Linkage and recombination
A testcross can reveal the gametes produced by a heterozygous parent because the other parent supplies only recessive alleles. The two most common classes often identify the parental allele combinations; less common reciprocal classes can identify recombinants. For short intervals, 1% recombination approximates one map unit. Observed recombination frequencies do not exceed 50% for ordinary two-locus mapping, and distant loci on the same chromosome can appear unlinked because multiple exchanges obscure the history.
In AB/ab × ab/ab, offspring Ab/ab and aB/ab are recombinant classes. If 72 of 600 offspring are recombinants, the estimated distance is 12 map units.
A 50% recombination frequency does not prove genes are on different chromosomes. Widely separated loci on one chromosome can give the same result.