Development of Sexuality and Selection of Sex in Mammals

In mammals, sexual differences are observed at several levels of development. These events include : 1) chromosomal or genetic sex, 2) gonadal sex, 3) hormonal sex, 4) phenotypic or somatic sex, 5) brain or psychic sex, 6) behavioral sex. Generally the development of sexuality in mammals consists of a series of positive sequential events in males, and the absence of these events results in complete or partial feminization depending on the stage of development where the absence occurred. The presence of a Y chromosome in mammals results in the development of a testis, and in the absence of a Y chromosome, an ovary will develop. The sex determination occurs through induction of testis development by a gene located on the Y chromosome. The Y-encoded testis-determining gene is testis determining factor(TDF) in humans and testis determining Y chromosome(Tdy) in mice. Recently it has been possible to determine gender by identification of Y chromosome- linked markers at the molecular level. It is possible to identify X- and Y-chromosome-bearing spermatozoa and to select sex before embryo transfer. Methods for embryo sexing include immunological use of male-specific antigen, measurement of X-linked enzyme, cytogenetic methods, and use of Y-specific DNA probes.