Lecture 6 Bio325 Fall 07

Lecture 6 Bio325 Fall 07 - 1 Linkage: YyRr gametes? If each...

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Unformatted text preview: 1 Linkage: YyRr gametes? If each locus segregates independently, then there should be 4 types of gametes: YR Yr yr yR If each locus did not segregate independently, then there should only be 2 types of gametes: YR yr (F1) ? Why would each locus not segregate independently? → Because the two genes are near each other on the same chromosome, and are thus linked YR + yr (gametes) Y y R r Y R Y r y R y r meiosis 4 types of gametes R Y r y meiosis 2 types of gametes, same as parental genotype Y and R loci on different chromosomes, therefore independent assortment: Y and R loci on the same chromosome, therefore linked: r y R Y 2 example in Drosophila: brown vs. black body color and straight vs. curved wings black, straight-wing x brown, curved wing all brown, straight-wing x black, curved-wing testcross: 2934 black, straight-wing (40%) 2768 brown, curved-wing (37%) 871 black, curved-wing (12%) 846 brown, straight-wing (11%) example in Drosophila: brown (b + ) vs. black (b) body color and straight (c + ) vs. curved (c) wings where b + and c + are dominant Fig. 5.4 expected if unlinked: ~ 1850 b c + / b c ~ 1850 b + c / b c ~ 1850 b c / b c ~ 1850 b + c + / b c difference between the expected distribution and the actual distribution is a sign of linkage. The # of parentals will always be larger than the # of recombinants. b c + c b + 3 Where do recombinants come from? → crossing over (recombination) between homologous chromosomes during meiotic prophase I Fig. 5.7 Proof of physical recombination came through experimental use of cytological markers on chromosomes: Fig. 5.6 4 an example of linkage on the X chromosome: red eyes (w + ) vs. white eyes (w) brown body (y + ) vs. yellow body (y) w y + y w + ~ 2250 w y + / Y ~ 2250 w + y / Y ~ 2250 w y / Y ~ 2250 w + y + / Y expected:...
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This note was uploaded on 06/19/2008 for the course BIO 325 taught by Professor Saxena during the Fall '08 term at University of Texas at Austin.

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Lecture 6 Bio325 Fall 07 - 1 Linkage: YyRr gametes? If each...

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