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9.8.S: Linkage and Mapping (Summary)
Opening the book…
Recombination is defined as any process that results in gametes with combinations of alleles that were not present in the gametes of a previous generation.
The recombination frequency between any two loci depends on their relative chromosomal locations.
Unlinked loci show a maximum 50% recombination frequency.
Loci that are close together on a chromosome are linked and tend to segregate with the same combinations of alleles that were present in their parents.
Crossovers are a normal part of most meioses, and allow for recombination between linked loci.
Measuring recombination frequency is easiest when starting with pure-breeding lines with two alleles for each locus, and with suitable lines for test crossing.
Because recombination frequency is usually proportional to the distance between loci, recombination frequencies can be used to create genetic maps.
Recombination frequencies tend to underestimate map distances, especially over long distances, since double crossovers may be genetically indistinguishable from non-recombinants.
Three-point crosses can be used to determine the order and map distance between of three loci, and can correct for some of the double crossovers between the two outer loci.
Key Terms
linkage
recombination
independent assortment
crossover
recombinant genotype
parental genotype
unlinked
recombination frequency (RF)
complete (absolute) linkage
incomplete (partial) linkage
syntenic
synapsis
coupling (cis) configuration
repulsion(trans) configuration
repulsion
map units (mu)
centiMorgans (cM)
genetic map
conserved synteny
double-crossover
three-point cross
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