newucsdlink

newucsdlink - Independent assortment occurs for genes on...

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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
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Linkage Genes located on the same chromosome do not follow Mendelian principle of independent assortment Will not get expected (9:3:3:1) ratio for crossing two double heterozygotes
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Linkage Will generate mostly two parental genotypes Crossing over can separate linked genes and produce a smaller number of new genotypes/phenotypes
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Linkage Genes which are on the same chromosome and are located close to one another tend be be inherited together.
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Linkage The greater the distance between genes which are on the same chromosome the greater the likelihood of crossover
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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; if linked close to 1:1 ratio , with smaller amount of other two phenotypes (due to crossing over)
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M,normal leaves m,mottles leaves D,tall d,dwarf If genes sort independently, get = numbers of recombinant and non-recombinant progeny
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M,normal leaves m,mottles leaves D,tall d,dwarf If genes linked (no crossing over) get non-recombinant progeny only
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No crossing over expected for closely linked genes The greater the distance of separation, the greater % of crossing over
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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
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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%
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Recombination frequency= (# recombinant progeny/total # progeny)X100% [(8+7)/ (55+53+8+7)}X100 %=12.2% or 0.122
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Coupling and Repulsion Configuration of Linked Genes Coupling (cis configuration): Wild type alleles are found on one chromosome; mutant alleles are found on the other chromosome. Repulsion (trans configuration): Wild- type allele and mutant allele are found on the same chromosome.
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Note: In repulsion, the nonrecombinants have produced new combination of traits when compared to parents
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Genes located on different chromosomes Independent assortment and combine randomly Produce = # of recombinant and recombinant prodigy New combinations of alleles due to interchromosomal recombination (random assortment of alleles at metaphase I) 50% recombinants;50% nonrecombinants
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Genes located on same chromosome Closely linked : produce only nonrecombinants
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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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newucsdlink - Independent assortment occurs for genes on...

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