Types of Fertilization in Sexual Reproduction:

Fertilization: Sperm and Egg
This a colored scanning electron micrograph (SEM) of sperm clustered around a human egg (ovum) during fertilization. The round egg (green) is seen on human tissue (brown). Sperm attached to its surface are hair-like structures (yellow).

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In sexual reproduction, two parents donate genes to their offspring through a process called fertilization. The resulting young receives a combination of inherited genes. In fertilization, male and female sex cells or gametes fuse to form a single cell called a zygote. A zygote grows and develops by mitosis into a fully functioning individual.

Fertilization is necessary for all organisms that reproduce sexually and there are two mechanisms by which fertilization can take place. These include external fertilization in which eggs are fertilized outside of the body and internal fertilization in which eggs are fertilized within the female reproductive tract.

Sexual Reproduction

In animals, sexual reproduction comprises the fusion of two distinct gametes to form a diploid zygote. Gametes, which are haploid are produced by cell division called meiosis. In most cases, a male gamete (spermatozoan) is relatively motile and usually has a flagellum to propel itself. A female gamete (ovum) is non-motile and often larger than a male gamete.

In humans, gametes are found in male and female gonads. Male gonads are testes and female gonads are ovaries. Gonads also produce sex hormones, which are required for the development of primary and secondary reproductive organs and structures.

Hermaphroditism

Some organisms are neither male nor female and these are known as hermaphrodites. Animals such as sea anemones may have both male and female reproductive parts. It is possible for hermaphrodites to self-fertilize, but most mate with other hermaphrodites to reproduce. In these cases, since both parties involved become fertilized, the number of offspring is doubled.

Hermaphroditism solves the problem of mate scarcity. The ability to change sex from male to female (protandry) or from female to male (protogyny) also mitigates this issue. Certain fish like wrasses may change from female to male as they mature. These alternative approaches to sexual reproduction are successful—fertilization does not need to be between a natural-born male and female to yield healthy offspring.

External Fertilization

External fertilization occurs mostly in aquatic environments and requires both a male and female organism to release or broadcast gametes into their surroundings (usually water). This process is called spawning. Amphibians, fish, and coral reproduce by external fertilization. External fertilization is advantageous because it results in a large number of offspring. However, due to various environmental hazards such as predators and adverse weather conditions, offspring produced in this way face numerous threats and many even die.

Animals that spawn do not typically care for their young. The degree of protection an egg receives after fertilization directly impacts its survival. Some organisms hide their eggs in the sand, others carry them around in pouches or in their mouths, and some simply spawn and never see their young again. An organism that is nurtured by a parent stands a much better chance of living.

Internal Fertilization

Animals that use internal fertilization specialize in developing and protecting an egg. Sometimes the offspring itself is encased in an egg upon its birth and sometimes it hatches from an egg before it is born. Reptiles and birds secrete eggs covered in a protective shell that is resistant to water loss and damage in order to protect them.

Mammals, with the exception of egg-laying mammals called monotremes, protect an embryo or fertilized egg within the mother as it develops. This extra protection increases the chances of survival by supplying the embryo with everything it needs until it is born via live birth. Organisms that internally fertilize care for their young for anywhere from a few months to several years after they are born.