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 reproduction, capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism.
..... Click the link for more information. in which specialized reproductive cells (gametes) form and, when united by fertilization Cross-fertilization indicates fusion of a sperm of one hermaphroditic plant or animal with an ovum of another, as distinguished from self-fertilization, in which ovum and sperm of the same individual are fused.
..... Click the link for more information. , produce a zygote (fertilized egg) that develops into a new individual. The female gamete is called an egg or ovum ovum (ō`vəm), in biology, specialized plant or animal sex cell, also called the egg, or egg cell.
..... Click the link for more information. , and the male gamete a sperm sperm or spermatozoon (spûr'mətəzō`ən, –zō`ŏn)
..... Click the link for more information. .

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 pistil (pĭs`tĭl), one of the four basic parts of a flower , the central structure around which are arranged the stamens, the petals,
..... Click the link for more information. and the male organ is the stamen stamen, one of the four basic parts of a flower . The stamen (microsporophyll), is often called the flower's male reproductive organ. It is typically located between the central pistil and the surrounding petals.
..... Click the link for more information. . 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 pituitary gland, small oval endocrine gland that lies at the base of the brain . It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system ).
..... Click the link for more information. ).

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 chromosome (krō`məsōm')
..... Click the link for more information. 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 meiosis (mīŏ`sĭs)
..... Click the link for more information. ), 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 mutation, in biology, a sudden, random change in a gene , or unit of hereditary material, that can alter an inheritable characteristic. Most mutations are not beneficial, since any change in the delicate balance of an organism having a high level of adaptation to its
..... Click the link for more information. ) 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 Human Genome Project, international scientific effort to map all of the genes on the 23 pairs of human chromosomes and, to sequence the 3.1 billion DNA base pairs that make up the chromosomes (see nucleic acid ).
..... Click the link for more information. 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.

Bibliography

See study by J. Maynard-Smith (1978).

sex

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. See also reproductive behaviour.

SEX - /seks/ [Sun Users' Group & elsewhere] 1. Software EXchange. A technique invented by the blue-green algae hundreds of millions of years ago to speed up their evolution, which had been terribly slow up until then. Today, SEX parties are popular among hackers and others (of course, these are no longer limited to exchanges of genetic software). In general, SEX parties are a Good Thing, but unprotected SEX can propagate a virus. See also pubic directory.

2. The mnemonic often used for Sign EXtend, a machine instruction found in the PDP-11 and many other architectures. The RCA 1802 chip used in the early Elf and SuperElf personal computers had a "SEt X register" SEX instruction, but this seems to have had little folkloric impact.

DEC's engineers nearly got a PDP-11 assembler that used the "SEX" mnemonic out the door at one time, but (for once) marketing wasn't asleep and forced a change. That wasn't the last time this happened, either. The author of "The Intel 8086 Primer", who was one of the original designers of the Intel 8086, noted that there was originally a "SEX" instruction on that processor, too. He says that Intel management got cold feet and decreed that it be changed, and thus the instruction was renamed "CBW" and "CWD" (depending on what was being extended). The Intel 8048 (the microcontroller used in IBM PC keyboards) is also missing straight "SEX" but has logical-or and logical-and instructions "ORL" and "ANL".

The Motorola 6809, used in the UK's "Dragon 32" personal computer, actually had an official "SEX" instruction; the 6502 in the Apple II with which it competed did not. British hackers thought this made perfect mythic sense; after all, it was commonly observed, you could (on some theoretical level) have sex with a dragon, but you can't have sex with an apple.