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Genes that are found on the sex chromosomes influence sexually dimorphic brain development both by causing sex differences in gonadal secretions and by acting in brain cells themselves to differentiate XX and XY brains. Because it is easier to manipulate hormone levels than the expression of sex chromosome genes, the effects of hormones have been studied much more extensively, and are much better understood, than the direct actions in the brain of sex chromosome genes. Although the differentiating effects of gonadal secretions seem to be dominant, the theories and findings discussed above support the idea that sex differences in neural expression of X and Y genes significantly contribute to sex differences in brain functions and disease.
All of the sexually dimorphic signals, whether gonadal or brain-autonomous, are likely to induce sex differences in cellular functions that are adaptive in one organ or at one point of development, but maladaptive in another context.
Many neurological and psychiatric diseases differ in incidence or severity between the sexes. Some of these diseases are known to involve X-linked genes32. The vulnerability of males to mutations of X-linked genes is an obvious source of sex differences in diseases. However, more subtle variation of the same loci probably accounts for some of the differences in psychological and neural function among populations of males and females. Recent improvements in methods to manipulate and measure gene action will lead to further insights on the role of X and Y genes in brain gender.