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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; males have the control of the sex of any resulting zygote, 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.

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]

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.

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