Concept 9 Specialized chromosomes determine gender.

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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). 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. 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!

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Improper segregation of the sex chromosomes can occur during meiosis such that one gamete receives both sex chromosomes and the other receives none. If fertilized these gametes can develop into human females, XXX or X, or human males, XXY.


X and Y are mismatched chromosomes. How does the cell segregate these chromosomes properly during meiosis?