Two basic patterns of reproduction have been observed among animals and these are asexual and sexual.
In asexual reproduction an individual can give rise to daughter individuals by mitotic divisions of a part of its own body; no gametes are required.
Asexual reproduction is the characteristic feature of lower organisms such as the protozoans, sponges, coelenterates, certain flatworms, ascidians and others.
In sexual reproduction genetically distinct two special sex cells, called gametes, fuse to form one celled structure the zygote which inturn divides repeatedly to grow into a fully developed, new individual.
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The sex cells or gametes are of two types, the male gamete (also called spermatozoan or sperm) and the female gamete (also called ovum) and are usually produced in different gonads, i.e. the male gametes or sperms in the testes and the female gametes or ova in the ovaries, by reduction division, the meiosis.
The main differentiating feature between these male and female gametes is the presence of sex chromosomes of different nature.
The male gamete is usually small in size, motile and contains a very little cytoplasm and stored food, while female gamete on the other hand is large in size, immobile and contains massive cytoplasm and stored food materials (mostly fat, some proteins, minerals and a complete collection of vitamins).
Each of the gametes frequently comes from a different parent so that in many instances sexual reproduction requires the participation of two parents.
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This, however, is not always the case as in the example of certain animals such as parasitic flatworms (tapeworm and flukes), free living ringediworms (earthworm and leech) and certain molluscs, where single organism forms the two types of gametes that undergo fusion.
Such a organism that produces both types of gametes (i.e., seperftis and ova) is called hermaphrodite.
In most hermaphrodites the products of two sexes, i.e., sperms and ova, do not mature at the same time, so that self-fertilization does not usually occur; cross-fertilization is common and for successful cross- fertilization the meeting of two hermaphrodite animals is essential.
In brief sexual reproduction is often biparental but may also be uniparental depending on the species.
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Why is sexual reproduction so widespread? Asexual reproduction certainly appears to be an economical way to produce large number of new members of the species; new organism can be produced without the necessity of bringing together two reproductive cells and they are the-exact copies of the producing parent.
Then why is sexual reproduction so widespread? Asexual reproduction gives offspring with a gene combination identical-to that of the single parent.
This offers little opportunity for variation and variation in organisms is essential in order to cope with change.
The sexual reproduction on the other hand gives such variations and for this reason nearly all living organisms have developed two sexes and reproduce sexually.