Segmental tetrasomy of chromosome 12 in the mouse

An inbred strain of mice has been identified that is homozygous for a chromosomal segment attached to the centromeric region of Chromosome (Chr) 5. The altered chromosome arose spontaneously, and it does not appear to interfere with fertility or to have a phenotypic effect. Karyotypic analysis based on G-banding and quinacrine/Hoechst-banding studies revealed a morphological similarity between the additional segment of Chr 5 and the centromeric region of Chr 12. The mutant strain possessed silver-stainable nucleolar organizer regions, not only in the centromeric regions of Chrs 12, 15, and 18 but also on the additional segment of Chr 5. We investigated the relationship between the additional segment and the restriction fragment length polymorphisms (RFLPs) for the variable region of ribosomal DNA and found a correlation between a HincII RFLP and either the abnormal Chr 5 or Chr 12. These results indicate that the origin of the additional segment was Chr 12 and that the segmental tetrasomy of the present strain is therefore due to a nondetrimental translocation between pericentromeric regions.