<unk><unk>  is the process by which organisms equalize the expression of genes between members of different biological<unk> es. Across species, different<unk> es are often characterized by different types and numbers of<unk><unk> s. In order to neutralize the large difference in gene<unk>  produced by differing numbers of<unk><unk> s among the<unk> es, various evolutionary branches have acquired various methods to equalize gene expression among the<unk> es. Because<unk><unk> s 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. For example, in humans, females (XX) silence the transcription of one X<unk>  of each pair, and transcribe all information from the other, expressed X<unk> . Thus, human females have the same number of expressed X-linked genes as do human males (XY), both<unk> es having essentially one X<unk>  per cell, from which to transcribe and express genes. Different lineages have evolved different mechanisms to cope with the differences in gene copy numbers between the<unk> es that are observed on<unk><unk> s. Some lineages have evolved<unk><unk> , an epigenetic mechanism which restores expression of X or Z specific genes in the heterogametic<unk>  to the same levels observed in the ancestor prior to the evolution of the<unk><unk> . Other lineages equalize the expression of the X- or Z- specific genes between the<unk> es, but not to the ancestral levels, i.e. they possess incomplete<unk>  with<unk>  balance”. One example of this is X-inactivation which occurs in humans. The third documented type of gene dose regulatory mechanism is incomplete<unk>  without balance (sometimes referred to as incomplete or partial<unk><unk> . In this system gene expression of<unk> -specific loci is reduced in the heterogametic<unk>  i.e. the females in ZZ/ZW systems and males in XX/XY systems. There are three main mechanisms of achieving<unk><unk>  which are widely documented in the literature and which are common to most species. These include random inactivation of one female X<unk>  (as observed in Mus musculus; this is called X-inactivation), a two-fold increase in the transcription of a single male X<unk>  (as observed in Drosophila melanogaster), and decreased transcription by half in both of the X<unk> s of a hermaphroditic organism (as observed in Caenorhabditis elegans). These mechanisms have been widely studied and manipulated in model organisms commonly used in the laboratory research setting. A summary of these forms of<unk><unk>  is illustrated below. However, there are also other less common forms of<unk><unk> , which are not as widely researched and are sometimes specific to only one species (as observed in certain bird and monotreme species). Three main mechanisms of<unk><unk>  observed in common model eukaryotic organisms.
Sex-chromosome dosage compensation
XY sex-determination system
Genomic imprinting
Nuclear sexing
Germline mosaicism
Postzygotic mutation
Uniparental disomy
Robertsonian translocation