Genetics

"Don't worry, Mark, despite what you may look like, you are still a man at heart." Mark's mom told him as she adjusted his tie, "Your father and I were worried about you when you hit puberty, and you're body started changing in ways we didn't expect. The doctors said it was the first spontaneous gender change they had ever seen; it's apparently something in the genetics of your father and I."

Mark sighed, "Thanks mom. I just wish the school didn't have a dress code requiring girls to wear skirts!"