  sexual differentiation is the process of development of the differences between males and females from an undifferentiated zygote (fertilized egg). 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 from physiologically differentiating to simply statistical. 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==  humans, many mammals, insects and other animals have an xy sex-determination system. humans have forty-six chromosomes, including tw
Biology_of_gender Gender Sex