Sex chromosomes differ significantly in their gene content (Adler et al., 1997), which has led to the evolution of mechanisms of dosage compensation (LYON, 1962; Mary F. Lyon, 1961). At least three different mechanisms have been described to adjust sex chromosome gene expression dosage between males and females. The single male X-chromosome is hyper transcribed in flies; both X-chromosomes are partially repressed in worms, while in mammals one of the two X-chromosomes is silenced in each cell (Mary F. Lyon, 1961). In mammals, dosage compensation for X-linked gene products between the sexes is achieved by X-chromosome inactivation (XCI) in females (Mary F. Lyon, 1961). This process leads to the highly regulated transcriptional silencing of one of the two X-chromosome during early development, leading to the formation of the heterochromatic Barr body.
Keywords: animal breeding, different mammals, genetics specialization, mechanisms of X-chromosome
