Abstract
Some difficulties with the classical model for the evolution of a genetically invert Y chromosome are discussed. An alternative model is proposed, which is based on the principle of Mullers ratchet; this involves the accumulation of chromosomes bearing deleterious mutant genes in a finite population in the absence of crossing-over. This process would result in the gradual increase, with time, in the number of mutant loci carried in an average Y chromosome, although the frequency of individual deleterious alleles at most loci remains low. It is shown that this creates a selection pressure for differentially increasing the activity of the X chromosome in heterogametic individuals at the expense of that of the Y, leading eventually to a genetically inert Y chromosome and to the evolution of dosage compensation.