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Successful reproductive sex in animals results in the fusion of a sperm and egg cell.

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into a male or female variety (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.

Sexual reproduction

Main article: sexual reproduction
File:Sexual cycle.svg

Sexual reproduction is a process where organisms form offspring that combine genetic traits from both parents. Chromosomes are passed on from one parent to another in this process. Each cell has half the chromosomes of the mother and half of the father.[1] 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 a process called meiosis.[2] 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).[3] 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.[4]

Hoverflies mating midair

Hoverflies engaging in sexual intercourse


Most sexually reproducing animals spend their lives as diploid organisms, with the haploid stage reduced to single cell gametes.[5] 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 spermatozoan (produced within a testicle), is a small cell containing a single long flagellum which propels it.[6] 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.[7] 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.[8] 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".[9] 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.

Human reproduction

Main article: Human reproduction
The main article for this category is category:Human sexuality.

This article is intended to focus on the biological aspects of sex. If you are interested in articles specifically related to humans and sexuality please see the above links.

Sex among humans

See Human sexuality for information about sexual activities (having sex, making love), sexual sensation, sexual gratification, and sexual intimacy between human beings

In humans, sex is conventionally perceived as a dichotomous state or identity for most biological and social purposes, such that a person can only be female or male. However, when the criteria which are generally used to define femaleness and maleness are examined more closely, it becomes apparent that the assignment or determination of 'sex' occurs at multiple levels. Environmental, biological, social, psychological and other factors are all believed to have some role in this process, and the complex interaction of these factors is expressed in the diversity of biological and psychosocial 'states' or levels found amongst the human population. A significant fraction of the human population simply does not correspond exclusively to either 'female' or 'male' with regard to every level of definition expressed in the following table. This discordance is discussed in more detail below.

This table outlines the major levels at which society currently recognizes a difference between human females and males. Some criteria are dichotomous and some, such as body size, exhibit sexual dimorphism (i.e. characteristics which are statistically more likely to be found in one sex than the other). Some of the levels are more amenable to scientific study or measurement than others; some are "imputed" or assigned to individuals by the society of which they are members (e.g. whether human males must wear trousers is a result of social norms); and some seem to be generated within each individual as a subjective identity or drive.

"Primary" sexual characteristics are typically present at birth and directly involved in reproduction. "Secondary" sexual characteristics typically develop later in life (usually during puberty) and are not directly involved in reproduction.

Level of definition Female Male
Biological levels (Sex)
Primary sex characteristics (Sex)
Usual sex chromosomes XX in humans XY in humans
Usual gonads ovaries testes
Usual level of sex hormones oestrogen, gestagen testosterone
Usual anatomy of internal genitalia clitoral crura, vagina, uterus, fallopian tubes corpora cavernosa, urethra, prostate, seminal vesicles
Usual anatomy of external genitalia glans clitoris, labia, vulva, clitoral hood, perineal urethra glans penis, scrotum, phallus, foreskin fused perineum
Secondary sex characteristics (Sex)
Usually Breasts, menstrual cycle, development of "hourglass" body form (i.e., 8), relatively shorter height, relatively more body fat Facial and body hair, development of "triangular" body form (i.e., , , , or , depending on the occasion), relatively higher height, relatively less body fat, relatively lower voice
Usually both sexes Pubic hair, underarm hair
Psychosocial levels (Gender)
Usual Assigned sex "It's a girl" "It's a boy"
Usual Gender of rearing "You are a girl" "You are a boy"
Usual Gender identity "I am a girl/woman" "I am a boy/man"
Usual Gender role "feminine" social behavior "masculine" social behavior
Usual sexual orientation androphilic gynephilic

The relationship between the various levels of biological sexual differentiation is fairly well understood. Many of the biological levels are said to cause, or at least shape, the next level. For example, in most people, the presence of a Y chromosome causes the gonads to become testes, which produce hormones that cause the internal and external genitalia to become male, which in turn lead parents to assign 'male' as the sex of their child (assigned sex), and raise the child as a boy (gender of rearing). However, the degree to which biological and environmental factors contribute to the psychosocial aspects of sexual differentiation, and even the interrelationships between the various psychosocial aspects of differentiation, is less well understood (see the nature versus nurture debate).


As indicated above, the levels of this paradigm imply a certain level of 'discordance' amongst the human population, as a result of diversity amongst humans.

Some discordances are purely biological, such as when the sex of the chromosomes (genetic sex) does not match the sex of the external genitalia (anatomic sex). This type of discordance is fairly well understood and is described briefly in the next section, and more fully in the article on intersex.

Discordances between the biological and psychosocial levels, such as when the gender identity does not match the anatomic sex, or between the various psychosocial levels, such as when the gender role does not match the gender identity, are even more common, but less well understood, generally speaking. These levels of definition and discordance are described below and in individual articles.

Understanding of discordance is important for several reasons. We can learn much about the processes of sexual differentiation, both biological and psychosocial, from people with biological discordances. Some of the levels of discordance have enormous significance to the lives of those affected and their relationships with society. In some cases, the causes of the discordances have acquired controversial political significance. Societies vary on the values placed on some discordances. In the last several decades, the public consensus of many Western societies has come to view some discordances as less undesirable and more tolerable than much of the rest of the world, although this view may exhibit a certain level of cultural imperialism.

Biological varieties of discordance

Human variability occurs in all the levels by which sex and gender are defined. Discordance at the biological levels is often referred to as an intersex condition. For example, some women may have an XY karyotype (chromosomal constellation); these women usually have a condition known as Complete Androgen Insensitivity Syndrome. Some boys may have a rudimentary uterus, or an extra X chromosome (Klinefelter's syndrome). In a small subset of boys and girls with intersex conditions, the external genitalia may be undervirilized or overvirilized. If the degree of virilization is "in-between", the genitalia are described as "ambiguous". Many people with intersex conditions do not have ambiguous genitalia. However, for these people, the relationships between biological factors (such as hormones i.e. progesterone, estrogen, and testosterone), environmental factors, and the psychosocial levels of sexual identity, such as gender identity and sexual orientation, have proven to be complex, with numerous exceptions to proposed theoretical systems. For example, there have been cases of people with male genetic/chromosomal sex, but with female external genitalia, assigned and raised as female, but discovering or deciding upon a male gender identity by adolescence. The degree to which a person's gender identity is affected by hormones, by genetic factors distinct from hormones, by early education, by social factors, and by "existential choice" remains imperfectly understood and a subject of superfluous contention.


File:Drosophila XY sex-determination.svg

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.[10] 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.

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.[11] 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.[12]

Many insects use a sex determination system based on the number of sex chromosomes. This is called XX/XO sex determination—the O 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.[13] 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 XO individuals are fertile males (and half their offspring are male).[14]

Other insects, including honey bees and ants, use a haplodiploid sex-determination system.[15] 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.

File:Ocellaris clownfish.JPG


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.

In some ferns the default sex is hermaphrodite, but ferns which grow in soil that has previously supported hermaphrodites are influenced by residual hormones to instead develop as male.[16]

Sexual dimorphism

Male and female pheasant

Common pheasants are sexually dimorphic in both size and appearance.

Main article: sexual dimorphism

Many animals have differences between the male and female sexes in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphisms are often associated with sexual selection - the competition between individuals of one sex to mate with the opposite sex.[17] 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 in size; in mammals species with high sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males.

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.[18] Occasionally this dimorphism is extreme, with males reduced to living as parasites dependent on the female.

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.[19] 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.

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.

Psychological, behavioral, and cultural varieties of discordance

In contrast to the small percentage of people with biological discordances of sex, a fairly large proportion of human beings may be "discordant" in one or more behavioral or psychological dimensions. The vast majority of these people who are discordant in some aspect of psyche or behavior do not have any detectable biological intersex condition, although some recent studies point towards biological factors in at least some of those conditions. Human societies respond to, or accommodate, these behavioral and psychological discordances in many different ways, ranging from suppression and denial of difference to acknowledging various forms of "third sex".

It may be significant that some societies identify youths with atypical behavioral characteristics and, instead of giving them corrective therapy or punishing them, socialize them in such a way that their individual characteristics let them provide a needed and/or useful function for the society in a recognized and respected role (e.g. individuals who take on the role or customs of shaman, medicine man or tong-ki).

Pictograms of men and women are often used to indicate the respective toilets designated for each sex. An example of this in the article pictogram shows the man with broader shoulders (sex dimorphism) and the woman in clothing that is, in the western world, rarely worn by men, and which functions as a gender signal. (Presumably these "male human" and "female human" pictograms are not used in countries where men wear dress-like clothing.) In many current societies, it is considered improper for a person of one sex to misrepresent himself or herself as a member of the opposite sex by donning gender-specific clothing of that sex, thereby practicing transvestism or cross-dressing. Such behavior receives severe social and/or legal sanctions in some cultures, whilst being tolerated or even celebrated in others.

See also berdache, hijra, xanith and transgender.

Such complex situations have led some scientists to argue that the two sexes are cultural constructions. Some people have sought to define their sexuality and sexual identity in non-polar terms, in the belief that the simple division of all humans into "males" and "females" does not fit their individual conditions. A proponent of this movement away from polar oppositions, Anne Fausto-Sterling, once suggested we recognize five sexes: male, female, merm (male pseudo-hermaphrodite), ferm (female pseudo-hermaphrodite) and herm (true hermaphrodite). Although this theory was quickly rejected by many as a bizarre flouting of human nature and social reality, inimical to the interests of those whom she was attempting to champion, it expresses the difficulty and imperfection of the current social responses to these variations.

Social and legal considerations

Forms of legal or social distinction or discrimination based on sex include sex segregation and sexism. Notably, some businesses, public institutions, and laws may provide privileges and services for one sex and not another, or they may require different sexes to be physically separated. Recently, western societies have moved towards greater sexual equality.

In gender theory, the term "heteronormativity" refers to the idea that human beings fall into two distinct and complementary categories, male and female; that sexual and marital relations are normal only when between two people of different genders; and that each gender has certain natural roles in life.


Main article: Evolution of sex

Sexual reproduction first appeared about a billion years ago, evolved within ancestral single-celled eukaryotes.[20] The reason for the initial evolution of sex, and the reason 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, and that sex helps in the removal of disadvantageous traits.

Sexual reproduction is a process specific to eukaryotes, organisms whose cells contain a nucleus and mitochondria. In addition to animals, plants, and fungi, other eukaryotes (eg. the malaria parasite) also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between bacteria; while not the same as sexual reproduction, this also results in the mixture of genetic traits.

What is considered defining of sexual reproduction 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.[21][22][23]

See also


  1. Alberts et al. (2002), U.S. National Institutes of Health, "V. 20. The Benefits of Sex".
  2. Alberts et al. (2002), "V. 20. Meiosis", U.S. NIH, webpage: V. 20. Meiosis.
  3. Gilbert (2000), "1.2. Multicellularity: Evolution of Differentiation", NIH, webpage:1.2.Mul.
  4. Alberts et al. (2002), "V. 21. Caenorhabditis Elegans: Development as Indiv. Cell", U.S. NIH, webpage: V. 21. Caenorhabditis.
  5. Alberts et al. (2002), "3. Mendelian genetics in eukaryotic life cycles", U.S. NIH, webpage: 3. Mendelian/eukaryotic.
  6. Alberts et al. (2002), "V.20. Sperm", U.S. NIH, webpage: V.20. Sperm.
  7. Alberts et al. (2002), "V.20. Eggs", U.S. NIH, webpage: V.20. Eggs.
  8. Alberts et al. (2002), "V.20. Fertilization", U.S. NIH, webpage: V.20. Fertilization.
  9. Ritchison G. Avian Reproduction. Eastern Kentucky University. URL accessed on 2008-04-03.
  10. Dellaporta SL, Calderon-Urrea A (1993). Sex Determination in Flowering Plants. The Plant Cell 5: 1241–1251.
  11. Smith CA, Katza M, Sinclair AH (2003). DMRT1 Is Upregulated in the Gonads During Female-to-Male Sex Reversal in ZW Chicken Embryos. Biology of Reproduction 68: 560–570.
  12. Evolution of the Y Chromosome. Annenberg Media. URL accessed on 2008-04-01.
  13. Yoshimura A (2005). Karyotypes of two American field crickets: Gryllus rubens and Gryllus sp. (Orthoptera: Gryllidae). Entomological Science 8 (3): 219–222.
  14. Riddle DL, Blumenthal T, Meyer BJ, Priess JR (1997). 'C. Elegans II, Cold Spring Harbor Laboratory Press.9.II. Sexual Dimorphism
  15. Charlesworth B (2003). Sex Determination in the Honeybee. Cell 114 (4): 397–398.
  16. Tanurdzic M and Banks JA (2004). Sex-Determining Mechanisms in Land Plants. The Plant Cell 16: S61–S71.
  17. Darwin C (1871). The Descent of Man and Selection in Relation to Sex, Murray, London.
  18. Stuart-Smith J, Swain R, Stuart-Smith R, Wapstra E (2007). Is fecundity the ultimate cause of female-biased size dimorphism in a dragon lizard?. Journal of Zoology 273 (3): 266–272.
  19. Zahavi, A. and Zahavi, A. (1997) The handicap principle: a missing piece of Darwin's puzzle. Oxford University Press. Oxford. ISBN 0-19-510035-2
  20. Book Review for Life: A Natural History of the First Four Billion Years of Life on Earth. Jupiter Scientific. URL accessed on 2008-04-07.
  21. Amanda Schaffer, "Pas de Deux: Why Are There Only Two Sexes?", Slate, updated 2007-09-27.
  22. Laurence D. Hurst, "Why are There Only Two Sexes?", Proceedings: Biological Sciences, 263 (1996): 415-422.
  23. ES Haag, "Why two sexes? Sex determination in multicellular organisms and protistan mating types", Seminars in Cell and Developmental Biology, 18 (2007): 348-9.

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