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Sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote. As male and female individuals develop from zygotes into fetuses, into infants, children, adolescents, and eventually into adults, sex and gender differences at many levels develop: genes, chromosomes, gonads, hormones, anatomy, and psyche. Sex differences range greatly and include physiologically differentiating. Sex-dichotomous differences are developments which are wholly characteristic of one sex only. Examples of sex-dichotomous differences include aspects of the sex-specific genital organs such as ovaries, a uterus or a phallic urethra. In contrast, sex-dimorphic differences are matters of degree (e.g., size of phallus). Some of these (e.g., stature, behaviors) are mainly statistical, with much overlap between male and female populations. Nevertheless, even the sex-dichotomous differences are not absolute in the human population, and there are individuals who are exceptions (e.g., males with a uterus, or females with an XY karyotype), or who exhibit biological and/or behavioral characteristics of both sexes. Sex differences may be induced by specific genes, by hormones, by anatomy, or by social learning. Some of the differences are entirely physical (e.g., presence of a uterus) and some differences are just as obviously purely a matter of social learning and custom (e.g., relative hair length). Many differences, though, such as gender identity, appear to be influenced by both biological and social factors ("nature" and "nurture"). The early stages of human differentiation appear to be quite similar to the same biological processes in other mammals and the interaction of genes, hormones and body structures is fairly well understood. In the first weeks of life, a fetus has no anatomic or hormonal sex, and only a karyotype distinguishes male from female. Specific genes induce gonadal differences, which produce hormonal differences, which cause anatomic differences, leading to psychological and behavioral differences, some of which are innate and some induced by the social environment. ==Sex determination system== (詳細はXY sex-determination system. Humans have forty-six chromosomes, including two sex chromosomes, XX in females and XY in males. It is obvious that the Y chromosome must carry at least one essential gene which determines testicular formation (originally termed ''TDF''). A gene in the sex-determining region of the short arm of the Y, now referred to as ''SRY'', has been found to direct production of a protein, testis determining factor, which binds to DNA, inducing differentiation of cells derived from the genital ridges into testes. In transgenic XX mice (and some human XX males), ''SRY'' alone is sufficient to induce male differentiation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「sexual differentiation」の詳細全文を読む スポンサード リンク
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