Allelic exclusion of immunoglobulin expression is not caused by somatic segregation

We have investigated the karyotype of immunoglobulin-producing cells in heterozygous animals. Using a karyotypic marker for one homolog of a chromosome carrying immunoglobulin genes, we established that immunoglobulin-producing cells are heterozygous with respect to this chromosome. Therefore, allelic exclusion of immunoglobulin expression cannot be caused by somatic chromosome segregation.