On 10 October 1970, the day she was born, she was named Dorothy Maree Alaniz -- a baby girl. Curiously, though, no one filled out a birth certificate that day. When the certificate was finally filed on 5 November the name on it was Rudolph Andrew Alaniz. Within less than one month after her birth, this girl became a boy. Every year in the United States, over two thousand children are born with an intersex condition or disorder of sex development. What makes someone a boy or a girl? Is it external genitalia, chromosomes, DNA, environment, or some combination of these factors? Not even doctors or scientists are entirely clear. What is clear is that sex is not an either-or proposition: not girl/boy, XX/XY, switching between two poles like an on-off switch on a radio. Rather, sex is like the bass and treble knobs on that radio. Between XX and XY provides a fascinating look at the science of sex and what makes people male or female. There are people born XXY, XXXY, or XXXXY, or with any number of variations in X or Y chromosomes, but those who do not fit into society's preconceived notions about sex often face a difficult path in life. Dr Callahan explores why humans are so attached to the idea of two sexes, and examines our obsession with sex and sexual intercourse through the ages.