Summary Of Map Distance Versus Observed Recombination Fraction

We assume that the relative physical distance between two loci can be measured by the proportion of crossovers. The probability of a crossover event increases as the physical distance between loci increases. 50% recombinant gametes equals 100% probability of a crossover event. The recombination fraction cannot exceed 50%, because both single and double crossover events result in 50% recombination gametes. "A recombination frequency of 50% indicates that these loci are sufficiently far apart to enable free recombination between them" (Strickberger, pg. 338). When the map distance is 50cM, the recombination fraction is 32%. Therefore, a distance of 50cM does not mean that two loci segregate independently.

The smaller the recombination fraction, the shorter the relative distance between loci. Cytogeneticists have shown that the frequency of crossover events is reduced near the centromere. Thus, the above assumption is not strictly true.

The proportion of crossover events is commonly measured by the observed recombination rather than counting chiasma formation using root-tip smears. If there were only single crossover events and double crossover events never occurred, the observed recombination fraction crossover would be one-half the observed relative frequency. A complication arises when we consider that we have fifty percent recombinant gametes resulting from the average of all types of double crossover events. Due to this complication, the recombination fraction does not equal the map distance, except for distances between loci of less than 10cM.

For short distances, the probability of double crossovers is very small and can be ignored. Then the recombination fraction is an estimate of map distance. We can add the map distances between pairs of loci that are located 10cM or less apart, because double crossover events are too rare to be considered. Recombination fractions are not additive when the map distance between loci are more than 10cM. In this case, we must convert recombination fractions to map distances and then we can add the map distances. The map distances have been adjusted for the probability of double crossover events. When there is more than ten percent recombination, then the recombination fraction is an underestimate of map distance.