Genetic Linkage & Map Distance Calculator

Calculate recombination frequency, map distance in centimorgans, gene order, double crossovers, coefficient of coincidence, and interference from two-point or three-point testcross data.

Background

Linked genes sit on the same chromosome and tend to be inherited together. The farther apart two genes are, the more likely a crossover occurs between them. In genetics problems, recombination frequency is used as an estimate of map distance: 1% recombination = 1 map unit = 1 cM.

Enter genetic cross data

What this calculator can show

Two-point recombination frequency and map distance
Three-point gene order from parental and double-crossover classes
Interval map distances in cM
Observed vs. expected double crossovers
Coefficient of coincidence and interference

Calculator mode

Two-point mapping — calculate recombination frequency and map distance Three-point mapping — find gene order, map distances, and interference

Use two-point mode when you only have parental and recombinant offspring counts.

Two-point testcross data

Enter total parental and recombinant offspring.

Parental offspring count

Recombinant offspring count

Three-point testcross data

Enter all 8 offspring classes. By default, the calculator treats the two largest classes as parental and the two smallest as double crossovers. You can also manually label classes.

#	Genotype	Count	Optional label

Optional labels help when a textbook or lab already tells you which classes are parental, SCO, or DCO.

Display options

Show step-by-step

Show plain-English interpretation

Show chromosome map visual

Show “how the middle gene was found” visual

Rounding

Quick picks

Result

No results yet. Enter cross data and click Calculate.

How to use this calculator

Choose two-point mapping if you only need recombination frequency between two genes.
Choose three-point mapping if you have 8 offspring classes from a three-gene testcross.
For three-point problems, enter each genotype class and its offspring count.
Use optional labels if your class already identifies parentals, single crossovers, or double crossovers.
Click Calculate to get gene order, map distances, interference, and steps.

How this calculator works

Recombination frequency: (recombinants ÷ total offspring) × 100
Map distance: 1% recombination = 1 cM
Three-point gene order: compare the parental classes with the double-crossover classes. The gene that switches is the middle gene.
Interference: compares observed double crossovers with expected double crossovers.

Formula & Equations Used

Recombination frequency: RF = recombinant offspring / total offspring × 100

Map distance: map distance = RF% cM

Expected DCO: expected DCO = RF₁ × RF₂ × total offspring

Coefficient of coincidence: COC = observed DCO / expected DCO

Interference: I = 1 − COC

Example Problems & Step-by-Step Solutions

Example 1 — Two-point map distance

A testcross gives 840 parental offspring and 160 recombinant offspring.

Total offspring = 840 + 160 = 1000
Recombination frequency = 160 / 1000 × 100 = 16%
Map distance = 16 cM

Example 2 — Three-point mapping

In a three-point testcross, the largest two classes are usually parental and the smallest two classes are usually double crossovers. Comparing parental and double-crossover genotypes reveals the middle gene.