the content of a chromosome would be changed mainly by mutation after duplication of the chromosome and translocation with other chromosomes. however, in mammals, since the chromosomal sex-determination mechanism would have been established in their earlier stages of evolution, polyploidy would have not occurred due to its incompatibility with the sex-determining mechanism. moreover, x-autosome translocation would have been prohibited because it might have resulted in detrimental effects for survival to the organism. thus in mammals, the content of x chromosomes has been conserved after typical 2 round duplication events at early ancestral stages of evolution, at the fish or amphibia (2r hypothesis).