What Is Sex All About?

In biology, sex is a process of combining and mixing genetic traits, often resulting in the specialization of organisms into male and female reproductive roles. Sexual reproduction involves combining specialized cells (called gametes) to form offspring that inherit traits from both parents. Gametes can be identical in form and function, but in many cases an asymmetry has evolved such that two types of gametes exist: male gametes are small and are optimized to transport their genetic information, while female gametes are large and carry the nutrients necessary for the development of the child organism.

An organism's sex is defined by the gametes it produces: males produce male gametes (sperm) while females produce female gametes (egg cells); organisms which produce both male and female gametes are hermaphrodites. Frequently physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience. In some cases female organisms also have the role of carrying offspring through the first part of development, a process called gestation.

Sexual reproduction is a process where organisms form offspring that combine genetic traits from both parents.[1] Genetic traits are contained within the DNA of chromosomes вЂ" by combining a set 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 that randomly inherit one of the two different chromosomes, a process called meiosis.[2] Meiosis also involves a stage of chromosomal crossover, in which regions of DNA are exchanged between the two chromosomes to form a new pair of mixed chromosomes. These processes result in the recombining of different genetic traits.

In many organisms the haploid stage has been reduced to specialized cells called gametes specialized to recombine and form a new diploid organism. Sometimes gametes are externally similar, particularly in size (isogamy); often, however, an asymmetry has evolved such that the gametes are different in size and other aspects (anisogamy).[3] By convention, the larger gamete (called an egg cell or ovum) is considered female, while the smaller gamete (called a sperm) 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 many cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second organism.[4]

Hoverflies engaging in sexual intercourse

Hoverflies engaging in sexual intercourse

Animals

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 formsвЂ"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 flagella 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 use 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 cloacaвЂ"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 spermвЂ"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 inseminationвЂ"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).

Flowers are the sexual organs of flowering plants, containing both male and female parts.

Flowers are the sexual organs of flowering plants, containing both male and female parts.

Plants

Main article: Plant sexuality

Like animals, plants have developed specialized male and female gametes.[10] 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.

Female (left) and male (right) cones are the sex organs of pines and other conifers.

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 carpelsвЂ"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.

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