Judah had 3 sons that are not mentioned in the geneology.
They were Er (meaning Observant), Onan (meaning power) and Shelah (meaning breaks, unties, undresses). Judah gave Er a canaanite bride Tamar.
Tamar ~ means beautiful date palm (I will explain the date palm after this.)
Er was very evil and so God killed Er before he could have children with Tamar. Tamar was then married to Onan who was by Hebrew law suppose to give his deceased brother an heir. Onan refused to give Tamar a child because then it would be heir to Judah instead of him. Because of Onan's disobedience God killed him too. Shelah was too young to marry Tamar and give her a child. Instead of waiting, Tamar disguised herself and met with Judah. Judah thought she was a beautiful prostitute, not knowing it was his daughter in law, so he slept with her. She concieved Phares and Zara, twins.