The ancient Jews had a law of “levirate marriage” whereby if a married man died without a son his brother must marry his widow and try to father a son with her. This would be the legal son and heir of the dead brother; this son would thus carry on the dead brother’s bloodline and would be responsible for the care of his mother when he came of age. Judah’s oldest son Er died childless, leaving his wife Tamar a childless widow. Judah ordered his second son Onan to marry her, but he too died childless.