Doonesbury by Garry Trudeau for October 28, 1970
Transcript:
Mike: B.D., since we're college roommates this year, I think it's time we got to know each other better... B.D.: Oh, boy. Mike: I thought for openers I'd show you photos from my wallet... let's see. This is my mother, my sister, and can you guess who this is? Can you? Well, since you can't, I'll tell you! It's my girl Elaine, the most beautiful woman in the entire world! B.D.: I'm glad I didn't guess. I thought it was his dog.
I guess his sister had a sex change operation, since Mike has a brother who becomes a salesman for Mr. Whoopee Condoms.