Abstract
In recent years, increasing evidence suggests that sex differences in the phenotype of all tissues is influenced by the inequality of effects of sex chromosome genes in the two sexes. In birds, genes on the Z chromosome are not well dosage compensated, so that most Z genes are expressed higher in ZZ male cells than in ZW female cells. The sex difference in expression of Z and W genes is likely to cause sex differences within cells, in addition to the sex differences caused by different levels of testicular and ovarian hormones. The sexual imbalance in cell physiology has implications for aviculture and novel developments in the poultry industry.