Sex

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For the article about the act of "sex", see sexual intercourse.

[change] Biological type

Butterflies are among those animals that reproduce sexually

When a biologist looks at living beings (plants and animals), there will be two different kinds to tell apart:

1. Those animals and plants that reproduce by growing new cells, and by splitting of the newly created organism at some point. This is called asexual reproduction. Some germs, and yeast do that, some spiders do that as well. Such organisms are usually all of the same kind, and they are called asexual (they do not have a sex)
2. Other plants and animals reproduce by a union of two different kinds of organisms. Each kind makes special cells used for reproduction. The type of cells each type makes are different. One kind of them is usually called male, the other is usually called female. The new organism is made when both types of cells are united (in what is usually called a zygote). Organisms that can make both kinds of cells for reproduction are called hermaphrodites. Most snails are hermaphrodites.
   • Those organisms that produce sperm are called male, those that produce ova ("eggs") are called female.
   • Some animals and plants are capable of reproducing either sexually or asexually.

[change] Benefits and drawbacks

Spider plants with small Stolons. They are a way of asexual reproduction

Asexual reproduction is easier than sexual reproduction, but there are benefits and drawbacks to both:

• Offspring produced asexually are identical to its parent
• Offspring produced sexually are different from either parent. It looks like being different from one's parent is an advantage when it comes to adapting to a changing environment (evolution plays its part)

[change] Humans

Human female and male

A person's sex is either male or female. For example: John's sex is male. Jane's sex is female. This difference is sometimes called gender. Not all living things have a sex. There are not male or female germs or yeast. Only animals that make babies using an ovum and sperm have a sex.

In humans, the sex of a person depends on what sex chromosomes that person has in his or her cells. All of a woman's ova already contain one X chromosome. A man's sperm contains either an X or a Y chromosome. When a sperm and ova combine to form a baby, the baby may get either of these chromosomes from its father. If the baby gets two X chromosomes, it will usually develop into a girl. If the baby gets one X and one Y, it will usually develop into a boy. There are exceptions to this rule, though. For example, sometimes a baby with XY chromosomes will not respond to the Y and develop as a girl. Sometimes mutations will occur when an individual would have 3 or more X chromosomes, or 2 or more Y chromosomes. These people are usually Early Diagnosed with Ability Deficit Hyperactivity Disorder (ADHD), Obsessive Compulsive Disorder (OCD), mental retardation, or Schizophrenia. These people will also have a weaker immune system, and might not be fertile.

Before babies are born, they grow either male sex parts or female sex parts.

[change] Evolution

Sexual reproduction first appeared about a billion years ago. It evolved within single-celled eukaryotes.^[1] The scientific community still discusses why it appeared, and why it still exists. Reasons commonly given include:

• Sex creates variation in offspring. If there are traits that give an advantage to the organism, they spread more easily. Traits that give a disadvantage tend to be removed quickly.

Sexual reproduction is a process that can only be found in eukaryotes. These cells have a nucleus and mitochondria. There are other kinds of organisms (other than animals, plants and fungi), the other eukaryotes - such as the malaria parasite- that also engage in sexual reproduction. Some bacteria use conjugation to transfer genetic material between bacteria. This is not the same as sexual reproduction, but it also results in the mixture of genetic traits.

In sexual reproduction, the cells used for reproduction, called gametes are different. Fertilisation needs two different such cells. If there are many different types of gametes in a species, this would still be called Sexual reproduction. However, no third gamete type is known in multicellular animals.^[2][3][4]

[change] Sex determination

Sex can be determined in different ways:

• Perhaps the simplest solution is that all organisms produce both male and female gametes. An organism that does this is usually called hermaphrodite. This system can be found in some animals, for example snails, and in most flowering plants.^[5]
• In many cases, there is a specialization of sex. This means that an organism either produces male gametes or female ones, but not both at the same time. The biological cause for an organism developing into one sex or the other is called sex determination.
• Most of the time when there is sex determination, the organism is either male, or female, but not both. There are a few exceptions though. In the roundworm for example, both sexes are hermaphrodite, and also male. This is known as androdioecy
• Sometimes an organism's is between male and female. This is called intersex, and very rare. Even though such organisms are called hermaphrodites this is not totally correct, because in intersex individuals either the male or the female aspect is sterile. Intersex individuals are exceptions, they are not the norm.

When there is sex determination there are basically two cases:

• The sex is determined through the genes the organism inherits from its parents.
• The sex is determined through the environment.

[change] Genetic

Like humans and other mammals, the common fruit fly has an XY sex determination system.

One way to determine sex is to use genes. That way, an organism's sex is determined by the genome it gets. With genetic sex determination, most alleles or genes that influence sexual development are on the same chromosome. That chromosome is then called sex chromosome. Sex is determined either by the fact that there is a sex chromosome (which can be missing), or by the number of them. Because genetic sex determination is determined by matching chromosomes, there are usually the same number of male and female offspring.

[change] Different systems to genetically determine sex

Humans and other mammals have an XY sex determination system: the Y chromosome carries factors responsible for male development. The default sex, in the absence of a Y chromosome, is female. XX mammals are female and XY are male. XY sex determination is also found in other organisms, including the common fruit fly and some plants.^[5] 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.

Birds have a system that works the other way round: It is called ZW sex-determination system. The W chromosome has factors for female development. By default (if the chromosome is missing), the organism will be male.^[6] 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.^[7]

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.^[8] 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).^[9]

Other insects, including honey bees and ants, use a haplodiploid sex-determination system.^[10] 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.

Clownfish are initially male; the largest fish in a group becomes female.

[change] Nongenetic

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 life. This phenomenon is 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 female at birth 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 hormones remaining to instead develop as male.^[11]

[change] References