Reproductive biologist Kathryn McClelland explores the complexities of how humans develop male and female characteristics. While traditional biology often teaches that XX and XY chromosomes determine sex, the reality is far more intricate. McLean explains the key developmental stages of sex differentiation, the role of hormones, and how genetic variations lead to diverse expressions of sex. Highlighting real-world examples—such as individuals with androgen insensitivity syndrome—she challenges the notion of a strict male-female binary. With 1.7% of the population having a chromosomal or anatomical variation, this talk underscores the natural diversity of human biology.