 An explanation for this theory is that the X-chromosome simply inactivates in the presence of another X-chromosome; this causes XX-chromosome humans to have a lower frequency of the regulatory gene (given that both X and Y chromosomes have an equal frequency of the regulator) and so the expression of the male trait is prevented from appearing in the phenotype.Theories like the one above have become redundant now, however.In the past, there wasn't much evidence supporting the idea that X-chromosome inactivation occurred due to dosage compensation.