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Gene dosage refers to the number of copies of a gene present in an organism's genome, or complete "library" of genetic information. Many organisms, including humans, store genetic information on paired chromosomes. Each member of a pair of chromosomes contributes a "single dose" of the genes contained on that chromosome. Sex chromosomes, however, tend to differ between males and females; human males have a single X chromosome and a single Y chromosome while human females have a pair of X chromosomes. Various regulatory processes known as "dosage compensation" are in place to ensure that gene dosage remains at proper levels in both males and females despite the genetic imbalance caused by different genes.
Generally speaking, more copies of a gene — or higher gene dosage — will result in increased expression of the proteins for which the genes code. To a significant extent, however, the genes on male and female sex chromosomes are expressed at comparable levels despite the difference in gene dosage. If this were not the case, females with their two X chromosomes could overexpress certain genes, or males with their single X and single Y chromosomes could underexpress certain genes. Either of these alternatives could cause severe mutations or death, so it is important that the genes are expressed at comparable levels in spite of the difference in gene dosage.
A set of regulatory mechanisms and processes known as dosage compensation are responsible for maintaining the expression of genes at appropriate levels. Different organisms have different means of regulating the expression of their genes, and some even make use of multiple methods of dosage compensation. Gene expression in human females is regulated through X-inactivation, through which one of the female's two X chromosomes becomes an inactive "Barr body." The result of X-inactivation is that males and females each only have a single X-chromosome that is actually expressing its genetic information and contributing to gene dosage.
In some organisms, such as the fruit fly or Drosophila melanogaster, the expression of genes on the male X chromosome is doubled to match the gene dosage of the female's two X chromosomes. The roundworm, or C. elegans, presents an interesting case, as it exists most commonly as a hermaphrodite with two X chromosomes, though some have only a single sex chromosome, X, and are classified as male. Dosage compensation in C. elegans results in the partial repression of the expression of genes on both of the X chromosomes in hermaphrodites.