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Birds represent an unprecedented case in which genes on one sex chromosome are expressed on average at constitutively higher levels in one sex compared with the other.Sex-chromosome dosage compensation is surprisingly ineffective in birds, suggesting that some genomes can do without effective sex-specific sex-chromosome dosage compensation mechanisms.

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Non-translated RNAs are also involved in SSDC in other species, such as the ].

In XX-XY systems, the male-specific Y genes spend all of their evolutionary history in males, and have evolved male-specific functions.

Male:female ratios of expression were significantly higher for Z genes than for autosomal genes in several finch and chicken tissues.

In contrast, in mouse and human the male:female ratio of expression of X-linked genes is quite similar to that of autosomal genes, indicating effective dosage compensation even in humans, in which a significant percentage of genes escape X-inactivation.

M: F ratios of expression were calculated from hybridization of male versus female samples.

In each of four tissues (adult brain, kidney, liver, and post-hatch day 1 (P1) brain), the log Distributions of male-to-female (M: F) ratios of gene expression based on microarray studies of birds.

Dosage compensation is not understood in birds, in which females (ZW) and males (ZZ) differ in the number of Z chromosomes.

Using microarray analysis, we compared the male:female ratio of expression of sets of Z-linked and autosomal genes in two bird species, zebra finch and chicken, and in two mammalian species, mouse and human.

X-chromosome genes are subject to competing evolutionary pressures.

Although X genes good for males are immediately subject to positive selection because of their hemizygous exposure in males, X genes also spend twice as much of their evolutionary history in females as in males, so that they may be under differential selection to be good for females.

(a) M: F ratios in zebra finches, in adult brain, liver, and kidney, and brain of post-hatch day 1 (P1).

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