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 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.

Sperm-egg distinction

Eggs are relatively few, large, and do not move, 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.

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.^[2] For birds, the female ovum determines the sex of the offspring, through the ZW sex-determination system.^[2]

Side note

There is some research that indicates it is the egg that allows the sperm to enter thereby choosing which sperm fertilizes it. Thus, even though the sperm is the carrier of either the X,X or the X,Y, it could be in fact the case that the egg is the gamete that chooses the sex by defining which sperm is included.^{[citation needed]}

Notes and references

1. ^ Dusenbery, David B. (2009). Living at Micro Scale, Ch. 20. Harvard University Press, Cambridge, Mass. ISBN 978-0-674-03116-6.
2. ^ ^{a b} Jay Phelan (30 April 2009). What Is Life?: A Guide to Biology W/Prep-U. Macmillan. pp. 237–. ISBN 9781429223188. http://books.google.com/books?id=F4okAxTXPtIC&pg=PA237. Retrieved 8 October 2010.