Non-random Inactivation of the X Chromosome in the Female Mule

Abstract

WE were interested that Mukherjee et al.¹ have a further mule in which the donkey X chromosome (X^P, X^D of our report)² is late replicating in 50 per cent of the cells. This is in agreement with the report by Mukherjee and Sinha³ and at variance with our findings². We were extremely careful not to speculate on the significance of our observations², and although we stressed that our findings suggested preferential activity of the paternal X we refrained from speculating about the origin of this phenomenon in “either inactivation or subsequent cell selection”. We agree that cell selection between inactivation and the time when we looked at the cells could account for our results. If this is so it is extremely interesting that in our three animals strong selective pressures seemed to favour activity of the X^M(X^D) as opposed to the X^P(X^H). We believe that our alternative hypothesis² could equally account for our results. It is interesting that in the animal studied by Mukherjee et al.¹ 57.9 per cent of cells had a late replicating X^P, while 42.1 per cent had a late replicating X^M. Although this distribution is significantly different from ours and not significantly different from 50 per cent, it reveals a similar trend. It is a pity that they¹ do not report results on glucose-6-phosphate dehydrogenase isoenzyme patterns for this animal.