Model for X-chromosome dosage compensation in Drosophila, C. elegans and mouse. X-chromosome transcription is generally upregulated (green arrow). X;AA tissues thus avoid unbalanced expression of X-chromosome genes. In order to avoid overexpression of X-chromosome genes (red symbols), females either destroy the compensation machinery used in males (Drosophila soma), or deploy a counteracting mechanism to reduce expression from both hyperactive X chromosomes (C. elegans) or eliminate expression from one hyperactive X chromosome (mouse). The mechanism used in the XX;AA Drosophila germline is unknown, but given that X-chromosome expression in the female germline is bi-allelic, an X-inactivation mechanism is unlikely.