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Stevens, Nettie (Maria)

chromosomes sex fusion male

(1861–1912) US cytologist: elucidated the chromosomal basis of sex determination.

For over 2000 years speculation and some experiments had been directed to an obvious biological question: what determines whether a living organism (including the human) is male or female? Only in the early 20th-c was it shown that this is fixed at the point of fertilization and depends on the chromosomes of ovum and sperm, and not, for example, on the external conditions of early growth. Of the several biologists concerned, Nettie Stevens was almost certainly the first to carry out the crucial experimental work and to clearly appreciate its result.

She was 35 when she became a student at Stanford University, having saved money from working as a teacher, and after graduation she went to Bryn Mawr in 1900 to research for a doctorate. She was fortunate that the small college for women then had the eminent geneticist to teach biology: later he was to claim her as the most talented of his many graduate students.

Although had observed spermatozoa with a primitive microscope in the 1670s, it was not until the late 19th-c that the use of new staining and fixing methods, together with improved compound microscopes allowed cytologists to observe the entry of the sperm nucleus into the ovum (egg) cell and its fusion with the egg nucleus. Sexual reproduction could now be seen as involving the fusion of two sets of chromosomes, one from each parent (meiosis).

Nettie Stevens obtained her PhD in 1903, studied in Europe for a year, partly with , and returned to Bryn Mawr to work on ‘the question how sex is determined in the egg’ as she put it, at first by studying the chromosomes of several insects and comparison with the sex of the progeny. Her success came in 1904 with the common mealworm Tenebrio molitor . She found that the sperms were of two kinds; their nuclei had either 10 large chromosomes, or nine large and one small. The egg nuclei all had 10 large chromosomes. The somatic cells of the female offspring have 20 large chromosomes, those of the male have 19 large and one small; Stevens concluded that the former result from X,X fusion and the latter from X,Y fusion. Studies of some other species gave similar results, since X,X fusion to give a female embryo and X,Y to give a male is a widespread pattern. E B Wilson (1856–1939) found essentially similar results at about the same time, but was later than Stevens in seeing their generality and significance.

This understanding of sex determination, and its linkage with work, which she recognized, is a basic result in biological science of Nobel-prizeworthy importance. But Stevens died from breast cancer in 1912, before her reputation or the importance of sex chromosomes in genetics was fully established. By the time science in this area was clarified, credit had perhaps inevitably focused on T H Morgan, who brought together ideas on genes, chromosomes and genetics, and it was he who received a well-deserved Nobel prize in 1933.

Stevens, Rosemary A. (1935–) - History of Medicine [next] [back] Stevens, John - Overview, Personal Life, Career Details, Social and Economic Impact, Chronology: John Stevens

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