Two strains of S. commune characterized by different levels of high recombination frequency in a particular region of the genome (between the two subunits, α and β, of the B incompatibility factor) were crossed, and their progeny tested for recombination frequency in the same region. The difference between the strains in recombination frequency is found to be due to some factor located within the recombining region itself.
The segregation among the progeny indicates that the factor consists of a number of sites, with additive effects. This and the dominance relationships suggest that these sites may be recognition sites which comprise a part of the fine control of recombination.