Matthew Poole and continuators on Genesis 38:8

8 And Judah said unto Onan, Go in unto thy brothers wife, and marry her , and raise up seed to thy brother .

This, as also divers other things, was now instituted and observed amongst Gods people, and afterwards was expressed in a written law, Deut. 25. 5, 6. See also Numb. 36. 6, 7. Ruth 1. 11. Mat. 22. 24,

Beget a child which may have thy brothers name and inheritance, and may be reputed as his child. So it was with the first child, but the rest were reputed his own.