Set8 linkage and recombination

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Unformatted text preview: 50 Null hypothesis: 2 gene pairs are assorting independently Observed values deviate from values predicted by our hypothesis, but is this significant? 35 Are genes A and B linked in testcross AB/ab x ab/ab ? progeny AaBb aabb Aabb aaBb observed expected 17 25 31 14 8 25 19 11 Sum of each class χ2= (#observed - #expected)2 #expected P R (19-25)2 χ2= (31-25)2 + 25 25 Σ# χ2= 1.44 + 1.44 χ2= 2.88 36 Degrees of freedom = # classes - 1 =2-1 =1 for χ2 = 2.88 and df = 1, table shows p is greater than 0.05 (~0.09) degrees of freedom will always be 1. Therefore we accept the null hypothesis that the genes are not linked 37 cutoff value is 3.84. 38 Why do we use a 5% (p=0.05) cutoff? = 2 SD (some researchers use 1% (p=0.01) = 3 SD) SD SD Any points (data) falling in this range is considered to be there NOT by chance unexpected, therefore reject null hypothesis; it’s a SIGNIFICANT difference! 39 For 3 degrees of freedom 40 p value high, likely hypothesis being tested is correct •  deviation from expected is insignificant •  genes are not linked •  p value very low, observed deviation from expected •  results become significant •  unlikely hypothesis being tested explains data •  genes are linked 41 Are genes A and B linked in testcross AB/ab x ab/ab ? progeny class AaBb aabb Aabb aaBb χ2= (62-50)2 50 + observed expected 34 62 28 50 16 38 22 50 100 (38-50)2 50 χ2= 2.88 + 2.88 χ2= 5.76 df = 1 Linked or unlinked????? we can’t say for sure that they are linked. 42 Recombination results when crossing-over during meiosis separates linked genes Reciprocal exchanges are the physical basis of recombination Proven through use of physical markers: cytologically visible abnormalities that make it possible to keep track of specific chromosome parts from one generation to the next 43 Recombination results when crossing-over during meiosis separates linked genes 1931- Harriet Creighton and Barbara McClintock, studies with corn Curt Stern, studies with Drosophila Demonstration that genetic recombination depends on the reciprocal exchange of parts between maternal and paternal chromosomes 44 Evidence for recombination 45 Recombination observed through the light microscope -Thomas Hunt Morgan first proposed chiasmata were the points of crossing over that resulted in recombination 46 Recombination frequencies of pairs of genes reflect the distances between them along a chromosome The probability of a crossover occurring between two genes increases with the distance separating them student of morgan’s A.H. Sturtevant proposed that the percentage of recombination, or recombination frequency (RF), could be used as a gauge of the physical distance between any two genes on the same chromosome 1% RF = 1 centimorgan (cM) = 1 map unit (m.u.) 47 Recombination frequencies provide the basis of genetic mapping 48 Experimental recombination frequencies between two genes are never greater than 50% 49 Recombination frequencies of 50% between two genes indicates the genes are either not on the same chromosome or are at a great distance from each other on the same chromosome. In this case, one way to determine if the genes are on the same chromosome is to show linkage with other genes that lie between them. 50 Next topic Map distances Chapter 7 pp 141-148 51...
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This document was uploaded on 02/25/2014.

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