Calculate recombination fraction from offspring counts and convert between recombination fraction and cM using Haldane or Kosambi maps.

Recombination percentage approximates cM only for short intervals. Haldane and Kosambi are model estimates; 50% recombination does not determine a finite map distance.

Genetic Distance Formula

The following formulas calculate observed recombination fraction (r) and percentage. Recombination percentage approximates map distance in cM only over short intervals; it is not a general distance conversion. 

r = RG / G
RF(%) = 100 × RG / G
  • Where r is the observed recombination fraction (unitless); standard two-locus population models range from 0 to 0.5
  • RF(%) is the observed recombination percentage; a finite sample can exceed the population-model expectation of 50%
  • RG is the total number of recombinant gametes (or recombinant offspring scored)
  • G is the total number of gametes (or total offspring scored)

To calculate the recombination fraction, divide the number of recombinant gametes by the total number of gametes. To express it as a percentage, multiply by 100. For standard two-locus population models, r approaches a maximum of 0.5 (50%). A sample above 50% requires checking sampling variation, parental phase and scoring; it does not imply a map distance above 50 cM. Also, using recombination percentage as “distance” (≈ cM) is only a good approximation for small distances because multiple crossovers can cause recombination to underestimate map distance; mapping functions (e.g., Haldane or Kosambi) are used to convert between r and map distance (cM).

For 0 ≤ r < 0.5, Haldane gives d = −50 ln(1 − 2r) cM and assumes independent crossovers without interference. Kosambi gives d = 25 ln((1 + 2r)/(1 − 2r)) cM under a particular interference model. Their inverses are r = (1 − exp(−d/50))/2 and r = 0.5 tanh(d/50), respectively. Genetic map distance may exceed 50 cM; at exactly 50% recombination no finite distance is determined by either model. Independent assortment can arise from separate chromosomes or far-apart loci on one chromosome.

How to Calculate Genetic Distance?

The following example problems outline how to calculate Genetic Distance.

Example Problem #1:

  1. First, determine the total number of recombinant gametes.
    • The total number of recombinant gametes is given as 90.
  2. Next, determine the total number of gametes.
    • The total number of gametes is provided as 250.
  3. Finally, calculate the Genetic Distance using the equation above: 

r = RG / G

The values given above are inserted into the equation below and the solution is calculated:

r = 90 / 250 = 0.36 (36%)

Example Problem #2: 

For this problem, the variables needed are provided below:

total number of recombinant gametes = 7

total number of gametes = 51

This example problem is a test of your knowledge on the subject. Use the calculator above to check your answer. 

r = RG / G = ?