 The XY, XX and ZW, ZZ system of sex determination allows accelerated fixation of alleles that are sex-linked recessive, male-beneficial and female-detrimental due to constant exposure to positive selection acting on heterogametic sex (XY, ZW) as compared to purifying selection removing the alleles only in homozygous state.In case of partial or completely dominant sex linked traits which are detrimental to male, the probability of selecting for the allele would be 2/3 as compared to selecting against probability of 1/3.Considering the above scenario it is likely that X and W chromosomes would harbor many sexually antagonistic alleles.