Dosage compensation: making 1X equal 2X

Animals that have XX females and XY or XO males have differing noses of X-l inked genes in each sex. Overcoming this is the most immediate and vital as pect of sexual differentiation. A number of systems that accurately compens ate for sex-chromosome dosage have evolved independently: silencing a singl e X chromosome ill female mammals, downregulating both X chromosomes in her maphrodite Caenorhabditis elegans and upregulating the X chromosome in male Drosophila all equalize X-linked gene expression. Each organism uses a lar gely non-overlapping set of molecules to achieve the same outcome: 1X = 2X.