High frequency of lymphocyte chimerism XX/XY and an analysis of hereditary occurrence of placental anastomoses in Booroola sheep

Cytogenetic studies of a closed herd of Booroola Merino sheep were carried out over six consecutive years and covered 167 (90 females and 77 males) sheep originating from 77 heterosexual multiple births. Lymphocyte 54XX/XY chimerism was revealed in 24 litters including 20 (11.98%) ewes and 22 (13.17%) rams, i.e. 25.14% of the karyotyped group of animals and 11.17% of all animals weaned in the herd. Familial and pedigree relations revealed in the study indicate a hereditary tendency to develop placental anastomoses in sheep. Fifteen chimeric litters were sired by six rams. Three rams were carriers of XX/XY chimerism and each sired a chimeric litter. Coefficients of inbreeding ranged from 0.06525 to 0.125 with a mean of 0.094. The pedigree analysis up to the fifth generations showed that two rams were ancestors of all chimeric litters. Based on the analysis of the reproductive performance of dams and sires which produced 32 litters of which three-quarters were chimeric suggested, that the formation of anastomoses is controlled by a dominant gene.