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Chapter 4new - Chapter 4 Eukaryotic Chromosome Mapping by...

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Chapter 4 Eukaryotic Chromosome Mapping by Recombination Suggested Problems: #1, 2, 4, 5, 6, 7, 8, 11a, 12(a,b), 13, 16, 18, 19, 20(a,b), 25, 42(a, b), 49, 50
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Inheritance Patterns of Linked Genes Thomas Hunt Morgan testcross 1 : 1 : 1 : 1 independent assortment 415 92 88 405 actual
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Linked alleles tend to be inherited together
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Independent assortment produces 50 percent recombinants
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For linked genes, recombinant frequencies are less than 50 percent more parentals that recombinants parental genotypes present at a 1:1 ratio in a testcross recombinant genotypes are rare recombinant genotypes present at a 1:1 ratio in a testcross
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parental recomb. recomb. parental
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Chiasmata are the sites of crossing over Morgan noted the presence of chiasmata in Morgan noted the presence of chiasmata in meiosis I. meiosis I. He proposed that when homologous He proposed that when homologous chromosome pair, they occasionally break chromosome pair, they occasionally break and exchange parts and exchange parts chiasmata are chromosomes that are chiasmata are chromosomes that are crossing over crossing over
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Crossing over produces new allelic combinations
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Proof that crossing over involves breaking and re-joining of chromosomes Harriet Creighton and Barbara McClintock (1931), studying corn Crossing-over Recombinants Progeny in which the genes had recombined always had separation of the two abnormal chromosome ends
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Crossing over is between chromatids, not chromosomes How do we know? In neurospora, can get four different ascospore genotypes in one ascus.
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Crossovers can involve more than two chromatids A B C x a b c Abc Abc aBC abc Abc Abc aBC abC 2 crossover events involving 3 chromosomes 2 crossover events involving 4 chromosomes
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Recombinants Produced by two cellular processes: independent assortment and crossing over Linked genes: -recombinants produced by crossing over -frequency of recombinants is less than 50% Unlinked genes: -recombinants produced by independent assortment -frequency is 50% - recombinants of unlinked genes can also be produced by crossing over!
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Calculating recombination frequency 1. If unlinked: expect 1:1:1:1 parental combinations: 2 recombinants: 2 2/4=50% recombinant 1. Actual (linked): parental: 415 + 405 =820 recombinants: 92 + 88 =180 total number flies: 414 +405 +92 +88 =1000 recombination frequency: 180/1000 = 0.18 or 18% P R R P
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Alfred Sturtevant’s “aha” moment Recombination frequencies MEAN something! They are proportional to the actual distance between genes on a chromosome.
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This note was uploaded on 05/28/2011 for the course BIO 220 taught by Professor Dr.leatherman during the Spring '10 term at N. Colorado.

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Chapter 4new - Chapter 4 Eukaryotic Chromosome Mapping by...

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