Dosage compensation occurs in Drosophila, C. elegans, and mammals [8,9]. If the expression level of each pair of autosomes (gray for both males and females) is set to 1.0, then the expression level of the two X chromosomes in females (pink) and the single X chromosome in males (blue) is also equal to 1.0. To achieve this dosage compensation, the single X chromosome in the Drosophila male soma and germline, C. elegans male soma, and mammalian male soma is upregulated (dark blue, up-arrow). In Drosophila female soma, the X chromosomes are both expressed and thus not upregulated (light pink). According to Gupta et al. , each X chromosome in the Drosophila female germline is probably upregulated, and yet they must also be downregulated somehow, in order to prevent functional tetrasomy (light pink, double arrow). The X chromosomes in C. elegans hermaphrodite soma are presumably upregulated, but they are also known to be downregulated by half (light pink, double arrow). In mammalian females, one of the two X chromosomes is active and upregulated (dark pink, up-arrow), while the other X chromosome gets inactivated (white, down-arrow). The haploid germ cells of mammals express but do not upregulate their X chromosomes to achieve the same level of autosomal expression (0.5; light pink for female and light blue for male) . Primary mammalian oocytes, which have two non-upregulated X chromosomes, and spermatocytes, in which the X chromosome is largely silenced, are not depicted.