Recombination frequencies
Recombination frequencies can be used to determine the order of the genes on a
chromosome and calculate the approximate distances from one gene to another.
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In genetic maps, 1% recombination equals 1 map unit, or 1 centiMorgan.
Double crossovers between two genes go undetected, so map distances between
distant genes tend to underestimate genetic distances.
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Two point test cross mapping
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The following testcross produces the progeny shown: Aa Bb × aa bb
10 Aa Bb,
40 Aa bb,
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40 aa Bb,
10 aa bb.
Were the A and B alleles in the AaBb parent in coupling or in
repulsion?
Ques- AaBb * aabb
AaBb - 1200
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Aabb - 100
aaBb - 100
aabb - 1200
Find rf?
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Three Point Test cross mapping
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Steps in determining gene order in a three-point cross
1.Identify the non recombinant progeny .
2.Identify the double-crossover progeny .
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3.Compare the phenotypes of double-crossover progeny with the
phenotypes of non recombinant progeny. They should be alike in
two characteristics and differ in one.
4.The characteristic that differs between the double crossover and the
non recombinant progeny is encoded by the middle gene.
Q:- A three-point testcross is carried out between three linked genes.
The resulting non recombinant progeny are s+ r+ c+ and s r c and the
double-crossover progeny are s+ r+ c. Which is the middle locus?
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DETERMINING THE LOCATIONS OF CROSSOVERS
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Average number of crossovers
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Coefficient of coincidence
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Interference
Degree to which one crossover interferes with additional crossovers.
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Interference = 1 − COC
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