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Double Crossover Detection
Double crossover can be detected when there is a third locus between
the two flanking loci which are of interest for mapping.
Hartl, pgs 68-69
In corn the following testcross was made:
Progeny from 3-point testcross
| Genotype |
Number |
Phenotype |
| Lzlz,
Glgl, Susu |
286 |
wild
type |
| lzlz,
Glgl, Susu |
33 |
lazy |
| Lzlz,
glgl, Susu |
59 |
glossy |
| Lzlz,
Glgl, susu |
4 |
sugary |
| lzlz,
glgl, Susu |
2 |
lazy,
glossy |
| lzlz,
Glgl, susu |
44 |
lazy,
sugary |
| Lzlz,
glgl, susu |
40 |
glossy,
sugary |
| lzlz,
glgl, susu |
272 |
lazy,
glossy, sugary |
When genes are linked, the parental or non-recombinant
classes are the most frequent. In the above testcross,
LzGlSu and lzglsu are the parental types with 286 and
272 progeny, respectively. Also, the double-cross over
gametes are the least frequent because single-crossovers
are more frequent than double-crossovers. Therefore,
Lzlz, Glgl, susu and lzlz, glgl, Susu are the double-crossover
classes with 4 and 2 progeny, respectively.
Parental
double crossover
double crossover
double crossover
parental
parental
parental
ABC
Abc
aBc
abC |
} |
1/4 observed double crossover |
The crossover genotype of the least frequent classes are in the middle
of the other two flanking genes.
Copyright
2000©, Ted Helms
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