CHIASMA DISTRIBUTION IN THE 1ST BIVALENT OF MICE CARRYING A DOUBLE INSERTION OF HOMOGENEOUSLY-STAINING REGIONS IN HOMOZYGOUS AND HETEROZYGOUS STATES

An examination of the meiotic pattern of chromosome 1 isolated from a feral mouse population and containing a double insertion (Is) of homog eneously-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 du ring early pachytene and heterosynapsed at midpachytene. No synaptic d isturbances were revealed in homozygotes in this study. Chiasmata numb er per first bivalent in heterozygous (1.87) and homozygous (1.88) mal es was shown to be higher than in normal ones (1.6 1). In normal males a single chiasma is located in the medial part of chromosome 1. In he terozygotes this segment is heterosynapsed and unavailable for recombi nation. This leads to a significant decrease in the frequency of bival ents bearing a single chiasma and an increase in the frequency of biva lents bearing double chiasmata located mostly at subcentromeric and su btelomeric regions of the chromosome. In homozygous males the frequenc y of double chiasmata is also increased, and even triple chiasmata bec ome possible because of the increase in the physical length of the biv alents. Thus insertion of heterochromatic regions, which are inert wit h respect to recombination, leads to an increase in the length of the genetic map of the chromosome because of relaxation of interference re strictions.