gamete

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A gamete (from Ancient Greek γαμέτης; translated gamete = wife, gametes = husband) is a cell that fuses with another gamete during fertilization (conception) in organisms that reproduce sexually. In species that produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual that produces the larger type of gamete — called an ovum (or egg) — and a male produces the smaller tadpole-like type — called a sperm. This is an example of anisogamy or heterogamy, the condition wherein females and males produce gametes of different sizes (this is the case in humans; the human ovum is approximately 20 times larger than the human sperm cell). In contrast, isogamy is the state of gametes from both sexes being the same size and shape, and given arbitrary designators for mating type. The name gamete was introduced by the Austrian biologist Gregor Mendel. Gametes carry half the genetic information of an individual, 1n of each type.

In humans, an ovum can carry only an X chromosome (of the X and Y chromosomes), whereas a sperm may carry either an X or a Y; hence, it has been suggested that males have the control of the sex of any resulting zygote^{[citation needed]}, as the genotype of the sex-determining chromosomes of a male is XY, and a female's is XX. In other words, because the Y chromosome can only be present in the sperm, it is that gamete alone which can determine whether an offspring will be a male or female.

Gametogenesis

Scheme showing analogies in the process of maturation of the ovum and the development of the spermatids.

The production of gameotypes is termed gametogenesis, during which phase gametocytes divide by meiosis into gametes. Meiosis reduces the number of sets of chromosomes from 2n to 1n (i.e., produces haploid gametes from diploid gametocytes). Organs that produce gametes are called gonads in animals, and archegonia or antheridia in plants.

Gamete cycle

An organism of 2n generation ultimately creates 1n gametes in the next generation, but still keeping the same quantity of genetic information.

Gametes are haploid cells; that is, they contain half(n) of a complete set of chromosomes (the actual number varies from species to species). When two gametes fuse (in animals typically involving a sperm and an egg), they form a zygote — a cell that has a complete set of chromosomes(2n) and therefore is diploid. The zygote receives one set of chromosomes from each of the two gametes through the fusion of the two gamete nuclei. After multiple cell divisions and cellular differentiation, a zygote develops, first into an embryo, and ultimately into a mature individual capable of producing gametes.

Sperm-egg distinction

Eggs are relatively few, large, and immobile, whereas sperm are many, small, and mobile. The size difference is mostly (but not entirely) accounted for by the very large cytoplasm of the egg. Eggs awaiting zygote formation may be anchored either to something in the environment or by an organ that contains them; sperm may rely solely on their own motility or may be relayed into place by an organ such as pollen to reach the place of zygote formation. Typically many more sperm than eggs are created and wasted, in the sense of never fusing with a partner gamete.

The sperm-egg distinction is the basis for distinguishing between males and females. Since some algae and fungi have sexual reproduction by combining two identical gametes, there is no male/female distinction in these species. This raises the question as to why most large/familiar species reproduce by sperm and egg. One theory for why the male/female distinction is so common is that it facilitated encounters between gametes, in ancestral marine species.^[1]

Dissimilarity

In contrast to a gamete, the diploid somatic cells of an individual contain one copy of the chromosome set from the sperm and one copy of the chromosome set from the egg; that is, the cells of the offspring have genes expressing characteristics of both the father and the mother. A gamete's chromosomes are not exact duplicates of either of the sets of chromosomes carried in the somatic cells of the individual that produced the gametes. They can be hybrids produced through crossover (a form of genetic recombination) of chromosomes, which takes place in meiosis. This hybridization has a random element, and the chromosomes tend to be a little different in every gamete that an individual produces. Additionally, base pairs in chromosomes often undergo random mutations resulting in modified DNA (and subsequently, new proteins and phenotypes). This mutation, recombination, and the fact that the two chromosome sets ultimately come from either a grandmother or a grandfather on each parental side account for the genetic dissimilarity of siblings.

Notes and references