The Difference between the Sexes - From Biology to Behaviour

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David Bainbridge

University of Cambridge, United Kingdom

Biography

David Bainbridge is a vet, reproductive biologist and popular science writer. He is Clinical Veterinary Anatomist at Cambridge University, and a fellow of St. Catharine’s College, where he is also Admissions Tutor in the Arts and Humanities. He has Cambridge degrees in zoology (1989) and veterinary medicine (1992) and a PhD in reproductive biology from London (1997). As well as clinical veterinary work, he has undertaken research in reproductive biology in deer, horses, cattle, mice and humans at Cornell, Sydney, London and Oxford Universities as well as the Institute of Zoology at London Zoo. His particular interest is the interactions between the mammalian mother and her developing fetus.

His science books are aimed at the general reader – often using zoological and evolutionary insights to explain human biology and behaviour. His books cover topics such as pregnancy (Making Babies, 2000), the brain (Beyond the Zonules of Zinn, 2008), human teenagers (Teenagers: A Natural History, 2009) and a forthcoming book will tackle Middle Age: A Natural History. His books have led to appearances on television and radio, and invited articles in newspapers and popular science magazines.

His 2003 book, The X in Sex: How the X Chromosome Controls our Lives, was an attempt to explain the history and cultural impact of the study of sex determination, especially in humans. It won the American Medical Writers’ Association prize.

Abstract

Deciding to be a boy? The history and prehistory of sex determination

Throughout recorded history, natural philosophers have wondered what makes women and men different. The duality of the sexes and its importance in the continuation of life has fascinated humans for thousands of years, as have the ‘extra’ differences between men and women which do not themselves seem absolutely necessary for the continuation of the species. We are now fortunate enough to understand many of the mechanisms controlling maleness and femaleness in animals, yet we still do not fully comprehend why sex determination systems have evolved into their present form.

The ancient Greeks posed many of the questions which still dominate the study of sex determination today – are the sexes equal counterparts? or is one default and one derived? is the assignment of sex a battle between male and female influences? is it the mother who determines the sex of a baby? or is it the father? or both? Surprisingly often, the theories put forward by classical thinkers have been validated by modern research, if not in humans, then in other species.

The modern study of sex determination started with the discovery of the X chromosome by Hermann Henking in 1890, although its full importance was not realised until the early years of the twentieth century, when the Y chromosome was also discovered. There followed a century-long process by which were discovered the sequential stages leading from the inheritance of particular sex chromosomes to the acquisition of male or female bodies, brains and behaviour.

One exciting implication of all this research is that the XX/XY system of sex determination present in humans is essentially asymmetrical, and thus has fundamentally different effects on the lives of men and women. For example, men must survive with only one X chromosome – an extremely precarious situation with potentially disastrous genetic consequences. Conversely, women must ‘cope’ with the presence of two X chromosomes, and this has unexpected and bizarre effects on their biology.

A second exciting corollary of the last century of study is that the human XX/XY system is just one of many systems of sex determination present in the animal kingdom. Sometimes it seems that for every conceivable method of sex determination, there will be at least one real-life species somewhere in the world which uses it! This extreme diversity has intriguing implications for the evolution of sex, and for the reasons why humans have evolved our own distinctive sexual biology.