Previous work has shown that recombination between alleles of the ftr cistron is disturbed by two "marker effects", one reduces the recombination frequency and the other increases it. These effects have been studied further. The results show that both enhancement and reduction are controlled by single genes which seem to be independent of one another. The genes are symbolized recE (for recombination enhancement) and recR (recombination reduction). Both genes are fully dominant, non-additive, and segregate readily from one another and from the ftr cistron. Recombination between any pair of ftr alleles is increased when recE is part of the genotype, but recE has no effect on recombination between alleles of the me-5 locus. On the other hand, the reduction of recombination caused by recR in the ftr cistron is polarised and alley-specific, but recR increases the frequency of recombination between me-5 alleles. The data are interpreted on the basis that the rec-gene products may be involved in chromatid pairing and that polymorphic variants of them cause differences in pairing which, by altering the opportunities for recombination, are observed as differences in allelic recombination frequency.

No abstract