Dosage compensation

Dosage Compensation is the equalization of gene expression between the males and females of a species. Because sex chromosomes contain different numbers of genes, different species of organisms have developed different mechanisms to cope with this inequality. Replicating the actual gene is impossible; thus organisms instead equalize the expression from each gene. In humans, the females (XX) silence the transcription of one X chromosome of each pair, and transcribe all information from the other, expressed X chromosome. Thus, human females have the same number of expressed X-linked genes as do human males (XY), both genders having essentially one X chromosome per cell, from which to transcribe and express genes.


XX/XY System
Several different dosage compensation methods are included in the XX/XY sex system. In the mammalian version, the females (XX) silence their extra X chromosome so gene expression matches that of the males (XY). In Drosophila flies, the males (XY) double the expression from their single X chromosome to match that of the females (XX).

In the mammalian XX/XY sex system, the X chromosome has more genetic information than the Y chromosome, and organisms have two (known) approaches to rectifying this. In humans, the females silence their extra X chromosome into inactive Barr bodies using the X-ist gene. This condenses almost all of the genetic material into untranscribable heterochromatin, leaving only pseudoautosomal portions free. These portions are similar to portions on the Y chromosome and allow division in meiosis.
In Drosophila flies, the opposite occurs. Instead of the females silencing their extra X chromosomes, the males double the expression from their X chromosomes. Several binding sites exist on the Drosophila X chromosome for the dosage compensation complex (DCC), a ribonucleoprotein complex; these binding sites have varying levels of affinity, presumably for varying expression of specific genes. The Male Specific Lethal complex, composed of protein and RNA binds and selectively modifies hundreds of X-linked genes, increasing their transcription to levels comparable to female D. melanogaster. This over-expression on the sole male X chromosome is particularly remarkable, as it has yet to be observed in another species.
In addition to humans and flies, some plants also make use of the XX/XY dosage compensation systems. Silene latifolia plants are also either male (XY) or female (XX), with the Y chromosome being smaller, with fewer genes expressed, than the X chromosome. Two separate studies have shown male S. latifolia expression of X-linked genes to be about 70% of the expression in females. If the S. latifolia did not practice dosage compensation, the expected level of X-linked gene expression in males would be 50% that of females, thus the plant practices some degree of dosage compensation but, because male expression is not 100% that of females, it has been suggested that S. latiforia and its dosage compensation system is still evolving. Additionally, in plant species that 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.