Does one centimorgan always equal the same number of base pairs?

No. The relationship between genetic and physical distance varies across the genome because recombination rates differ between regions; on average about one centimorgan corresponds to roughly a megabase in the human genome, but this is only an approximation.

Why are mapping functions needed at all?

Because double crossovers between distant loci can be invisible in the offspring, the raw recombination frequency underestimates the true crossover count; mapping functions convert recombination frequency into an additive distance that accounts for this.

Genetic Map Distance and Centimorgan

Genetic map distance expresses how far apart two loci are in terms of how often recombination separates them, rather than in physical base pairs. Its unit is the centimorgan, defined so that one centimorgan corresponds to a one percent recombination frequency; it converts an observable inheritance statistic into an additive measure of position along a chromosome.

Definition

Genetic map distance is a measure of the separation between two loci derived from recombination frequency, expressed in centimorgans (cM), where one centimorgan equals a one percent probability that a recombination event occurs between the loci in a single meiosis.

Scope

The entry covers the definition of the centimorgan, how recombination frequency is translated into additive distance, the mapping functions that correct for multiple crossovers, and the distinction between genetic and physical distance. It is a reference topic on the quantitative basis of gene mapping.

Core questions

What does one centimorgan mean, and why is it not a physical distance?
Why is recombination frequency not simply additive over long intervals?
How do mapping functions relate recombination frequency to map distance?
How does genetic distance differ from physical distance?

Key concepts

Centimorgan (map unit)
Recombination frequency as a distance proxy
Additivity of map distances
Mapping functions (Haldane, Kosambi)
Crossover interference
Genetic versus physical distance

Mechanisms

Sturtevant's insight was that the recombination frequency between two loci can serve as a proxy for their distance, because crossovers occur more often between widely separated loci. One map unit, the centimorgan, is defined as the distance giving one percent recombinant offspring. Over short intervals, recombination frequency is approximately equal to map distance and distances are additive; over longer intervals, double crossovers can return chromatids to the parental arrangement, so the observed recombination frequency underestimates the true number of crossover events and saturates near 0.5. Mapping functions correct for this: Haldane's (1919) function assumes crossovers occur independently (no interference), while Kosambi's (1943) function incorporates positive interference, in which one crossover suppresses another nearby. Because recombination rates vary along the genome, genetic distance in centimorgans does not correspond to a constant number of base pairs of physical distance.

Clinical relevance

Genetic map distance is the scale on which linkage and disease-gene localization are reported, so centimorgans appear when describing how close a marker lies to a disease locus. This entry is reference background on the measurement and is not a basis for individual diagnosis or treatment.

History

Sturtevant (1913) first used recombination percentages as distances to draw a genetic map, and the unit of map distance was later named the centimorgan after Thomas Hunt Morgan. As it became clear that recombination frequency does not increase linearly over long intervals, mapping functions were introduced: Haldane (1919) derived a function assuming no interference, and Kosambi (1943) provided one accommodating crossover interference, both still used to build genetic maps.

Key figures

Alfred Sturtevant
J. B. S. Haldane
Damodar Dharmananda Kosambi
Thomas Hunt Morgan

Seminal works

sturtevant-1913
haldane-1919
kosambi-1943

Frequently asked questions

Does one centimorgan always equal the same number of base pairs?: No. The relationship between genetic and physical distance varies across the genome because recombination rates differ between regions; on average about one centimorgan corresponds to roughly a megabase in the human genome, but this is only an approximation.
Why are mapping functions needed at all?: Because double crossovers between distant loci can be invisible in the offspring, the raw recombination frequency underestimates the true crossover count; mapping functions convert recombination frequency into an additive distance that accounts for this.