One day Dave and his twin brother

enlisting in the Army & were getting their physicals.
During the inspection, the doctor was surprised to discover that both
of them possessed incredibly long, over sized penises. "How do you
account for this?" he asked them.
"It's hereditary, sir," Dave replied.
"I see," said the doctor, writing in his file. "So, your father's the
reason for your overly elongated penises?"
"No sir, our mother." Dave replied.
"Your mother? You idiot, women don't have penises!"
"I know that, sir," replied Dave, "but she only had one arm, and
when it came to getting us in and out of the bathtub as infants, she had to manage as best as she could sir."