Dosage compensation is a genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. Dosage compensation also occurs in other organisms like the fruit fly Drosophila melanogaster and the round worm Caenorhabditis elegans.
Species can have different mechanisms of dosage compensation. In human females (XX), one chromosome is inactivated (see X-inactivation), resulting in a heterochromatic and largely genetically inactive Barr body. Drosophila males (XY) double the expression of genes along the X chromosome. In C. elegans hermaphrodites (XX), both X chromosomes are partially repressed. Any of these mechanisms results in balancing the relative gene expression between males and females (or, in the case of C. elegans, hermaphrodites and males).
In plants (which lack dimorphic sex chromosomes), dosage compensation can occur when aberrant meiotic events or mutations result in either aneuploidy or polyploidy. Genes on the affected chromosome may be upregulated or down-regulated to compensate for the change in the normal number of chromosomes present.
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