Autosomal polymorphism in Donryu strain rats

1) Normal karyotype of Donryu strain rat was determined according to the classification of KURITA et al. (8). Namely, the number of chromosomes was 42 in diploid cells, and chromosomes were divided into 3 groups (A, B and C) according to the position of centromere. A.group was consisted of 7 pairs of metacentric chromosomes, B-group 4 pairs of sub· meta.subtelocentrics and C.group 10 pairs of telocentrics and Y. 2) Among all chromosome pairs a pair of the longest telocentric chromosomes (C.l), 4 pairs of all the B.group, and the Y chromosome were recognizable. 3) The presence of polymorphism was demonstrated in the smallest submetacentric chromosomes (BA),: namely, (I) a homologous submeta. centric pair, (II) a homologous subtelocentric pair and (III) a heteromor. phic submeta and subtelocentric pair which seemed to be a hybrid from (I) and (II). To distinguish the polymorphism in their genotype from phenotype was impossible. 4) Animals with type III B·4 chromosomes were produced by type I and type II animals. 5) By checking the chromosomes of the inbred Donryu strain rats maintained over 40 generations by brother.sister mating at Nihon Rat Co., polymorphism in BA chromosomes was also recognized.