linkage - Linkage What is Linkage? Linkage is defined...

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Linkage
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What is Linkage? Linkage is defined genetically: the failure of two genes to assort independently. Linkage occurs when two genes are close to each other on the same chromosome. However, two genes on the same chromosome are called syntenic . Linked genes are syntenic, but syntenic genes are not always linked. Genes far apart on the same chromosome assort independently: they are not linked. Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in prophase of meiosis 1, where homologous chromosomes break at identical locations and rejoin with each other.
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Discovery of Linkage In 1900, Mendel’s work was re-discovered, and scientists were testing his theories with as many different genes and organisms as possible. William Bateson and R.C. Punnett were working with several traits in sweet peas, notably a gene for purple (P) vs. red (p) flowers, and a gene for long pollen grains (L) vs. round pollen grains (l).
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Bateson and Punnett’s Results PP LL x pp ll selfed F1: Pp Ll F2 results in table Very significant deviation from expected Mendelian ratio: chi-square = 97.4, with 3 d.f. Critical chi square value = 7.815. The null hypothesis for chi square test with 2 genes is that the genes assort independently. These genes do not assort independently. phen otype obs exp ratio exp num P_ L_ 284 9/16 215 P_ ll 21 3/16 71 pp L_ 21 3/16 71 pp ll 55 1/16 24
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B+P Genes in a Test Cross Purpose of a test cross: the offspring phenotypes appear in the same ratio as the gametes in the parent being tested. Here, we want to see how many gametes are in the original parental configuration (PL or pl) and how many are in the recombinant configuration (Pl or pL). The parental types have the same combination of alleles that were in the original parents, and the recombinant types have a combination of the mother’s and father’s alleles. Original parents: PP LL x pp ll F1 test cross: Pp Ll x pp ll Pheno- type obs purple long 392 purple round 116 red long 127 red round 365 total 1000
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More Test Cross Parentals: 392 PL + 365 pl = 757. 757/1000 total offspring = 75.7% parental Recombinant: 116 Pl + 127 pL = 243. 243 /1000 = 24.3% recombinant. If the genes were unlinked, 50% would be recombinant. These genes are linked, with 24.3% recombination between the P gene and the L gene. If the genes were right on top of each other, that is, the two phenotypes were both caused by the same gene (pleiotropy), then there would be 0% recombination between them. The percentage of recombinants is always between 0% and 50%, and the percentage of parentals is always between 50% and 100%.
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Better Symbolism We have been following Mendel’s tradition in writing the two alleles for each gene together, as in PP LL x pp ll. Now we need to start paying attention to the fact that genes are on
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This note was uploaded on 12/24/2011 for the course STEP 1 taught by Professor Dr.aslam during the Fall '11 term at Montgomery College.

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linkage - Linkage What is Linkage? Linkage is defined...

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