This F1 hybrid is produced by crossing a female of strain STOCK Rb(6.16)24Lub (Stock No. 000885) with a male of strain STOCK Rb(16.17)7Bnr (Stock No. 000615). The maternal parental strain (Stock No. 000885) is homozygous for the Robertsonian chromosome Rb(6.16)24Lub. The paternal parental strain (Stock No. 000615) is homozygous for the Robertsonian chromosome Rb(6.16)24Lub. The F1 hybrid is heterozygous for each of these Robertsonian chromosomes. This F1 hybrid is used to produce Chromosome 16 trisomy (Ts16) embryos, which have many of the fetal developmental features of Down Syndrome, by mating to usually C57BL/6J mice. Mice heterozygous for two Robertsonian chromosomes with a chromosome in common (monobrachial homology), when mated to mice with a normal 40 acrocentric karyotype, produce a higher than background frequency of trisomy selectively for the chromosome that the Robertsonians have in common.