Recombination frequency between two loci (percentage of recombinant gametes) is proportional to the distance between them on a chromosome. By analyzing the frequencies of different gamete types from a testcross or pedigree, geneticists can construct genetic maps showing relative positions and distances of genes.
Perform a testcross analysis: identify parental and recombinant classes, calculate recombination frequency, convert to map distance in centimorgans (1 cM = 1% recombination). Compare genetic and physical maps to understand that recombination rates vary across the genome.
From your study of meiotic recombination, you know that during meiosis I, homologous chromosomes pair up and exchange segments through crossing over. The key insight for genetic mapping is that the probability of a crossover occurring between two genes depends on how far apart they are on the chromosome. Genes that are very close together are almost always inherited as a unit because a crossover is unlikely to land in the short stretch between them. Genes that are far apart experience crossovers frequently, and at very large distances, they recombine so often that they behave as if they were on separate chromosomes (50% recombination — indistinguishable from independent assortment).
Recombination frequency is measured by performing a test cross: an individual heterozygous for two linked markers (Ab/aB, for example) is crossed with a homozygous recessive individual (ab/ab). Because the recessive parent contributes only recessive alleles, each offspring's phenotype directly reveals which type of gamete the heterozygous parent produced. Parental gametes (Ab and aB) carry the original allele combinations, while recombinant gametes (AB and ab) carry new combinations generated by crossing over. The recombination frequency is simply the number of recombinant offspring divided by the total number of offspring: if 8 out of 100 offspring are recombinants, the recombination frequency is 8%.
This frequency translates directly into map distance, measured in centimorgans (cM): 1% recombination = 1 cM. So the two genes in the example above are 8 cM apart. By performing pairwise crosses between many genes, geneticists can determine the relative order and spacing of genes along a chromosome, building a genetic map. The logic is additive: if gene A is 8 cM from gene B, and gene B is 12 cM from gene C, and A is 20 cM from C, then the order must be A—B—C. If instead A-C distance were less than the sum of A-B and B-C, the order would need rearranging — or multiple crossovers are complicating the count.
There is an important limitation: for genes far apart, double crossovers can occur — two crossover events between the genes, which restore the parental configuration and make a recombinant gamete look parental. This means observed recombination frequencies underestimate true genetic distance for loci more than about 20–30 cM apart, and recombination frequency never exceeds 50% regardless of actual physical distance. Mapping functions (like the Kosambi or Haldane functions) correct for this by estimating the true number of crossover events from the observed recombination frequency. Additionally, recombination rates are not uniform across the genome — recombination hotspots have rates 10–100 times the average, meaning genetic map distances and physical distances (in base pairs) do not scale linearly.