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set8 linkage and recombination - BIOL 239 Linkage and...

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BIOL 239 1 Linkage and recombination Chapter 3 pp 48-52 Chapter 7 pp 135-140, 150-153
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If genes on different chromosomes assort independently because of the independent segregation of homologs during meiosis, how do genes on the same chromosome assort ? w y w+ y+ w w+ y+ y w+ w y y+ w+ y w y+ w+ y+ w y gametes 2
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Some genes on the same chromosome assort together more often than not In dihybrid crosses, departures from a 1:1:1:1 genotypic ratio of F 1 gametes indicates that the two genes are on the same chromosome - not completely independent of each other -can see this in F 2 offspring In other words, they are in close proximity to one another and therefore tend to assort together = genetically linked 3
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Some genes on the same chromosome assort together more often than not -some genes on the same chromosome are more closely linked than others -how can we tell this? -a look at sweet peas …… .. 4
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Ratio: ~24: 1: 1: 7 5
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Total =1000 820 1000 x100 = 82% 180 1000 x100 = 18% Testcross -tracking her chromosomes! -can track genes on chromosomes by looking at F 2 offspring of a testcross 6
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Total =1000 820 1000 x100 = 82% 180 1000 x100 = 18% only concerned with phenotypes for classing X-over events 7
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We can tell if genes are linked by observing the F 2 generation or by pedigree analysis Law of independent assortment predicts all 4 phenotypes will occur in a dihybrid testcross (4 types of F 1 gametes). The departure from a 1:1:1:1 genotypic ratio indicates that the two genes are on the same chromosome -not completely independent of each other When genes are linked, parental combinations outnumber recombinant types 8
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To keep track of parental allelic combinations, we can represent the genotype of the F 1 as: vg b / vg + b + or vg b Represents alleles from father Instead of vg + vg b + b Represents alleles from mother vg + b + 9
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Another example ..only concerned with phenotypes for classing Test-cross genotype does not contribute to phenotype 10
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w + m + /Y or w + m + wm/wm or wm wm What will the F 1 s be? O + O ! 11
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What will the F 2 generation look like? Since the genes for eye colour and wing size are both on the X-chromosome, you might expect the F 1 female to produce only wm (from P female) and w + m + (from P male) gametes Therefore you would expect F 2 males to be ONLY white-eye/mini wing or red-eye/full wing wm /Y w + m + /Y 1:1 But . wm/w+m+ wm /Y 12
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Not 1:1:1:1 wm /Y w + m + /Y wm + /Y w + m /Y wm/w+m+ wm /Y Recall: only concerned with phenotypes for classing 13
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Unlinked genes assort independently Linked genes tend to assort together Recombination between non-sister chromatids during meiosis gives rise to recombinant phenotypes (trading of alleles)
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