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change in chromosome number.doc

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University of Waterloo
BIOL 239
Christine Dupont

Changes in chromosome number Aneuploidy - lost or gained a single chromosone - chromosome # is not same as haploid # - monosomic (2n-1) - trisomic (2n+1) - tetrasomic (2n+2) - trisomies of chromosome 1 and 2 usually result in spontaneous abortion - trisomy 21 less of an impact. downs syndrome. well tollerated. - polydactally (extra toes) is accociated with trisomy 13 chromosome 21 - at meiosis 1 - nondisjunction happens and chromosomes travel together into one egg. at meiosis 2 splits normally, but there are double the chromosomes in egg - at meiosis 2 - 1st meiosis occurs normally, but at meiosis 2, nondisjuction occurs and one set of chromosomes do not get pulled appart and travel into one of the eggs. - if happens in meiosis one, all gametes will be abnormal - if it happens in meiosis two, 50% of gametes will be normal, other two will not be Aneuploidy for X chromosome - XY male, XX female - must have 1 X for viability - need to have one copy of x chromosome because need copy of proteins - females - 2 x chromosomes - females have 2 genes dosages for x - have to bring it down to 1 bc we dont want double the amoint - x inactivation - expression of xlinked genes on all but one x chromosome - occurs in mammalian species - one x chromosome is randomly inactivated - exists as hi
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