“For the first six weeks of development in the womb, embryos are sexually indifferent. Regardless of sex chromosomes—the twenty-third pair responsible for a person’s sex characteristics—their gonads, internal reproductive tracts, and external genitalia are identical. After week six, embryos typically begin to sexually differentiate, starting first with the gonads. All gonads can form into either testes or ovaries, and that development is dictated by sex chromosomes, which hold the blueprints, so to speak. If the chromosomal sex is XY, then usually the gonads will become testes, and their internal and external genitalia will masculinize—and vice versa if the chromosomal sex is XX. I said typically and usually while describing this process because, as I found out that day in my class, it’s not always what happens. Sometimes an embryo rebels. Enter androgen insensitivity syndrome, or AIS. When a person is born with AIS, their sex chromosomes are XY—the typical chromosomes for babies who are assigned male at birth. But AIS infants are usually assigned female at birth because their external genitalia appear feminine. However, a pair of undescended testes (instead of ovaries) and a blind-ending vagina that doesn’t lead to a cervix or uterus are present. The AIS body is able to convert androgens—hormones, such as testosterone, that are responsible for the development of male sex characteristics—into estrogens, or hormones that are responsible for the development of female sex characteristics. This magic trick of sorts is partially able to occur because androgens, as it turns out, are precursors to estrogens.”