Sec3-segregation+-+disease09

Sec3-segregation+-+disease09 - Non-disjunction sex det J.L...

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Non-disjunction; sex det 10/5/09 J.L. Marsh Page 1 Section 3 - Chromosome segregation and genetic disease Is downs syndrome or Patau genetic? How does it differ from Mendelian expectations? Mendel I is based on segregation: what do we know about segregation? Road map non disjunction meiosis non disjunction mitosis Segregation problems and human disease birth defects pregnancy outcomes Sex Determination and developmental problems of sex caused by segregation abnormalities NONDISJUNCTION a] In 1916, 6 years after Morgan's work, C. Bridges was a grad student. Bridges crossed on a large scale: O w/w O X w/+ +/Y O X O w/Y O White O Red 1:2500 1/2000 sterile Thousands To explain exceptionals, Bridges proposed a MODEL which could be tested. Said, Errors occur whereby paired X's fail to separate (disjoin) at either 1st or 2nd meiotic division in females. What predictions does this explanation make? Ask what gamete types would be predicted!!
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Non-disjunction; sex det 10/5/09 J.L. Marsh Page 2 ProphaseI zygotene Anaphase I Reductional Telophase Equational ProphaseI zygotene Anaphase I Reductional Telophase Equational ProphaseI zygotene Anaphase I Reductional Telophase Equational NON-DISJUNCTION MEI I NON-DISJUNCTION MEI II XX XX O O XX O X X
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Non-disjunction; sex det 10/5/09 J.L. Marsh Page 3 Could also happen in MALES. PREDICTIONS: what predictions does this make? Y X + w X w X O w X w X O XXX XXY OY NORMALS ~2000 EXCEPTIONALS 1/2000 X + /O sterile,red white female red females white males gametes from 1] 1 ° exceptional F & M should cytologically be XXY & XO. They were. 2] 2 ° exceptional red M and white F should be XY and XXY. They were. 3] 1/2 of red daughters from XXY white 1°F should be XXY and 1/2 = XX This was genetically invisible but cytologically borne out 4] 1/2 of white sons from XXY white 1°F should be XYY and 1/2 = XY This also genetically invisible but found to be true The correlation between EYE COLOR & SEX CHROMOSOME constitution held up even in face of even very rare events. Proves beyond reasonable doubt that SPECIFIC GENES ON SPECIFIC C'SOMES QUESTION FOR CLASS? If nondisjunction can occur in males why didn't Bridges report exceptional red males which were fertile from the union of XY non-disjoined sperm and nullo nondisjoined eggs? DO AT HOME So ND is a natural error of biology & occurs ~1/2000 in fly
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Non-disjunction; sex det 10/5/09 J.L. Marsh Page 4 What about MITOSIS?? HOW DOES IT DIFFER FROM MEIOSIS? No pairing, no crossover (by legitimate mechanisms) Illustrate normal mitosis (below) What can go wrong? Failure to disjoin = non Disjunction (below) Failure to move = Anaphase lag XX zygote in mitosis XX XX normal daughter cells XXX X XX X ND gives twin abnormal clones Anaphase lag ND vs. Ana lag & twin spots When & where in life cycle does this occur & how to detect?
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Sec3-segregation+-+disease09 - Non-disjunction sex det J.L...

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