GoKawiilSex, Genes and Chromosomes: Their Discovery, Function and Evolution Jennifer A. Marshall Graves Cambridge Univ. Press (2026)
“Sex is the most profound normal difference between human individuals,” explains geneticist Jennifer Graves in Sex, Genes and Chromosomes. In this definitive and wide-ranging book, she outlines the discovery, molecular biology and evolution of sex chromosomes as well as the determination of sex across vertebrates — including the charged public discourse around genes, sex and gender.
Graves knows this terrain well, having contributed to many of the discoveries that she describes. She offers fresh insights for experts as well as clear explanations for lay readers. Genomics is notoriously hard to describe, however, and she apologizes for the complicated vocabulary: “Sorry, but you really can’t get sex unless you understand ploidy, meiosis, and recombination.” Students, take note.
AI tools can design genomes. Will they upend how life evolves?
In humans and most mammals, sex is determined at fertilization: if a sperm bearing an X chromosome fertilizes an egg that also contains an X, the result is typically female offspring; if it contributes a Y chromosome instead, the result is male offspring. But biology is rarely that simple.
Researchers know that it is the presence of the Y, rather than the absence of an X, that constitutes the first step in determining sex. Organisms with unusual combinations of sex chromosomes, such as XXY or XYY, develop as males, whereas those without a Y usually develop as females, including those with a single X, or even three or more X chromosomes.
There are exceptions. Male offspring with XX chromosomes can arise when a part of the Y chromosome is stitched onto X or one of humans’ other 22 pairs of chromosomes. And female offspring with XY chromosomes can arise when a part of the Y chromosome is lost.
Comparing those occurrences has enabled researchers to pinpoint the exact gene on the Y chromosome that determines sex — and that triggers the development of a testis instead of an ovary. In those exceptional cases in which parts of the Y chromosome are added or deleted, this gene is always present in an XX male and always absent in an XY female.
The decades-long hunt is one of the great detective stories of genetics. It illustrates how science advances through the systematic elimination of plausible but wrong hypotheses.
Making of a male
... continue reading