[Middle English, from Latin sexus.]
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Sex is a short word, but an immense concept, and many of us spend a lot of time thinking about it. Sex is the engine that drives creation, ensuring propagation of the race and ultimate survival of the species. The imperfection of biological machines means that after a certain period of time they become dysfunctional and obsolescent, and it becomes more economic and energy-efficient to replace them completely than to continue to renew the old ones.
So how can selfish organisms, intent only on their own survival, be persuaded to reproduce and hand on their heritage and their living space to an utterly new individual?
There are a number of instruments, such as the selfish gene theory and the maternal instinct but the chief contrivance is sex, and Mother Nature has been capriciously kind in allowing us this delightful inducement to procreation.
Apart from being great fun, sex allows the mixing of genetic material with its consequent crucial increase in adaptability; no two progeny are exactly alike, as the cocktail of genes will never mix in exactly the same way. This genetic diversity permits adjustment to any environmental changes, as any advantageous characteristics will promote survival and increase their chances of being further propagated. For example, if the weather gets colder, people with a tendency to wear warm pyjamas will be more likely to survive and pass on their cuddly proclivities; thus the human race will tend to become more huggable, and I say go for it.
The act of sex is also of immense symbolic significance, a gesture of trust and vulnerability. The woman lies on her back (usually) exposing herself without guard or guile to the frenzied thrusting of the man.
The man is also uniquely vulnerable at this time; at the point of ejaculation his back arches, his eyes close in rapture, and he is oblivious to all ordinary sensations. This vulnerability is exploited more cold-bloodedly in other species; the male spider and praying mantis pay the ultimate price for sex, for that gasping moment of desire, for the chance to perpetuate their genes; they form a tasty little post-coital snack for their partner.
And one of the consequences of the desire for sex is that sublimation of that desire has led to that finest of human emotions: romantic love, the passionate unconditional constant devotion to another person, the inspiration for great art, great literature, great poetry, great food, and great, great pop songs. Some authors, however, have argued that it is implausible to suggest that this remarkable harmony between the interests of the species and the ecstasy of the individual came about solely though evolutionary pressure; that the sheer joy of sexual love is far greater than can be explained by reference to biological utility or Freudian psychology — as Tina Turner rasped, ‘What's love got to do with it?’
As one of our great primal drives, cultures and religions have naturally developed many different ways to depict and control sex. For example, many traditional religions consider the act of sex without procreation to be sinful, whereas contemporary Western social mores and wider environmental concerns about over-population take the opposing view. In Western society, most forms of sex are now acceptable, so long as they involve consenting adults, and these increasingly bizarre forms of non-procreational sex may be a species response to overcrowding and an inherent, if unconscious, awareness of the dangers of over-population; they might be considered analogous to the legendary mass migrations and suicides undertaken by lemmings when their numbers become too great to support their food supply, though thankfully outre sex is much more diverting both to partake in and to watch.
So the bottom line, so to speak, is that sex represents a ferociously potent device for ensuring that our species continues to adapt and survive (and have great parties on the way).
— Liam Farrell
Definition: intercourse between animates
Antonyms: abstention, chastity
Marital relations are the wife's right and the husband's duty. This obligation he must fulfill at specific intervals, which vary according to his occupation and ability (Ket. 61b). Should he wish to change from one occupation to another that will demand longer absences from home, he must secure his wife's permission in advance, because a woman prefers a smaller income and a close relationship with her husband to a higher income and separation from him (ibid.). The "curse of Eve," that a woman's desire is for her husband and that he will rule over her (Gen. 3:16), is said to account for woman's sexual modesty and her inhibition against taking the initiative in sexual activity (Er. 100b).
Romantic love does not play the role in early Judaism that it does in later periods: "Isaac then brought her into his mother Sarah's tent, and took Rebekah, and she became his wife; and he loved her" (Gen. 24:67). Love came after marriage, when the couple had assumed their mutual responsibilities. In Rabbinic Judaism, the sexual relationship between husband and wife is governed by intimacy, continuity, and sensitivity to physical needs. Intimacy refers to mutual consent, with the law prohibiting a husband from compelling his wife to have intercourse with him. The law also prohibits intercourse when either spouse is drunk or when the woman is asleep. Mutual consent implies non-exploitation. The sages insisted that husbands and wives must not withhold themselves sexually and must not engage in intercourse when in a state of anger against one other. Sexual favors are not an object for barter. Intimacy also implies exclusivity: the sages forbade a person to have intercourse with his or her spouse while thinking of someone else. Intimacy thus demands a totality of relationship between the two parties.
A second component of marital life is continuity, an awareness of the ongoing character of their union (see Family; Family Purity). A marriage may not be entered into with the intention of terminating it, for this contravenes the essential pledge of continuity. A married couple should refrain from having intercourse in the fields---not because someone may see them but because such behavior lacks the essentials of continuity. A third component is sensitivity to physical needs. The sexual component cannot be negotiated out of marriage by mutual agreement, although economic factors can. For example, the partners could agree that the husband will not support the wife but that she will provide for herself, so that everything she earns will belong to her. If, however, a couple should agree to maintain only a platonic relationship, the marriage is invalid; for besides procreation, the aim of marriage is to develop a mature sexual relationship between the parties. Sensitivity to each other's sexual urges should be so fundamental to the relationship that neither partner need verbalize it.
The sages demand modesty and restraint in the sexual act. Overindulgence is to be avoided, as well as unnatural positions (Sanh. 37b). Intercourse should take place at night, in privacy, after tender, loving words have been expressed. According to the author of Iggeret ha-Kodesh, an anonymous ethical work of the 13th century, sex is "holy and pure when engaged in properly at the proper time and with the proper intentions ... Whatever God created cannot possibly be shameful or ugly." It is the misuse of man's body that creates ugliness, since every one of his organs is neutral. Accordingly, "when a husband is united with his wife in holiness, the Divine Presence abides with them."
The preferred time for intercourse is on Friday night, the holiest night of the week (Kit. 62b). The sages view man's sexual drive as an expression of the yetser ha-ra (the evil inclination), bad if uncontrolled but good if channeled. They believe that man is capable of restraining his libido. The ideal is not to deny or suppress a natural urge but to harness it for consecrated ends---hence the admonitions to engage in conjugal relations within certain parameters that foster intimacy, continuity, and sensitivity to one's partner's sexual needs. Such parameters include the laws of Menstruation (niddah). These laws prohibit marital intercourse during the menstrual period and for at least a week thereafter (Lev. 15:19-28).
Sexual OffensesIn Judaism, sexuality is bound up with the establishment of a family, with love and mutuality, and involves a natural act. All sexual acts that are not conducted within the framework of the marital relationship, or which are unnatural, must be shunned. Premarital sex is forbidden according to Maimonides (Yad, Ishut 1:4), so as to prevent immoral behavior among Jews. Sex with Gentile women is also banned (Sanh. 82a), to prevent lasciviousness, as is visiting prostitutes.
The law forbidding masturbation is usually derived from the Onan story (Gen. 38:8-10). Although this episode seems to indicate coitus interruptus, the ban was widened to include any act that rules out procreation, the Talmud calling it "adultery of the hand" (Nid. 13b). Female masturbation is not mentioned, as it does not involve a deliberate waste of semen. Ethically, masturbation is prohibited because it occurs outside the marital relationship and is not conducive to its enhancement and mutuality. This holds true also for pre- and extramarital relations, and for Homosexuality. The Bible categorizes extramarital relations entered into by a woman as Adultery (Ex. 20:13), while rabbinic law forbids men to engage in such relations. Male homosexuality is termed "an abhorrent act" (Lev. 20:13) and female homosexuality---lesbianism---is likewise prohibited (Yad, Issuré Bi'ah 21:8).
Rape constitutes a transgression. If a man rapes a betrothed girl, he is put to death; if she is single, he must marry her and she can never be divorced. The woman is put to death only if she is married and was a consenting party, or where she could easily have called for help and failed to do so (Deut. 22:22-29). In the Judaic ethic, sex is held to be an indispensable element of life. It is the means through which men and women find completion; it must be disciplined by rules; it is private and intimate; and, when culminating in procreation, it has cosmic significance.
Information and misinformation on sex, and jokes about it, must always have circulated informally, yet, paradoxically, it is literary scholars who give most insight into this semi-secret oral tradition. Elizabethan sexual slang and jokes are well documented, since they are central to Shakespeare's humour; so are the innuendoes in Restoration dramatists and 18th-century satirists; there are even anthologies of such minor genres as the bawdy limerick, a form of wit popular among educated men in the late 19th and 20th centuries, and the bawdy songs of all-male groups (students, rugby footballers, army, etc.). Dirty jokes are a flourishing oral genre (sometimes aggregating into joke cycles, e.g. about Viagra, or Essex girls), and are frequently found as graffiti.
Sexual folklore certainly exists, both on a serious and on a jocular level. One widespread notion, seriously taught by doctors and schoolmasters well into the 20th century, was that boys who masturbate go blind, or ‘soft in the head’. Girls' taboos about menstruation were equally unscientific, though less alarming. There was, and still is, a good deal of ‘folk wisdom’ about sexual characteristics, though it is hard to tell how seriously it is held. Thus, it is said that the size of a man's nose indicates that of his penis; of a woman's mouth, the size and tightness of the entry to her vagina. Big feet could have the same meaning. Baldness in a youngish man is supposed to indicate virility, especially if accompanied by thick bodily hair, so women allegedly find bald men sexy. Oysters, Spanish fly, valerian, and ground-up animal horn are all reputed to be aphrodisiacs.
In older folk beliefs, certain animals which were inherently virtuous, such as bees and lions, would respect a virgin, but attack an unchaste girl; hence, a virgin could walk through a swarm of bees unstung. Nowadays it is sometimes said that if a woman forgets to put salt on the table when setting it for a meal, it is a sign she has lost her virginity.
In the days when public behaviour had to be discreet, various rumours circulated about visual signals indicating sexual status and intentions. In the 1950s, it was said that a girl wearing a Robertson Marmalade golliwog badge meant ‘I've lost my virginity’, and any woman wearing a thin bracelet round her ankle was sexually available, or even a prostitute. Some said if a woman wore a red hat, or red shoes, or allowed her petticoat to show, it meant she was not wearing knickers. Prostitutes had ways of attracting clients without actually soliciting in the legal sense; some used to walk slowly, accompanied by a poodle, while others might stand in shop doorways jingling a bunch of keys to show they were available, and had a flat to take the client to. A red lamp in the window was the sign of a brothel, hence the phrase ‘red light district’.
In recent years, there has been similar talk about homosexual signals based on how one's tie is knotted or one's breast-pocket handkerchief folded, though probably the wisest guide is the folk saying, ‘It takes one to know one’. It is often said by ‘straight’ people that any man wearing one earring is gay, though they disagree as to whether it is the left or the right ear that matters; in any case, male earrings are now so common that they can hardly be significant.
Colloquial speech abounds in references to sexual acts and organs. Until recently they were completely taboo in general society, so for the male subgroups which did use them among themselves they were a powerful mark of ‘belonging’. Currently, they are used freely by far more people than at any previous period in England. They still have aggressive and insulting force, but in some contexts are exploited for humorous effect; they are common in minor verbal genres such as riddles, playground rhymes, and limericks, where the with may consist either in uttering the offensive word or unexpectedly avoiding it.
Few folklorists have yet done research on the topic; Sutton, 1992, is the only book based on English current material, from women informants. The present authors sent out a questionnaire in 1998, on which much of the present entry is based.
See also CONCEPTION, MENSTRUATION, PREGNANCY.
The difficulties the western tradition has had with sexual desire are spectacularly voiced by Kant: ‘Taken by itself [sexual love] is a degradation of human nature; for as soon as a person becomes an object of appetite for another, all motives of moral relationship cease to function, because as an object of appetite for another a person becomes a thing and can be treated and used as such by every one’ (Lectures on Ethics). Kant seems to be describing a gang rape rather than sexual love, but he thought the only, fragile, escape from the fate of being ‘cast aside as one casts away a lemon that has been sucked dry’ was a contractual relationship based on marriage, although he himself did not try it (nor, probably, sex). In Plato, sexual desire is a good, although only the first step on a ladder of perfection (see beauty, love, Dante). The movements of thought whereby Plato's view degenerated to the Calvinism of Kant include increasing disgust with merely material as opposed to spiritual existence, and the Pauline and Augustinian conviction that original sin is somehow associated with sexual desire (see concupiscence). But older classical and Judaic traditions also associated sexuality and especially female sexuality and menstruation with uncleanliness and pollution. Women came off worst in other ways also, since as in too many modern cultures, rape was thought to dishonour the victim.
A more optimistic view of the matter than Kant's was voiced by Hobbes: ‘The appetite which men call lust…is a sensual pleasure, but not only that; there is in it also a delight of the mind: for it consisteth of two appetites together, to please, and to be pleased; and the delight men take in delighting, is not sensual, but a pleasure or joy of the mind consisting in the imagination of the power they have so much to please’ (Human Nature, ix. 10). In this area, prophecies are apt to be self-fulfilling: it is predictable that if we side with Kant our sexual relationships will be a lot worse than if we understand Hobbes. The power of the tradition of sin is still visible in the ratio of writings that pay serious attention to Kant's view, as opposed to ones that start with Plato, or Hobbes. See also obscenity, pornography.
The biological and anatomical differences distinguishing females from males.
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.
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.
See study by J. Maynard-Smith (1978).
Having sexual relations in a dream or seeing others having sex may indicate repressed desires for physical or emotional love, as well as the urge to "bond" and create new life. Sexuality is too complex and confused an area of modern life to capture here its broad range of possible meanings.
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1. the fundamental distinction, found in most species of animals and plants, based on the type of gametes produced by the individual or the category to which the individual fits on the basis of that criterion. Ova, or macrogametes, are produced by the female, and spermatozoa, or microgametes, are produced by the male. The union of these distinctive germ cells results in the production of a new individual in sexual reproduction.
2. to determine the sex of an animal.
Classification of an individual as male or female on the basis of anatomic, functional, hormonal, and chromosomal characteristics.
In biology, sexual reproduction is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety, each known as a sex. Sexual reproduction involves combining specialized cells (gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function (known as isogametes), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist: male gametes are small, motile, and optimized to transport their genetic information over a distance, while female gametes are large, non-motile and contain the nutrients necessary for the early development of the young organism.
An organism's sex is defined by the gametes it produces: males produce male gametes (spermatozoa, or sperm) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic. Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.
It is considered that sexual reproduction in eukaryotes first appeared about a billion years ago and evolved within ancestral single-celled eukaryotes. The reason for the initial evolution of sex, and the reason(s) it has survived to the present, are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex evolved as an adaptation for repairing damages in DNA (see Evolution of sexual reproduction).
While there are a number of theories, there are two main alternative views on the evolutionary origin and adaptive significance of sex. The first view assumes that sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to sex in animals, plants, and fungi, there are other eukaryotes (e.g. the malaria parasite) that also engage in sexual reproduction. On this first view, the adaptive advantage that maintains sexual reproduction (in competition with asexual modes of reproduction) is the benefit of generating genetic variation among progeny. Furthermore, on this view, sex originated in a eukaryotic lineage. The earliest eukaryotes and the bacterial ancestors from which they arose are assumed to have lacked sex. For instance, some bacteria use conjugation to transfer genetic material between cells; and while not the same as sexual reproduction, this also results in the mixture of genetic traits. The reason that bacterial conjugation is not the same as sexual reproduction is that the numerous genes necessary for conjugation are not located on the bacterial chromosome, but on small circular DNA self-replicating parasitic elements called conjugative plasmids. Thus, conjugation arises from an adaptation of parasitic DNA for its own transmission.
The second alternative view on the evolutionary origin and adaptive significance of sex is that sex existed in early bacteria as the process of natural transformation, a well studied DNA exchange process still in existence in many present day bacterial species (see Transformation (genetics)). Transformation involves the transfer of DNA from a donor to a recipient bacterium. Recipient bacteria must first enter a special physiological state, termed competence, to receive donor DNA (see Natural competence). The numerous genes necessary for establishment of competence are located on the bacterial chromosome itself. Thus the process of transformation is likely beneficial to bacteria, and can be regarded as a simple form of sex. In general, competence is induced under stressful conditions, such as nutrient limitation and exposure to DNA damaging agents, as reviewed by a number of authors. Sex, on this view, was present in the earliest single-celled eukaryotes because they were descended from ancestral bacteria capable of transformation. Sex was maintained as an adaptation for repairing DNA damage (see Evolution of sexual reproduction). In particular, meiosis the key stage of the sexual cycle of eukaryotes, in which genetic information derived from different individuals (parents) recombines, was likely derived from the analogous, but simpler, genetic information exchange and DNA repair process that occurs during transformation in bacteria (and also see Meiosis, section: Origin and function of meiosis). Thus, by this view, sex appears to have evolved in bacteria as a way of repairing DNA damages induced by environmental stresses, was maintained through the prokaryote/eukaryote boundary, and continued to evolve in higher multicellular eukaryotes, in part, as a DNA repair process.
What is considered defining of sexual reproduction in eukaryotes is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, no third gamete is known in multicellular animals.
While the evolution of sex itself dates to the prokaryote or early eukaryote stage, the origin of chromosomal sex determination may have been fairly early in eukaryotes. The ZW sex-determination system is shared by birds, some fish and some crustaceans. Most mammals, but also some insects (Drosophila) and plants (Ginkgo) use XY sex-determination. X0 sex-determination is found in certain insects.
No genes are shared between the avian ZW and mammal XY chromosomes, and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals. A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.
Sexual reproduction in eukaryotes is a process whereby organisms form offspring that combine genetic traits from both parents. Chromosomes are passed on from one generation to the next in this process. Each cell in the offspring has half the chromosomes of the mother and half of the father. Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes – by combining one of each type of chromosomes from each parent, an organism is formed containing a doubled set of chromosomes. This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid". Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis. Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between matched types of chromosomes, to form a new pair of mixed chromosomes. Crossing over and fertilization (the recombining of single sets of chromosomes to make a new diploid) result in the new organism containing a different set of genetic traits from either parent.
In many organisms, the haploid stage has been reduced to just gametes specialized to recombine and form a new diploid organism; in others, the gametes are capable of undergoing cell division to produce multicellular haploid organisms. In either case, gametes may be externally similar, particularly in size (isogamy), or may have evolved an asymmetry such that the gametes are different in size and other aspects (anisogamy). By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. An individual that produces both types of gametes is a hermaphrodite; in some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.
Most sexually reproducing animals spend their lives as diploid organisms, with the haploid stage reduced to single cell gametes. The gametes of animals have male and female forms—spermatozoa and egg cells. These gametes combine to form embryos which develop into a new organism.
The male gamete, a spermatozoon (produced within a testicle), is a small cell containing a single long flagellum which propels it. Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called fertilization.
Female gametes are egg cells (produced within ovaries), large immobile cells that contain the nutrients and cellular components necessary for a developing embryo. Egg cells are often associated with other cells which support the development of the embryo, forming an egg. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.
Animals are usually mobile and seek out a partner of the opposite sex for mating. Animals which live in the water can mate using external fertilization, where the eggs and sperm are released into and combine within the surrounding water. Most animals that live outside of water, however, must transfer sperm from male to female to achieve internal fertilization.
In most birds, both excretion and reproduction is done through a single posterior opening, called the cloaca—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing". In many other terrestrial animals, males use specialized sex organs to assist the transport of sperm—these male sex organs are called intromittent organs. In humans and other mammals this male organ is the penis, which enters the female reproductive tract (called the vagina) to achieve insemination—a process called sexual intercourse. The penis contains a tube through which semen (a fluid containing sperm) travels. In female mammals the vagina connects with the uterus, an organ which directly supports the development of a fertilized embryo within (a process called gestation).
Because of their motility, animal sexual behavior can involve coercive sex. Traumatic insemination, for example, is used by some insect species to inseminate females through a wound in the abdominal cavity – a process detrimental to the female's health.
Like animals, plants have developed specialized male and female gametes. Within most familiar plants, male gametes are contained within hard coats, forming pollen. The female gametes of plants are contained within ovules; once fertilized by pollen these form seeds which, like eggs, contain the nutrients necessary for the development of the embryonic plant.
Many plants have flowers and these are the sexual organs of those plants. Flowers are usually hermaphroditic, producing both male and female gametes. The female parts, in the center of a flower, are the carpels—one or more of these may be merged to form a single pistil. Within carpels are ovules which develop into seeds after fertilization. The male parts of the flower are the stamens: these long filamentous organs are arranged between the pistil and the petals and produce pollen at their tips. When a pollen grain lands upon the top of a carpel, the tissues of the plant react to transport the grain down into the carpel to merge with an ovule, eventually forming seeds.
In pines and other conifers the sex organs are conifer cones and have male and female forms. The more familiar female cones are typically more durable, containing ovules within them. Male cones are smaller and produce pollen which is transported by wind to land in female cones. As with flowers, seeds form within the female cone after pollination.
Because plants are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many plants, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Other plants have heavier, sticky pollen that is specialized for transportation by insects. The plants attract these insects with nectar-containing flowers. Insects transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in pollination.
Most fungi reproduce sexually, having both a haploid and diploid stage in their life cycles. These fungi are typically isogamous, lacking male and female specialization: haploid fungi grow into contact with each other and then fuse their cells. In some of these cases the fusion is asymmetric, and the cell which donates only a nucleus (and not accompanying cellular material) could arguably be considered "male".
Some fungi, including baker's yeast, have mating types that create a duality similar to male and female roles. Yeast with the same mating type will not fuse with each other to form diploid cells, only with yeast carrying the other mating type.
Fungi produce mushrooms as part of their sexual reproduction. Within the mushroom diploid cells are formed, later dividing into haploid spores—the height of the mushroom aids the dispersal of these sexually produced offspring.
The most basic sexual system is one in which all organisms are hermaphrodites, producing both male and female gametes—this is true of some animals (e.g. snails) and the majority of flowering plants. In many cases, however, specialization of sex has evolved such that some organisms produce only male or only female gametes. The biological cause for an organism developing into one sex or the other is called sex determination.
In the majority of species with sex specialization, organisms are either male (producing only male gametes) or female (producing only female gametes). Exceptions are common—for example, in the roundworm C. elegans the two sexes are hermaphrodite and male (a system called androdioecy).
Sometimes an organism's development is intermediate between male and female, a condition called intersex. Sometimes intersex individuals are called "hermaphrodite"; but, unlike biological hermaphrodites, intersex individuals are unusual cases and are not typically fertile in both male and female aspects.
In genetic sex-determination systems, an organism's sex is determined by the genome it inherits. Genetic sex-determination usually depends on asymmetrically inherited sex chromosomes which carry genetic features that influence development; sex may be determined either by the presence of a sex chromosome or by how many the organism has. Genetic sex-determination, because it is determined by chromosome assortment, usually results in a 1:1 ratio of male and female offspring.
Humans and other mammals have an XY sex-determination system: the Y chromosome carries factors responsible for triggering male development. The default sex, in the absence of a Y chromosome, is female. Thus, XX mammals are female and XY are male. XY sex determination is found in other organisms, including the common fruit fly and some plants. In some cases, including in the fruit fly, it is the number of X chromosomes that determines sex rather than the presence of a Y chromosome (see below).
In birds, which have a ZW sex-determination system, the opposite is true: the W chromosome carries factors responsible for female development, and default development is male. In this case ZZ individuals are male and ZW are female. The majority of butterflies and moths also have a ZW sex-determination system. In both XY and ZW sex determination systems, the sex chromosome carrying the critical factors is often significantly smaller, carrying little more than the genes necessary for triggering the development of a given sex.
Many insects use a sex determination system based on the number of sex chromosomes. This is called X0 sex-determination—the 0 indicates the absence of the sex chromosome. All other chromosomes in these organisms are diploid, but organisms may inherit one or two X chromosomes. In field crickets, for example, insects with a single X chromosome develop as male, while those with two develop as female. In the nematode C. elegans most worms are self-fertilizing XX hermaphrodites, but occasionally abnormalities in chromosome inheritance regularly give rise to individuals with only one X chromosome—these X0 individuals are fertile males (and half their offspring are male).
Other insects, including honey bees and ants, use a haplodiploid sex-determination system. In this case diploid individuals are generally female, and haploid individuals (which develop from unfertilized eggs) are male. This sex-determination system results in highly biased sex ratios, as the sex of offspring is determined by fertilization rather than the assortment of chromosomes during meiosis.
For many species sex is not determined by inherited traits, but instead by environmental factors experienced during development or later in life. Many reptiles have temperature-dependent sex determination: the temperature embryos experience during their development determines the sex of the organism. In some turtles, for example, males are produced at lower incubation temperatures than females; this difference in critical temperatures can be as little as 1–2 °C.
Many fish change sex over the course of their lifespan, a phenomenon called sequential hermaphroditism. In clownfish, smaller fish are male, and the dominant and largest fish in a group becomes female. In many wrasses the opposite is true—most fish are initially female and become male when they reach a certain size. Sequential hermaphrodites may produce both types of gametes over the course of their lifetime, but at any given point they are either female or male.
Many animals and some plants have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sex differences in humans include, generally, a larger size and more body hair in men; women have breasts, wider hips, and a higher body fat percentage. In other species, the differences may be more extreme, such as differences in coloration or bodyweight.
Sexual dimorphisms in animals are often associated with sexual selection – the competition between individuals of one sex to mate with the opposite sex. Antlers in male deer, for example, are used in combat between males to win reproductive access to female deer. In many cases the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drive sexual dimorphism, such as in the case of the stalk-eyed fly.
Other animals, including most insects and many fish, have larger females. This may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm—larger females are able to produce more eggs. For example, female southern black widow spiders are typically twice as long as the males. Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female, such as in the anglerfish. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss Dicranum and the liverwort Sphaerocarpos. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females.
In birds, males often have a more colourful appearance and may have features (like the long tail of male peacocks) that would seem to put the organism at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle. This hypothesis says that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps.
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