ucsdchpt7 - Linkage, Recombination, and Gene Mapping...

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Linkage, Recombination, and Gene Mapping Independent assortment occurs for genes on separate chromosomes Random alignment and separation of chromosomes in metaphase I/ anaphase I Result is recombinant gametes containing new combinations of alleles Linkage Genes located on the same chromosome do not follow Mendelian principle of independent assortments Will not get expected (9:3:3:1) ratio for crossing two double heterozygotes Will generate mostly two parental genotypes Crossing over can separate linked genes and produce a smaller number of new genotypes/phenotypes Genes, which are on the same chromosome and are located close to one another tend to be inherited together. The greater the distance between genes which are on the same chromosome the greater the likelihood of crossover Test Cross to determine linkage Test cross between heterozygote and homozygous recessive can be used to determine linkage Traits of progeny reveal gene contribution from heterozygote For dihybrid cross, expect 1:1:1:1 if genes are not linked; close to 1:1 ratio, with smaller amount of other two phenotypes (due to crossing over) If genes sort independently, get = numbers of recombinant and non-recombinant progeny If genes linked (no crossing over) get non- recombinant progeny only M, normal leaves. m,mottles leaves. D, tall. d, dwarf.
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No crossing over expected for closely linked genes The greater the distance of separation, the greater % of crossing over. If crossing over occurs, expect 1/2 recombinant gametes;1/2 non recombinant gametes The maximum % of recombinants gametes is therefore 50% The frequency of recombinants is therefore 1/2 frequency of crossover The frequency of recombinants is therefore 1/2 frequency of crossover (Max of 50% if crossing over occurs every meiosis Recombination frequency=% of recombinant progeny (# Recombinant progeny/total # progeny) X100% Recombination frequency= (# recombinant progeny/total # progeny)X100% [(8+7)/(55+53+8+7)] X100%=12.2% or 0.122 * Recombinant Crossing over! Coupling and Repulsion Configuration of Linked Genes
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This note was uploaded on 01/04/2010 for the course BICD BICD100 taught by Professor Smith during the Fall '09 term at UCSD.

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ucsdchpt7 - Linkage, Recombination, and Gene Mapping...

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