Sex differences in fitness and selection for centric fusions between sex-chromosomes and autosomes

SUMMARY A model of centric fusions between the X or Y chromosome and an autosome carrying a selected locus is studied. It is assumed that fusions are not associated with any fitness effects, and that all chromosomes disjoin regularly from their homologues. It is shown that a necessary condition for the fusion to be favoured is that there is a selectively maintained sex difference in allele frequencies at the selected locus. If this condition is satisfied, the initial rate of increase of a rare Y -autosome fusion is about three times that of an X -autosome fusion, with the same parameter values. Computer calculations of the final equilibrium states reached by populations containing such fusions were done.