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Lecture 3

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McGill University
Biology (Sci)
BIOL 202
Daniel Schoen

Lecture 3 The Behaviour of chromosomes during meiosis (Nettie Stevens=one of the first researchers to describe the chromosomal basis of sex...worked with mealworms, have X and Y chromosomes, just like humans) For guys, XY, girls XX, generally accepted for humans, BUT there are other possible combos Y chromosome is too small to carry much heritable information, the X is responsible for carrying sex-linked traits Sex chromosomes were not known at the time of Mendel's work Thomas hunt morgan (developmental biologist at first, but interested at development of characteristics, like mendel), with fruit fly (drosophilia) genetics,Males=XY or X, Females =XX Morgan (first North American to receive the Nobel Prize) coined the term "genes" He observed that sex linked traits did not follow the expected mendelian ratios.-->discovered the role played by chromosomes in heredity See different crosses on slides for examples. Calvin Bridges was the first to suggest that chromosomes contained genes. Nettie Stevens proved it. There was an observation of nondisjunction of meiosis, eggs missing an X, so you get a white eyed female and a red eyed male. Autosomes and sex chromsomes are 2 different things (autosomes are basically all chromosomes that aren't sex chromosomes) Human sex chromosomes have 2 pseudoautosomal region, and a differential region of the X in the middle (for an X chromosome, Y has a differential region of the Y, for Y linked Genes). Y does determine gender here (as opposed to with fruit flies), The 2 pseudoautosomal regions are useful so they can pair up during meiosis. Y still contains very little genes, so sex-linked inheritance = X- linked inheritance (generally) Single gene traits in humans (Polymorphisms, tongue rolling is NOT controlled by genes) widows peak (dominant) Thalasse
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