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The normal gene dosage is dependent on the species; humans generally have two doses -- one copy from the mother and one from the father. Changes in gene dosage can be a result of copy number variation (gene insertions or gene deletions), or aneuploidy (chromosome number abnormalities). These changes can have significant phenotypic consequences.
Z-specific genes were over-expressed in males when compared to females, and a few genes had equal expression in both male and female Z chromosomes. [citation needed] In chickens, most of the dosage compensated genes exist on the Zp, or short, arm of the chromosome while the non-compensated genes are on the Zq, or long, arm of the chromosome.
For some rare genes, the normal diploid level of gene expression is essential to individual survival; fewer than two copies of such a gene results in lethality. In Drosophila , a single dose of the locus known as Triplolethal is in an otherwise diploid individual.
An example of this is seen in the case of Williams syndrome, a neurodevelopmental disorder caused by the haploinsufficiency of genes at 7q11.23. The haploinsufficiency is caused by the copy-number variation (CNV) of 28 genes led by the deletion of ~1.6 Mb. These dosage-sensitive genes are vital for human language and constructive cognition. [2]
Theoretically, X-inactivation should eliminate the differences in gene dosage between affected individuals and individuals with a typical chromosome complement. In affected individuals, however, X-inactivation is incomplete and the dosage of these non-silenced genes will differ as they escape X-inactivation, similar to an autosomal aneuploidy.
Coordinate inter-genomic gene expression Duplicated genes often result in increased dosage of gene products. Doubled dosages are sometimes lethal to the organism thus the two genome copies must coordinate in a structured fashion to maintain normal nuclear activity. [1] Many mechanisms of diploidization promote this coordination.
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The SHOX gene in the PAR1 region is the gene most commonly associated with and well understood with regards to disorders in humans, [17] but all pseudoautosomal genes escape X-inactivation and are therefore candidates for having gene dosage effects in sex chromosome aneuploidy conditions (45,X, 47,XXX, 47,XXY, 47,XYY, etc.).
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