YJX Home Revise: 4.1 Sexual and Asexual Reproduction

Asexual and Sexual Reproduction
■ Reproduction: production of a new generation of individuals(offspring) from pre-existing ones(parents).
■ Importance: ensure the continuity(连续性) or perpetuation(永久性) of the species.
■ Types of reproduction

	►	Sexual reproduction
	►	Asexual reproduction

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Sexual Reproduction

■ The production of offspring by the fusion(融合) of specialised sex cells(gametes)
■ The male gamete(sperm) fuses with the female gamete (ovum or egg) by a process known as fertilisation.
■ This results in the formation of a zygote(受精卵). The zygote develops into a new individual.
■ Living organisms which carry out sexual reproduction: humans, all vertebrates, insects and flowering plants.
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■ Types of fertilisation:

	►	Internal fertilisation The sperms are deposited in the female reproductive tract and fertilisation occurs within the body of the female. The zygote develops internally and live individuals are born. Occurs in insects, reptiles, birds and mammals.
	►	External fertilisation Limited almost entirely to aquatic organisms, sperms and eggs are shed into the water in great numbers simultaneously. Only a small number of the eggs are fertilised by sperms. Occurs in most fish amphibians and aquatic plants.

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Asexual Reproduction
■ This is a rapid method of increasing the number of new individuals.
■ The new individuals are formed from one parent and they are genetically identical to each other and to the parent.