This is from Chicken Soup for a Mother's Soul:

A young woman named Mary gave birth to her first child, and because her husband was on military duty, she spent a couple of weeks after the birth at the home of her parents.

One day Mary mentioned to her mother that she was surprised the baby’s hair was reddish, when both she and her husband were blonde.

“Well, Mary,” said her mother, “you must remember, your daddy’s hair is red.”

“But Mamma,” said Mary, “that doesn’t make any difference because I’m adopted.”

With a little smile, Mamma said the loveliest words that her daughter had ever heard: “I always forget.”