SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Portrait of Maria 'Nettie' Stevens (1861-1912), US biologist. Now largely forgotten, Stevens was the first to establish that the sex of an animal offspring was determined by the type of chromosome from the sperm transferred in the fertilisation of the egg. Her early death meant that her work was developed and expanded by others, notably the head of her laboratory at the California Institute of Technology, Thomas Morgan.
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