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Lecture 12

BIOL 207 (10/07/13) Lecture 12: Sex Chromosomes (con't)

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University of Alberta
Biology (Biological Sciences)
Mike Harrington

BIOL 207 (October 7, 2013) Lecture 12: Sex Chromosomes Ploidy Level  Ex. Hymenoptera (Bees, wasps, ants)  Monoploid = male  Diploid = Female Sex Chromosomes: XY System  Ex. Drosophila  X/X = female  X/Y = male (males have heterozygous genotype) Sex Chromosomes: ZW system  Reverse of XY system o Z/W = Female (females have heterozygous genotype) o Z/Z = Male Interesting Example o Clownfish o One female o Many males o How? What are sex chromosomes o It was discovered that beetles had mismatched chromosomes that pair up with eachother during meiosis I (discovered by Nettie Stevens) o Given the name ‘X’ and ‘Y’ chromosomes  ‘X’ for mysterious, ‘Y’ because it is the next letter after ‘X’ o Why do they match up even though they are different?  Millions of years ago, they were identical (homologous with eachother)  Then ‘Y’ chromosome lost MOST of its genes  The ‘X’ chromosome still has most though o Also, sex chromosome have evolved many times:  That why there is also a ZW sex chromosome system  They are defective XY chromosomes Sex Chromosomes in Mammals o How do the sex chromosomes pair up? o They rely on a pseudoautosomal region to recognize each other and pair up o The genes SRY and spermatogenesis do not occur
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