Map distance from recombination (cM, basic)
A genetic map orders genes along a chromosome and reports the distance between loci using
map units, called centiMorgans (cM). In an introductory two-point mapping model, map distance is estimated
from the recombination frequency (RF), the fraction of offspring that are recombinant in a suitable cross.
Key definitions
In a two-gene testcross (heterozygote × double-recessive), offspring phenotypes (or genotypes) reveal the
gametes produced by the heterozygous parent. Two offspring classes are usually parental (non-recombinant)
and two are recombinant. The recombination frequency is the recombinant fraction expressed as a percent.
\[
\%RF = 100\cdot\frac{\text{recombinants}}{\text{total offspring}}
\]
Basic mapping assumption used in this calculator
For small distances, a common approximation is:
map distance (cM) ≈ %RF. This works reasonably well when loci are close together because most meioses have
at most one crossover between the genes, and recombinants closely track crossover probability.
\[
\text{Map distance (cM)} \approx \%RF
\]
RF → cM conversion
Under the basic approximation, converting recombination frequency to map distance is a direct substitution.
For example, if 12.5% of the offspring are recombinant, then the map distance is approximately 12.5 cM.
\[
\%RF = 12.5\% \quad\Longrightarrow\quad d \approx 12.5\ \text{cM}
\]
cM → RF conversion (basic)
The same approximation can be read in reverse: an estimated distance of d cM corresponds to an expected
recombination frequency of about d%, with an important biological constraint:
in a simple two-locus setting, observed recombination frequency cannot exceed 50%.
\[
\%RF \approx d
\]
\[
0\% \le \%RF \le 50\%
\]
Why the approximation breaks down for large distances
When loci are far apart, multiple crossovers can occur between them in a single meiosis.
A key point is that an even number of crossovers can restore the parental allele combination,
producing offspring that appear non-recombinant even though crossovers occurred.
As a result, the observed %RF underestimates the true number of crossover events for distant genes.
This is why recombination frequency saturates toward 50%: as genes get farther apart, the proportion of
recombinant gametes approaches (but does not exceed) 1/2, making far-apart linked genes behave like they assort
independently.
\[
\text{As distance increases, } \%RF \to 50\%
\]
How to interpret your result
Use these introductory interpretations:
small %RF (near 0%) suggests strong linkage and close loci,
moderate %RF suggests weaker linkage,
and %RF near 50% indicates the genes behave like independent assortment (unlinked or very far apart on the
same chromosome).
What the visuals show
The mini genetic map strip displays two gene markers (Gene A and Gene B) with a formatted distance label
(for example, 12.5 cM), making the result easy to read and share.
The optional RF vs distance curve is a conceptual panel showing why RF approaches 50% for large distances,
reinforcing the warning that cM ≈ %RF is mainly reliable for small distances.
Limitations
This calculator uses a basic two-point mapping approximation and does not correct for multiple crossovers,
interference, or perform statistical testing. For more accurate mapping at larger distances, genetics courses
introduce mapping functions and multi-point (three-gene) analysis.
