Homozygous deficiency: Syrian hamsters with only 42 chromosomes.

By crossing individuals of a line of Syrian hamsters (1SH), 2n = 43, which lack one derivative chromosome (der 11) of a reciprocal translocation T(11;20), two homozygous deficient males were obtained. These animals have only 52 chromosomes and completely lack the centromere and short arm of chromosome 11 and most of the long arm of chromosome 20. Phenotypically and anatomically these animals are apparently normal. Meiosis in both animals was regular with 21 bivalents, and mature sperm were produced. One of the animals sired two litters when mated to normal females. Detailed cytogenetic studies gave no evidence of mosaicism, and the kinetic behaviour of blood and skin cells in short-term culture was identical to control (2n = 44) cells. The detailed kinetics of DNA synthesis of fibroblasts in vitro did not differ from those of normal cells. The missing material, which constitutes about 2.5% of the total chromosome length, synthesizes DNA late, replicating bands being absent until the last sub-phase (Sk5) of the hamster S phase.