The occurrence of sex differences prior to the introduction of hormones implies that genes on the X or Y sex chromosomes have direct effects on sexual differentiation. Sex chromosome genes may directly produce products that contribute to sexual dimorphism or regulate transcription on autosomal genes (Malone & Oliver, 2008; Ober, Loisel, & Gilad, 2008). One method of exploring this hypothesis has been the development of a transgenic mouse model in which the sex chromosome complement is dissociated from the gonadal phenotype. The four potential combinations include XX and XY mice without the SRY genes, who develop ovaries, and XX and XY mice with the SRY gene, who develop testes (Arnold, 2009). Studies of this mouse model have revealed dosages of sex chromosome genes affecting a variety of brain structural and behavioral features, including gene expression in multiple tissues (Arnold, 2009; Reisert & Pilgrim, 1991), regional densities of tyrosine hydroxylase and vasopressin neurons, and behaviors such as habit formation (Quinn, Hitchcott, Umeda, Arnold, & Taylor, 2007) and responses to intruders (Arnold, 2009).
