Generally speaking, the majority of F1 hybrids between the horse and donkey are sterile and do conform to the hypothesis of gametogenic breakdown. The results of our own research, nevertheless, lead us to conclude that there is variability in degree of sterility and fertility for both F1 and B1 individuals, with a gradual advance from sterility to fertility being seen. This is the subject we will discuss from the points of view of oestrus, ovulation, mating, pregnancy, spermatogenesis, endocrine activity, isozymes and karyotypic analysis in this paper.

Eight B1 progeny produced by backcrossing fertile female mules and hinmes with donkeys or horses, were assessed for levels of esterase isozyme in their sera. Four bands are found in horses, three fast moving (E3) and one slow (E1), while donkeys show only the slow E1 band. Mules and hinnies, the F1 hybrids of the horse and donkey, show both the E1 and E3 bands. Among the eight tested backcross (B1) progeny, a variety of different combinations are found, depending on whether or not the horse band (E3) has been trasmitted by the mule or hinny parent. The significance of this result is because it shows that irregular patterns of segregation of the horse and donkey chromosomes exist between different F1 hybrids, contrary to some hypotheses proposed previously.