Nettie Stevens (1861 – 1912)
How the sex of a child is determined at conception has puzzled many grey cells more powerful than mine over the centuries. Aristotle thought it was all about environmental heat and advised males who were looking to sire sons to copulate in the summer. A popular theory going the rounds in Europe during the 19th century was that it was all about nutrition. A good diet produced girls whilst a poor one resulted in males. That was one way of keeping down the food bill.
A more drastic course of action was promulgated by the 18th century French anatomist, Michel Procope-Couteau (1684 – 1753), who in The Art of Having Boys revived Parmenides and Anaxagoras’ theory that the testicles and ovaries were either male or female. Excision of the unwanted reproductive organ would ensure the birth of a child of the desired sex. I’m not sure too many followed his strictures and he did come up with a more practical alternative. The female should lie on the correct side and let gravity take care of the rest.
It was only at the turn of the 20th century that we had a clearer idea of how sex was determined and this is where some insects and our latest inductee into our illustrious Hall of Fame, Vermont-born geneticist Nettie Stevens comes in. A late entrant into the groves of academe she was awarded a doctorate in cytology by Bryn Mawr College in Pennsylvania in 1900 and continued as a researcher, looking into the subject of sex determination.
Drosophila melanogaster, to give the fruit fly its Latin tag, is often used in research because they can be bred readily in laboratory conditions, breed quickly and lay a large quantity of eggs. Of particular interest to our Nettie was the fact that they only have four sets of chromosomes and it was these that she studied under her microscope in 1905. She quickly discovered that the chromosomes differed between the sexes.
Transferring her attentions to the mealworm, Stevens identified and isolated a chromosome she called Y, realising that it was linked to and the opposite of the X chromosome discovered by and so named by Hermann Henking in 1890. Extending her research to include egg tissue and the fertilisation process, Nettie realised that the X and Y chromosomes always existed in pairs and that it was the presence or absence of the Y that determined the gender of the result of the fertilisation process. The sex of a baby had nothing to environmental factors – it was down purely to genetics and the Y chromosome.
But Nettie was not working in a vacuum – Edmund Wilson was also carrying out researches into how sex was determined. His methods differed from Nettie’s – he concentrated on species where the male had one fewer chromosome than the female and concentrated on the testes as eggs were too fatty for his staining methodology. It is almost certain that Wilson had access to Nettie’s results and although he concluded that environmental factors also had a hand in sex selection and was less adamant in its conclusions, his paper was published first and being a chap, he was credited with discovering the chromosomal basis for sex determination.
The other villain of the piece is the prominent geneticist, Thomas Hunt Morgan. He wrote the first text book on genetics and there is evidence that he corresponded with Nettie, asking for more and more details of her experiments. When she died in 1912 of cancer, Morgan was dismissive of her contribution, inferring she was more of a researcher than a scientist. There was no mention of Stevens in his magnum opus and to make matters worse in 1933 Morgan and Wilson were awarded the Nobel Prize in Physiology and Medicine.
Although Stevens’ theory could not be proven at the time, it turned out to be right and it is only now that her contribution is beginning to be recognised. Her period in obscurity makes her a worthy inductee.
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For more enquiring minds, try Fifty Curious Questions by Martin Fone