The father could transfer the birthright if he had a good reason, giving the firstborn’s inheritance to a younger son. In the instances of this noted in the Bible, it was not through whim or favoritism, but there was a basis on which the father determined to make the change in the birthright inheritance. Ishmael, as Abraham’s oldest son, was prospective heir for about 14 years. (Ge 16:16; 17:18-21; 21:5) But at Sarah’s request and with Jehovah’s approval, Abraham dismissed Ishmael, then about 19 years of age. Isaac then possessed the firstborn’s right and later received everything that Abraham had, with the exception of gifts that Abraham gave to sons later born to him by Keturah. (Ge 21:8-13; 25:5, 6) Reuben the firstborn of Jacob lost his birthright inheritance by reason of fornication with his father’s concubine. (Ge 49:3, 4; 1Ch 5:1, 2) Jacob gave the greater blessing to Ephraim, Joseph’s younger son, rather than to Manasseh, the older.​—Ge 48:13-19.

The firstborn son inherited a double portion (two parts) of the property as compared with that allotted to other sons.

Under the Law a father was prevented from constituting the son of a better-loved wife his firstborn at the expense of his actual firstborn from a wife less loved. He had to give to the firstborn a double portion of everything that he owned. (De 21:15-17)