gamete

A gamete (from Ancient Greek γαμέτης gametes "husband" / γαμετή gamete "wife") is a cell that fuses with another cell 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 has approximately 100,000 times the volume of a single human sperm cell^[1][2]). 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.

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.

Plants

Plants which reproduce sexually also have gametes, however, they are produced in the anther and ovary. They produce pollen and ovules by meiosis, in a similar way to animals.

Sex determination

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; thus the male sperm determines the sex of any resulting zygote, if the zygote has two X chromosomes it will develop into a female, if it has an X and a Y chromosome, it will develop into a male.^[3] For birds, the female ovum determines the sex of the offspring, through the ZW sex-determination system.^[3]

Notes and references