Schematic representation of different dosage-compensation systems. (a) Drosophila melanogaster, (b) Homo sapiens, (c) Caenorhabditis elegans. Combinations of chromosomes in the diploid somatic cells of males and females are shown. The sex chromosomes are symbolized by the letters X and Y, autosomes as A. Dosage-compensated chromosomes are colored: red indicates activation, blue repression. The sizes of the As indicate the average expression level of an autosome in a diploid cell. The sizes of the X chromosomes reflect their activity state (see text). The arrows represent the activating and repressive factors that determine the activity of the corresponding sex chromosome. In Drosophila (a), the male X chromosome is transcriptionally activated twofold in the male to match the total level of expression from the two female X chromosomes. In mammals (b), X chromosomes are hypertranscribed in both sexes, and to equalize X-chromosomal gene expression between the sexes, one of the two X chromosomes is inactivated in females. In C. elegans (c), males do not have a Y chromosome (O indicates its absence) and XX individuals are hermaphrodites. Worms also overexpress X-linked genes in a sex-independent manner, as indicated by the red-colored Xs, but subsequently halve the expression levels of the genes from both X chromosomes in the hermaphrodite (indicated by the blue Xs) to equalize gene dosage between the sexes.