Pedigree analysis of 22 heterosexual litters demonstrating the leukocyte chimerism (XX/XY) was carried out. The analysis revealed the familial occurrence of the chimerism in 15 litters. Eight litters were obtained from four ewes and the other seven ones were sired by three rams. Moreover, it was shown that ten chimeric litters had a common male ancestor. The coefficients of inbreeding ranged from 0.0 to 0.0625, with a mean of 0.019. These results suggest that the development of placental anastomoses in sheep is genetically controlled. A possible mode of the genetic control is discussed.