A Robertsonian translocation in the heterozygous condition was observed in a phenotypically normal Friesian bull. This chromosomal abnormality was probably transmitted by the dam, which also displayed heterozygosity. It is suggested that this cow could have been the original carrier. Chromosome studies on forty-six animals in the pedigree of the heterozygous bull are described.
Karyotypically normal cows (139) were inseminated with frozen semen from the bull and ninety-one calves were born. The distribution of the translocation and the sex ratio in the progeny group did not significantly deviate from 0·5. One heterozygous male calf died from a septal cardiac defect but the association between the deformity and heterozygosity was considered coincidental. The fertility of the bull was normal and meiotic studies were undertaken after slaughtering.