Is it a boy or a girl? Dr. Page looks at how we define male and female and summarizes the development of human sex characteristics. He then explains the role of the sex chromosomes, X and Y, and, in particular, the SRY gene. Dr. Page demonstrates the differences between species that reproduce sexually and those that reproduce clonally without sex. A likely major advantage of sexual reproduction is that meiotic recombination and subsequent natural selection can weed out deleterious mutations.