Dosage compensation

Dosage compensation comprises regulatory processes that reduce differences in gene expression caused by different gene copy numbers between sexes or karyotypes.

X-chromosome inactivation: The largely chromosome-wide silencing of one X chromosome in many female mammals. It compensates for two X chromosomes in XX mammals by reducing expression from one.
Sex chromosome: A chromosome involved in determining or differentiating biological sex in a species. Sex chromosomes create the unequal gene-copy numbers that compensation addresses.
Dosage imbalance: A difference in gene expression or function caused by unequal genomic copy numbers. It is the expression disparity that compensation can reduce, but does not always eliminate.
X-inactivation center: A region of the X chromosome that coordinates initiation and control of X-chromosome inactivation in mammals. Its regulatory elements determine which X chromosome becomes largely silent.
X-chromosome upregulation: Increased transcription from the single X chromosome in many male flies. It raises male X-linked expression to approach the output of two female X chromosomes.
Gene dosage: The number of copies of a gene or genomic region present in a cell. Dosage compensation concerns expression differences arising from unequal gene copy number.
Mammalia: The vertebrate class that includes mammals, characterized by hair and mammary glands. Mammals commonly silence one X, unlike flies, which increase output from one X.
Escape from X-inactivation: The expression of genes from an X chromosome that remains active after chromosome-wide inactivation. Escape explains why some genes retain unequal dosage despite mammalian X silencing.
Dosage compensation complex: A protein complex that binds the male X chromosome in Drosophila and increases its transcription. It is the molecular machinery behind fly X-chromosome upregulation.
Transcription: The synthesis of RNA from a DNA template by RNA polymerase. Many compensation systems adjust gene output by changing transcription rates.

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