Early-replication DNA patterns in a derivative chromosome in a Syrian hamster with only 42 chromosomes

From crosses within a 2n=43 line of Syrian hamsters (Mesocricetus auratus) lacking one derivative (der 11) of an 11;20 reciprocal translocation we have obtained homozygotes with only 42 chromosomes. These animals are homozygous deficient (nullisomic) for the centromere and short arm of chromosome 11 and for the bulk of the long arm of chromosome 20. -During cytogenetic studies, we investigated the frequency patterns of early-replicating bands in the surviving derivative (der 20) at two cytologically defined sub-phases of S using short-term fibroblast cultures. These patterns were compared with those observed in the component, untranslocated arms in normal 2n=44 cells at the same two sub-phases. -Very close agreement was found, indicating that neither the nullisomy, nor the new arm combination has interfered detectably with the pattern or programme of early band replication.