Diploid-triploid chimeras have been observed both in man and in a number of laboratory and livestock animals. The mechanism(s) of their origin remains enigmatic. One approach is to calculate for each proposed mechanism the expected frequencies of zygotes bearing different gonosomic complements in the two cell lines. Observed samples are then compared with the expectations. The mechanisms that have been considered include: (1) fertilization of a blastomere, (2) absorption of the second polar body into a blastomere, (3) fertilization of the first polar body, (4) independent fertilization of both nuclei in binucleated oocytes, (5) fertilization of the second polar body as well as the egg, and (6) fusion of two eggs. The sample of minks comprised three preimplantation embryos, nine postimplantation embryos, and three neonatal pups, with gonosomic complements of 7 XX/XXX, 3 XX/XXY, 4XY/XXY, and 1 XY/XYY; the chicks comprised 13 embryos at 1 day of incubation, 1 embryo at 4 days, and one adult bird, with gonosomic complements of 5 ZZ/ZZZ, 1 ZZ/ZZW, 1 ZW/ZZZ, 3 ZW/ZZW, and 5 ZW/ZWW. If it is assumed that within each species all, or most, of the 2n/3n chimeras arise from the same mechanism, then the occurrence of a type that has an expected frequency of zero for a given proposed mechanism effectively eliminates that mechanism as a source. All of the chicks could have resulted from only one mechanism, viz., independent fertilization of both nuclei in binucleated oocytes. The sample of minks could have resulted from the same mechanism or from fertilization of a blastomere of a two-cell, 2n embryo.