∞ generated and posted on 2016.12.03 ∞
Measure of the degree of linkage between two loci.
|Frequency of recombination is a measure of the degree to which recombinant types are found following crosses (matings) between specific parental types and is a measure of the likelihood of crossing over events occurring between the two genetic loci being considered during these test crossings.|
Frequency of recombination is measured during specially prepared crosses where recombinant offspring can be identified particularly as phenotypically different from either of the two parents (i.e., different from the "parental types").
Here these phenotypes take into account those associated with two different genetic loci where a recombinant type possesses the phenotype associated with one parent at one locus but at the same time also the phenotype associated with the other parent at the other locus.
The following video illustrates simply the process of crossing over (i.e., recombination or molecular recombination), without consideration of the frequency of that process between two locations on a chromosomes:
If the two loci in question are not linked then the frequency of recombination will be equal to 50%, meaning that half of the progeny will be recombinant types. If the two loci are linked then the frequency of recombination will be less than 50%. In either case, frequency of recombination is equal to the total number of recombinant-type progeny divided by the total number of progeny, i.e., such that half recombinant types is equal to 50%:
Frequency of Recom. = recom. types / (recom. types + parental types)
If there are 100 progeny, for example, and 27 were recombinant types, then the frequency of recombination would be 27%. If in the same example there were 50 recombinant types then the frequency of recombination would be 50% and the two loci would be assumed either to not be located on the same chromosome or instead to be on the same chromosome but quite distantly separated. A frequency of recombination of greater than 50% is achieved only through error.
Frequency of recombination information can be used to generate what are known as genetic maps, a.k.a., linkage maps. Prior to extensive DNA sequencing, and particularly whole genome sequencing, it was primarily through frequency of recombination determinations that the relative positions of genes on chromosomes was determined.
The following video does a good job of going through both what frequency of recombination means and how to calculate it:
Note though that there are slight stumbles in the above video when explaining just what exactly frequencies of recombination of 17% or 50% mean, which are not necessarily 17% of the length of the chromosome (5:58) nor necessarily on different chromosomes (6:45), respectively.
See also the concept of linkage disequilibrium.