Abstract
Among 22 F2 offspring from matings of the Oceanian (2n=38, _??_) and the Ceylonese type (2n=40, _??_) black rats, two XO females and one XXY male were obtained. The karyotype of the two XO females showed the Oceanian type, but the chromosome number was 37 by the X monosomy. The XXY male showed the Ceylonese type, but the chromosome number was 41 by the sex chromosome aneuploidy. The external features of the rats with these anomalies were apparently normal, although their fertility could not be determined. Based on the occurrence of sex chromosomal anomalies in both female (XO) and male (XXY) in the F2 hybrids, the anomalies are assumed to be caused by non-disjunction of the sex chromosomes in gametogenesis of the F1 hybrids.
