The firstborn does not receive a double portion of property that was not yet in his father's possession at the time of death, (even if when it would have eventually come into his possession, he would usually inherit it).

How is this? If his father died within the life of his grandfather, he only receives a double portion of his father's estate, but not of his grandfather's estate. But if his father was a firstborn, he receives from his grandfather in the same way as his father.