Linkage and Gene Mapping
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Transcript of Linkage and Gene Mapping
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Linkage and Gene Mapping
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Mendel’s Laws: Chromosomes
• Locus = physical location of a gene on a chromosome• Homologous pairs of chromosomes often contain
alternative forms of a given gene = alleles• Different alleles of the same gene segregate at meiosis I• Alleles of different genes assort independently in gametes• Genes on the same chromosome exhibit linkage:
inherited together • Linked genes are not always inherited together
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Gene Mapping
• Gene mapping methods use recombination frequencies between alleles in order to determine the relative distances between them
• Recombination frequencies between genes are inversely proportional to their distance apart
• Distance measurement: 1 map unit = 1 percent recombination (true for short distances)
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• Genes with recombination frequencies less than 50 percent are on the same chromosome = linked)
• Linkage group = all known genes on a chromosome• Two genes that undergo independent assortment have recombination
frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked
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• The closer together two genes are, the more likely they will be inherited together.
• Cross-over frequencies are related to distances between genes.• The higher the frequency, the further the genes are apart
• Linkage maps show the relative locations of genes.
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Recombination
• Recombination frequency is specific for a particular pair of genes
• Recombination frequency increases with increasing distances between genes
• No matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50 percent.
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Gene Mapping
• Recombination results from crossing-over between linked alleles.
• Recombination changes the allelic arrangement on homologous chromosomes
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Gene Mapping
• The map distance (cM) between two genes equals one half the average number of crossovers in that region per meiotic cell
• The recombination frequency between two genes indicates how much recombination is actually observed in a particular experiment; it is a measure of recombination
• Over an interval so short that multiple crossovers are precluded (~ 10 percent recombination or less), the map distance equals the recombination frequency because all crossovers result in recombinant gametes.
• Genetic map = linkage map = chromosome map
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• Cross-over frequencies can be converted into map units.• Ex: A 5% cross-over frequency
equals 5 map units.• gene A and gene B cross over
6.0 percent of the time• gene B and gene C cross over
12.5 percent of the time• gene A and gene C cross over
18.5 percent of the time
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Example
• Draw a linkage map based on the following cross over percentages:
• A – B = 8%• B – C = 10%• A – C = 2%
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