Understanding how traits are inherited requires knowing the relative positions of genes on a chromosome. This process, known as gene mapping, relies on a quantitative measure of genetic distance. Map units provide this measurement, defining the spacing between gene locations, or loci, along a chromosome. These units are used for predicting how likely two traits are to be passed down together.
Defining Map Units and Genetic Linkage
Map units are the standard measure for genetic distance, frequently referred to as centiMorgans (cM). This measurement is based on genetic linkage, which describes the tendency of genes located near each other on the same chromosome to be inherited together. If genes are linked, the alleles tend to remain in their original combinations in the offspring.
The distance measured in map units is directly related to the probability that recombination, or crossing over, will occur between the two genes during the formation of reproductive cells. One map unit (one centiMorgan) is defined as the distance corresponding to a one percent chance of recombination occurring between two loci. Therefore, the closer two genes are physically located, the lower the chance of a recombination event separating them.
Map units represent a relative genetic distance based on recombination frequency, not a physical distance like the number of base pairs of DNA. While there is a general correlation, this relationship is not perfectly uniform across the entire chromosome. A genetic map unit distance of 10 cM indicates a 10% chance of recombination.
Calculating Recombination Frequency
Determining the map distance between two genes requires calculating the recombination frequency (RF) from experimental data. This calculation typically involves a test cross between an individual heterozygous for two genes and an individual homozygous recessive for both. Offspring are analyzed to determine which inherited the original parental combination of alleles and which inherited a new, recombinant combination.
Offspring displaying phenotypes matching the original parents are called Parental types, meaning no detectable crossing over occurred between the genes. Recombinant types exhibit new combinations of traits not seen in the parental generation. These new combinations result from a crossing over event between the two gene loci.
The recombination frequency is calculated by dividing the number of recombinant offspring by the total number of offspring produced. For example, if a cross yields 1,000 total offspring and 150 are recombinant types, the frequency is 150 divided by 1,000, equaling 0.15. This frequency provides the probability of recombination occurring between the two genes.
Converting Frequency to Map Units
Once the recombination frequency (RF) is calculated, the conversion to map units is straightforward. One percent recombination equates directly to one map unit. To obtain the map distance in centiMorgans (cM), the calculated recombination frequency must be multiplied by 100.
Using the prior example where the recombination frequency was 0.15, multiplying this value by 100 yields 15. This means the two genes are separated by 15 map units, or 15 cM. This distance signifies a 15% chance that a recombination event will occur between the two gene loci during meiosis.
Building and Analyzing Genetic Maps
The map unit distance calculated between any two genes is referred to as a two-point cross, and multiple such calculations are used to build a comprehensive genetic map. By performing two-point crosses for several genes on the same chromosome, their relative order and spacing can be determined. These distances are generally additive, meaning the distance between gene A and gene C can be approximated by summing the distances between A and B, and B and C, if gene B lies between them.
However, the accuracy of this direct conversion begins to decrease significantly as the distance between genes increases. Genetic maps derived solely from recombination frequency are reliable only up to a distance of approximately 50 map units. Beyond this distance, or even approaching it, the calculated recombination frequency often underestimates the true genetic distance.
This underestimation occurs because of a phenomenon known as a double crossover. A double crossover involves two separate recombination events occurring simultaneously between the two genes, which can result in the recombinant offspring appearing to have the original parental combination of alleles. Because these double crossover events are not counted as recombinants in a simple two-point cross, the total number of observed recombinant offspring is artificially lowered. This makes the calculated map unit distance appear smaller than it should be, which is why geneticists rely on multiple, shorter-distance calculations to construct accurate linkage maps.