Expression of the IV (reversed and/or heterotaxic) phenotype in SWV mice

Approximately 50% of iv/iv mice have situs inversus (mirror image reversal of viscera) and 40% have heterotaxia (anomalous arrangement of viscera). The occurrence of heterotaxia is independent of situs. Using the cross-intercross breeding system to put the iv gene on the SWV background, an occasional presumed iv/+ mouse was found that had an IV (situs inversus and/or heterotaxic) phenotype. Testcrosses of these reversed animals indicated an iv/+ genotype. Since iv is linked tightly to Igh-C on chromosome 12, we inferred the genotype with a polymorphism of Igh-C demonstrated using the polymerase chain reaction (PCR). This confirmed them to be iv/+. The expression of the IV phenotype in animals heterozygous for the iv gene may be due to an interaction of iv with an autosomal recessive gene found in SWV. We have not found the IV phenotype in heterozygous iv/+ mice following placement of the iv gene on six other inbred strains. Rarely, we also found that presumed SWV +/+ mice had the IV phenotype. Test matings of these phenodeviants, corroborated by PCR, have confirmed them to be +/+. Although the phenotypes of the affected SWV +/+ and iv/+ mice resembled those found in iv/iv mice, the occurrence of situs inversus and heterotaxia were not independent of each other, and most of the SWV mice with the IV phenotype had heterotaxia with situs solitus. This infrequent dominant expression of the iv gene has so far only been seen when iv is on the SWV background. These findings are consistent with the idea that this phenomenon is due to the interaction of the iv gene with another gene found so far only in the SWV strain.