(Internet connection required.)

Frequency of recombination

The frequency with which loci recombine depends on how far apart they are on the chromosome (loci on separate chromosomes cannot be linked). Usually (but not always*) physical distance is closely related to linkage - loci which lie close together are more likely to remain linked. The number of recombination events observed among the progeny gives a measure of the distance between two loci. Thus recombination percentage = 100 x (no. of recombinants / total no. of progeny).

Completely independent assortment, such as the Punnett square results we saw earlier, gives a recombination percentage of 50% (recombination occurs half of the time), indicating the loci lie either on different chromosomes or far apart on the same chromosome. Any departure from this ratio suggests linkage between the two loci.

* Some areas of the genome may have more or less recombination than expected; for example, the region either side of the centromere often has an unexpectedly low level often have very low levels of recombination