Different dosage compensation mechanisms control the level of gene expression from the X chromosomes. (a) In mammals, one X chromosome of the female (XX) is inactivated, forming a compact structure called a Barr body. (b) In Drosophila, the single X chromosome of the male (XY) is transcribed at twice the level of each X chromosome in the female (XX). (c) In C. elegans, the two X chromosomes of the hermaphrodite (XX) are transcribed at half the level of the single X chromosome of the male (X0).