A lot of evidence from genetics over the years has made it apparent that gene dosage - the amount of gene product present in a cell - is quite important in animals (and other organisms). In humans, having an extra copy of chromosome 21 (trisomy 21) results in Down Syndrome and other trisomies either fail to develop or lead to very early death (only trisomy 13 and trisomy 18 ever are observed in live births). The effect of trisomy is a significant imbalance in gene dosage in these individuals. It seems that having an extra copy of just a few genes is usually not a problem, but an extra copy of all of the genes on a chromosome results in a dosage imbalance that has significant effects.

Dosage compensation refers to the phenomenon in which the expression of a gene - and, thus, its dosage - is altered depending on the number of copies of the gene in a cell. This is most frequently referred to in cases of sex-linked genes. Since XY and XX individuals differ in the number of copies of X-linked genes, expression levels are made consistent by dosage compensation.

This can either be accomplished by hyperactivation of genes on the X chromosome in the XY individuals (as, for example, in Drosophila), or by inactivation of one copy of the X chromosome (X-inactivation) in XX individuals (as in humans and mammals generally). X inactivation is covered on this page.

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