Nettie Stevens and Edmund Wilson explain how gender is determined by special chromosomes.

Hello, I'm Nettie Stevens. Dr. Edmund Wilson and I showed that gender is determined by special chromosomes. I did it by looking at cells of meal worms. [MEAL WORM LARVA] The following drawings are adapted from my 1905 paper. Body cells from female worms have 20 chromosomes, all similar in size. As expected, female gametes have a half set of chromosomes (10). [FEMALE BODY CELL GAMETE (EGG)] In male body cells, there are 19 similar chromosomes and one that is much smaller. I found that two types of male gametes are made during meiosis. Some sperm have the small chromosome, and some don't. [MALE BODY CELL GAMETES (SPERM)] I concluded that the "different" chromosome must be responsible for maleness in meal worms. This different chromosome Y and its larger partner X are sex chromosomes. Gender in most species is determined by these sex chromosomes. So, gender is inherited like a Mendelian trait. Human gender is determined this way... Females have two X chromosomes; Males have one X and one Y. Since females can only contribute an X chromosome, males are responsible for the sex of offspring. They can provide either an X chromosome for a female... XX: It's a girl! ....or a Y chromosome for a male. If meiosis and fertilization occur normally, there will be an equal number of males and females. XY: It's a boy!

sex chromosomes, x chromosomes,y chromosomes worm larva, edmund wilson, meal worms, male gametes, nettie stevens, fertilization, body cell, gamete

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