Global dosage-compensating mechanisms. Gene expression levels are indicated by chromosome width, levels of expected normal expression are indicated in dark blue, and hyper-transcription above this level is indicated in light blue. (a) In therian mammals, one X is inactivated in females (indicated by the thin white line) [15], then the active X is hyper-transcribed in both sexes to equalize expression levels with autosomal genes [37]. (b) In Drosophila, parity between the sexes is achieved through hyper-transcription of the male X chromosome, a process that also equalizes expression between the X and the autosomes [43,44]. (c) In C. elegans, hyper-transcription occurs for X chromosomes in both sexes [45], achieving X:A parity in males. However, X:A >1 in hermaphrodites, and a second hermaphrodite-specific chromosome-wide countering mechanism returns transcription in this sex to X:A parity (shown in white) [46,47].