An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homogeneously-staining regions (HSRs) was carried out. In a previous study is was shown that the region delineated by the proximal breakpoint of Is(HSR;1C5) 1Icg and the distal one of Is(HSR;1D)2Icg is unpaired during early pachytene and heterosynapsed at midpachytene. No synaptic disturbances were revealed in homozygotes in this study. Chiasmata number per first bivalent in heterozygous (1.87) and homozygous (1.88) males was shown to be higher than in normal ones (1.61). In normal males a single chiasma is located in the medial part of chromosome 1. In heterozygotes this segment is heterosynapsed and unavailable for recombination. This leads to a significant decrease in the frequency of bivalents bearing a single chiasma and an increase in the frequency of bivalents bearing double chiasmata located mostly at subcentromeric and subtelomeric regions of the chromosome. In homozygous males the frequency of double chiasmata is also increased, and even triple chiasmata become possible because of the increase in the physical length of the bivalents. Thus insertion of heterochromatic regions, which are inert with respect to recombination, leads to an increase in the length of the genetic map of the chromosome because of relaxation of interference restrictions.