Chromosome Map Distance Calculator
Convert recombination frequency into genetic map distance in centimorgans (cM) between two loci on a chromosome. Used in linkage analysis and genetic mapping experiments.
About this calculator
Genetic map distance measures how far apart two loci are on a chromosome based on the frequency of recombination between them. Recombination occurs when homologous chromosomes exchange segments during meiosis (crossing over). The map distance in centimorgans (cM) is defined as: Map Distance (cM) = Recombination Frequency × 100. A recombination frequency of 0.01 (1%) equals 1 cM, also called 1 morgan in honor of Thomas Hunt Morgan. One cM roughly corresponds to a 1% chance that a crossover event will separate the two loci in a single generation. Note that this linear relationship is only accurate for short distances (< 50 cM); at larger distances, multiple crossovers cause the observed recombination frequency to plateau near 50%, requiring mapping functions like Haldane's or Kosambi's for correction.
How to use
Suppose a testcross of a dihybrid organism yields 400 offspring: 180 parental class A, 180 parental class B, 20 recombinant class C, and 20 recombinant class D. Recombination frequency = (20 + 20) / 400 = 40 / 400 = 0.10. Enter 0.10 into the Recombination Frequency field. Map Distance = 0.10 × 100 = 10 cM. This means the two loci are 10 centimorgans apart, indicating they are linked but recombine in about 10% of meioses.
Frequently asked questions
What does a genetic map distance of 50 centimorgans mean for linkage?
A map distance of 50 cM means the two loci recombine in 50% of meioses, which is statistically indistinguishable from independent assortment (unlinked loci). Loci separated by 50 or more cM are considered unlinked, even if they are on the same chromosome, because the high frequency of crossovers between them makes them behave as if on separate chromosomes. In practice, a recombination frequency of exactly 50% is used as the upper limit of measurable linkage. Loci with frequencies below 50% are considered genetically linked.
Why is the centimorgan not a physical unit of DNA length?
The centimorgan is a measure of genetic distance based on recombination probability, not physical distance in base pairs. The relationship between cM and base pairs varies enormously across the genome and between species. Hotspots of recombination compress genetic distance relative to physical distance, while cold spots stretch it. For example, 1 cM corresponds to roughly 1 million base pairs (1 Mb) on average in humans, but this varies from tens of kilobases to several megabases in different genomic regions. This is why genetic and physical maps can differ substantially.
When should I use a mapping function instead of the simple recombination frequency formula?
The simple formula (map distance = recombination frequency × 100) is reliable only for short genetic distances, generally below 10–15 cM, where double crossovers are rare enough to be negligible. For longer distances, double crossovers between loci cancel out and are not detected in offspring phenotypes, causing the observed recombination frequency to underestimate true map distance. Mapping functions like Haldane's (assuming no interference) or Kosambi's (assuming positive interference) correct for this by modeling the expected number of crossovers mathematically, giving more accurate map distances for larger intervals.