sex therapy, treatment of sexual disorders and dysfunction, including impotence, orgasmic dysfunction, vaginismus (spasm of the muscles of the vagina), premature ejaculation, and lack of sexual responsiveness, not caused by a physical problem. Pioneered by Masters and Johnson in the early 1970s, sex therapy is based on the premise that sexual behavior is learned and that problems can be alleviated through sex education, sensitization exercises, and improved communication between sexual partners. Treatment is generally short-term and focused on the sexual problem, although in some cases sex therapists also address underlying problems in the relationship. It is most successful when it includes the patient's sexual partner.

sex, term used to refer both to the two groups distinguished as males and females, and to the anatomical and physiological characteristics associated with maleness and femaleness. Sex relates to the type of reproduction in which specialized reproductive cells (gametes) form and, when united by fertilization, produce a zygote (fertilized egg) that develops into a new individual. The female gamete is called an egg or ovum, and the male gamete a sperm.

Sexual Differentiation

Differentiation into two sexes appears in some members of all divisions of the plant and animal kingdoms. Even in species where little or no sexual difference has occurred anatomically, an implied separation exists in forms in which conjugation occurs (e.g., among different strains in paramecia and between plus and minus strains in molds). Many lower forms reproduce within the one individual two different kinds of cell that unite to form a new individual; in others, male and female cells form in different individuals. Among the vertebrates, the sexes are usually readily distinguishable by their primary sexual characteristics, i.e., the structure of their reproductive organs. In the highest group of plants, the seed-bearing plants, the female organ is the pistil and the male organ is the stamen. The stamens and pistil may appear in the same flower, in different flowers of the same plant, or in the flowers of separate plants. Secondary sexual characteristics include the bright coloration of many male birds and fish, the antlers of male deer, the beard and deepened voice of human males, and the mammary glands of female mammals. In higher animals, hormones released by the sexual organs under stimulation from the pituitary hormones play a dominant role in the control of sexual characteristics and the sexual processes of reproduction (see pituitary gland).

Genetic Basis of Sex and Sex-linked Traits

The modern science of genetics has provided a scientific explanation about how an offspring becomes either female or male. Based on the discovery that among the chromosomes present in the body cells, a special pair of sex chromosomes exist that bear the genes determining the sex of the offspring. In the human female, these chromosomes are identical and are called X chromosomes (indicated by XX). The male has one X chromosome and one smaller Y chromosome, which is dominant for maleness. During the process of producing reproductive cells (see meiosis), each of these chromosomes is segregated into a different gamete. Thus, when fertilization occurs, according to Mendelian law, 50% of the offspring will be XX (female) and 50% XY (male). Deviations from this rule do occur, but it is generally true.

The rule also helps to explain the inheritance of sex-linked characteristics such as hemophilia (a blood clotting disorder) and red-green color blindness, since the X chromosome also carries some genes for nonsexual traits. The Y chromosome carries very few genes for nonsexual traits; these few (including one for hairy ears) are called holandric genes. Certain inherited characteristics comprise X-linked traits, so called because a single X chromosome occurs in males. A recessive characteristic, e.g., when a gene leads to the expression of a disease such as hemophilia, may locate on the X sex chromosome in males and thus appear in that family.

Genetic Variability

Because of the myriad genes in the nucleus of every parent cell, the probability of two individuals inheriting identical characteristics is almost zero; thus, innumerable new variations (see mutation) constantly undergo testing for survival advantages in the individual's environment. The evolutionary flexibility that results from sexuality at some stage of the reproductive cycle seems not only beneficial but necessary in maintaining the adaptability of the species. The Human Genome Project is mapping and sequencing the approximately 30,000 human genes. The goal of this international scientific effort focuses on discovering the genetic basis for diseases in order to help humans avoid having children with severe or fatal genetic disorders.

Either of a pair of chromosomes that determine whether an individual is male or female. The sex chromosomes of mammals are designated X and Y; in humans, they constitute one pair of the total 23 pairs of chromosomes. Individuals possessing two X chromosomes (XX) are female; those having one X and one Y chromosome (XY) are male. The X chromosome is larger and carries more genetic information than the Y. Traits controlled only by genes found on the X chromosome (e.g., hemophilia, red-green colour blindness) are said to be sex-linked. Sex-linked traits occur far more frequently in males than in females, since a male inheriting an allele for a recessive (see recessiveness) trait on the X chromosome lacks a corresponding allele on the Y chromosome that might counteract its effects. Several disorders are associated with an abnormal number of sex chromosomes, including Turner syndrome and Klinefelter syndrome.

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Sum of features by which a member of a plant or animal species can be placed into one of two complementary reproductive groups, male or female. In both plants and animals, sex is determined by the reproductive cells (gametes) produced by the organism. The male produces sperm cells, and the female produces egg cells. Males and females may or may not have apparent structural differences, but they always have functional, hormonal, and chromosomal differences. Patterns of behaviour, sometimes elaborate, may also distinguish the sexes in some species. Seealso reproductive behaviour.

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