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.