Sex, Genes and the Human Condition

Sex is the most dramatic normal human polymorphism. A single gene triggers remarkable physical differences in gonads and gametes, in anatomy and behaviour of men and women via a network of genes that induce a ridge of cells to become either a testis or an ovary. These very different gonads make hormones that activate other whole networks of genes in far-flung tissues and organs. Here I explore the role of sex differences in health, longevity, reproduction and evolution, societal roles and discrimination. Sex is genetically complex, and wide variation results from variation in genes of the sex pathway, in hormone-producing genes, and in downstream genes that receive these messages in other tissues. This throws up ethical dilemmas of whether, or how, to treat, babies with atypical sexual development. And how to create a ‘level playing field’ for sport. Some variants may influence gender identity, and mate choice so that transgender and homosexual, may both be seen as simply the edges of normal curves of male and female sexual development. I conclude with the hope that we are heading for a more enlightened world in which variation in sexual development and behaviour, and sexual identity is accepted and celebrated.