Myra Benson was the mother of Biff Tannen.
In 1936, she gave birth to Biff, whose father was the prohibition era gangster, Kid Tannen. Kid wanted her to have an abortion, but she wouldn't do it, as it was illegal, and she felt that her mother would kill her if she did.
Kid and Myra married on December 6, 1936, simply for the purpose of making it seem like Biff wasn't born out of wedlock, with Biff being watched by a babysitter during the ceremony. The two parted ways immediately after the wedding, and neither parent wanted Biff. Because of this, Kid's mother, Gertrude Tannen, took custody of him. Before he left his son, Kid gave Gertrude a forged birth certificate that stated that Biff was born on March 28, 1937.