The ribosomal genes (rDNA) in mouse inbred strains have a multichromosomal distribution. Using a structural feature of rDNA [variable length rDNA segment (VrDNA)] that shows length polymorphism within and among inbred strains, we studied the chromosomal distribution of the variant ribosomal gene type through genetic analysis. Our results show that five of the length variant classes can be divided into three discrete linkage groups. The variants present on a particular chromosome pair appear to be unique to that pair and absent from nonhomologous chromosomes. The chromosomal location of particular variants appears to be the same in two unrelated inbred strains suggesting that the observed linkage patterns predate the origin of inbred mice. The nonrandom chromosomal distribution of these rDNA classes suggests that only a limited degree of genetic exchange occurs among nucleolus organizer regions on nonhomologous chromosomes. We have localized one particular VrDNA linkage group to chromosome 12. These and other restriction fragment polymorphisms can be used in the construction of detailed mouse linkage maps.
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