The Gene: An Intimate History

by Siddhartha Mukherjee

To Stevens, this suggested a simple theory of sex determination. When sperm was created in the male gonad, it was made in two forms—one bearing the nublike male chromosome, and another bearing the normal-size female chromosome—in roughly equal ratios. When sperm bearing the male chromosome—i.e., “male sperm”—fertilized the egg, the embryo was born male. When “female sperm” fertilized an egg, the result was a female embryo. Stevens’s work was corroborated by that of her close collaborator, the cell biologist Edmund Wilson, who simplified Stevens’s terminology, calling the male chromosome Y, and the female X. In chromosomal terms, male cells were XY, and females were XX. The egg contains a single X chromosome, Wilson reasoned. When a sperm carrying a Y chromosome fertilizes an egg, it results in an XY combination, and maleness is determined. When a sperm carrying an X chromosome meets a female egg, the result is XX, which determines femaleness. Sex was not determined by right or left testicles, but by a similarly random process—by the nature of the genetic payload of the first sperm to reach and fertilize an egg.