From “Study Confirms Males and Females Have at Least One Thing in Common: Upregulating X” ( ScienceDaily, Oct. 24, 2011), we learn:
Women have two X chromosomes, while men have one X and one Y. The lack of a ‘back up’ copy of the X chromosome in males contributes to many disorders that have long been observed to occur more often in males, such as hemophilia, Duchenne muscular dystrophy, and certain types of color blindness. Having only one copy of X and two copies of every other chromosome also creates a more fundamental problem — with any other chromosome, the gene number imbalance resulting from having only one copy would be lethal. How can males survive with only one X?
Turns out they upregulate, as needed.
In mammals — humans and mice — both males and females up-regulate X chromosome gene expression and females then equalize expression by turning off the one X chromosome. In roundworms (C. elegans) the both female X chromosomes stay active, but the genes on both Xs are down-regulated by half to compensate in the females. In fruit files (Drosophilia melanogaster), males increase the expression of X chromosome genes, with no upregulation of X in females.
Well, the guys had to be doing something.