July 7 - Happy Birthday, Nettie Stevens

  Posted on July 7, 2022     

This is an update of my post published on July 7, 2011:

Most famous for her discovery of the chromosomal difference that (usually) determines whether an animal is male or female, Nettie Stevens is one of the first American women actually recognized for her contribution to science!

Nettie Stevens was born in Vermont on this day in 1861. She studied at Westfield Normal School in Massachusetts, graduating at the top of her class, and she earned B.A. and M.A. Degrees from Stanford University, in California. Finally, at Bryn Mawr (in Pennsylvania), Stevens studied insects and was able to identify the Y chromosome in a species of mealworms. She completed her PhD at Bryn Mawr in 1903.

This is Nettie Stevens pictured with X (above)
and Y (below) chromosomes.

You might know that most mammals have two sex chromosomes. Oversimplifying a bit, males have one X and one Y chromosome, and females have two X chromosomes. (That's an oversimplification because, first, there are individuals who have only one sex chromosome and others who have three; second, there are some conditions in which individuals with XY chromosomes develop female reproductive structures and individuals with XX develop male reproductive structures. Add to all of that the fact that, in humans at least, there is a wider range of gender identity than just male/female.)

Getting back to animals' sex chromosomes: although some insects such as fruit flies also have X and Y chromosomes, many animals do not have sex chromosomes. In some cases a creature's sex is determined by the environment. For example, if an egg is kept at one particular temperature, the developing creature becomes male; at another temperature, the creature develops into a female. 

Some animals (about one third of all animal species, other than insects) have both male and female gametes and are either born male, later becoming female if necessary, or vice versa. Clownfish like Nemo fall into this category. Also, most plants are hermaphrodites.

Of course, some organisms are asexual - they don't have sexes at all.

An example of asexual reproduction:
a hydra budding off a "baby" hydra.

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