Sex, physical and behavioral difference that distinguishes individual organisms according to their functions in the reproductive process. Through this difference, termed male and female, a species can constantly reshuffle its genetic information, thereby creating genetically different offspring, some of which may be better adapted to changing environments.
Many organisms also reproduce asexually (the parent multiplying without prior union), as in the case of bacteria and protozoans, which divide through mitosis (see Cell) into separate individuals. Plants and hydras reproduce asexually by budding. Many other organisms-including plants, the water fleas Daphnia, and some wasps-reproduce by parthenogenesis, in which the unfertilized egg develops into an adult. Such asexual reproduction has the advantage of colonizing great populations of a species in a very short period; indeed, Daphnia and wasps switch from sexual reproduction to parthenogenesis to populate ponds and their nests within the short warm season. Such populations, however, are made up of genetic replicas of the parent, and should some adversity occur in the environment, the entire population or species risks extermination.
In animals, the sex of an individual is generally determined at the time of fertilization by the sperm cell. If a sperm cell carrying an X chromosome fertilizes the egg, the offspring will be female (XX); if a sperm cell carrying a Y chromosome fertilizes the egg, the offspring will be male (XY). See Genetics. The term primary sexual characteristics denotes the kind of gamete the gonad produces: The ovary produces egg cells in the female, and the testis produces sperm cells in the male. The term secondary sexual characteristics denotes all other sexual distinctions that play indirect roles in uniting sperm and eggs. Secondary sexual characteristics include everything from the specialized male and female features of the genital tract, to the brilliant plumage of male birds or facial hair of humans, to behavioral features such as courtship.
In mammals, the hormones that influence sexual differentiation and development are androgens (mainly testosterone), which stimulate later development of the ovary. In the sexually undifferentiated embryo, testosterone stimulates the development of the Wolffian duct system, the forerunner of the male genital tract. Later, testosterone, along with gonadotrophins released by the pituitary gland, stimulates spermatogenesis. The Müllerian duct system, the forerunner of the female genital tract in the female embryo, probably differentiates spontaneously without hormonal stimulus. After female sex is well defined, estradiol, produced in the ovaries and the placenta, plays a major role in the development and the functioning of the female reproductive tract.
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