Non-random x-inactivation in the female mule.

Abstract

THE mule (2n=63) is a hybrid between a jack donkey (Equus asinus, 2n=62) and a mare (E. caballus, 2n=64). The two chromosome complements are quite distinct, and in particular the X chromosomes differ morphologically; the donkey X (XD) is submetacentric and the horse X (XH) is metacentric (Fig. 1). It has also been shown that the glucose-6-phosphate dehydrogenase (G6PD) locus is X-linked in both1,2. The electrophoretic phenotype consists of a fast and slow band in both species, the G6PD of the donkey moving faster than the G6PD of the horse (Fig. 2). If one of the X chromosomes in the female mule is inactivated at random and if this inactivation can be recognized by a delay in its DNA synthesis3–5, then about 50 per cent of cells should show a late replicating XH and 50 per cent a late replicating XD. Furthermore, the G6PD pattern in the mule should consist of three bands, a fast G6PD-donkey, a mixed intermediate band corresponding to the slow G6PD-donkey plus the fast G6PD-horse and finally a slow G6PD-horse (Fig. 2).