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Asexual Reproduction, Sexual Reproduction in Animals and Reproductive Patterns- Biology Guide for Class 8

Sexual and Asexual Reproduction in Animals

Information about Asexual and Sexual Reproduction

Title

Asexual Reproduction, Sexual Reproduction in Animals and Reproductive Patterns

Class

Class 8

Subject

Class 8 Biology

Topics Covered

  • What is Reproduction?
  • Asexual Reproduction
  • Sexual Reproduction
  • Reproductive Patterns
  • External Fertilisation
  • Internal Fertilisation


Reproduction is the ability of the living organisms to produce young ones similar to themselves. It is one of the most important characteristics of life. All living things reproduce. All plants, animals and other living organisms exist because of reproduction. 

Reproduction in Animals

Animal reproduction is of two types:
  1. Asexual reproduction
  2. Sexual reproduction

1. Asexual Reproduction

  • The production of young ones by a single parent, without the formation and fusion of gametes is called asexual reproduction (Uniparental reproduction).
  • New individuals, so formed, are genetically identical to the parent. It is a primitive method of reproduction that is simpler than sexual reproduction.
Asexual reproduction takes place by the following processes:
  1. Binary fission
  2. Multiple fission
  3. Budding

1. Binary fission

  • It is usually observed in unicellular organisms.
  • It is common in organisms like bacteria and Amoeba.
  • In this process, a parent cell divides into two daughter cells and each cell, thus formed, grows to form an individual. 

2. Multiple fission

  • It is the repeated division of a parent cell into a number of small individuals.
  • For example, the Plasmodium (malarial parasite) parent cell undergoes repeated division.

3. Budding

  • The formation of a new individual, from a small protuberance, (called the bud), arising on the parent body, is called budding.
  • For example, budding takes place in organisms like Hydra and yeast.
  • The offspring remains attached to the parent during its growth.
  • It separates at an appropriate time to live as an independent organism.

Sexual Reproduction 

  • Production of young ones by the formation and fusion of special cells (called gametes), contributed generally by two parents, (male and female) is termed as sexual reproduction (Biparental reproduction).
  • Unlike asexual reproduction, the young ones are not genetically identical to the parent.
  • In multicellular organisms the needed special cells, or gametes, are produced by the reproductive organs. 
  • The male gametes, called the sperms, are produced by the male reproductive part and the female gametes, called the eggs or ova (singular-ovum), are produced by the female reproductive part. 
  • In most cases, these two types of gametes are produced by two different individuals. 
  • However, in some cases, like that of the earthworm, the same individual produces both types of gametes. The earthworm is called a hermaphrodite animal
  • When the two types of gametes, i.e. sperm and ovum unite, they form a single cell, called the fertilised egg, or zygote. This process is known as fertilisation. The zygote then develops into an offspring.

Reproductive Patterns

Different animals have been classified into three types on the basis of the sites of their fertilisation and development:
  1. External Fertilisation and External Development
  2. Internal Fertilisation and External Development
  3. Internal Fertilisation and Internal Development

1. External Fertilisation and External Development

  • Parents release sperms and eggs into the surrounding water where fertilisation occurs and zygotes develop into offsprings.
  • For this pattern to succeed, the male and the female must shed their gametes in large numbers at the same time and place. Frogs belong to this type of animals.

2. Internal Fertilisation and External Development

  • Sperms are passed from the male into the female body. The fertilisation takes place within the female body.
  • The zygote, after its formation, is surrounded by some secretions along with some reserve food and a shell. These help in the development and protection of the growing embryo.
  • For example, in animals, like the hen, the development of the chick takes place after the egg has been laid.

3. Internal Fertilisation and Internal Development

  • Both fertilisation and development take place in the female body.
  • This means that the formation of the zygote and the development of the young one takes place inside the body of the female.
  • Human beings, the elephant, the cow and many other animals belong to this category.

Some Important Points

  • If nothing stopped them, Amoebae would multiply so fast that they would cover up the entire planet. But Amoebae have predators; they also face competition for food and space.
  • Parthenogenesis is a form of asexual reproduction in which females produce eggs that develop without fertilisation. Parthenogenesis occurs naturally in aphids, daphnia, rotifers, some invertebrates and in many plants.
  • In many social insects, such as the honeybee and the ant, the unfertilised eggs give rise to the male drones and the fertilised eggs to the female workers and a queen. This has also been observed in some snakes, fish and monitor lizard.
  • A rabbit (which becomes an adult in 8 months) can produce 10-30 offsprings per year and a fruit fly (which becomes an adult in 10-14 days) can produce about 900 offsprings per year. Animals with few offsprings, devote more resources to the nurturing and protection of each individual offspring. This reduces the need for many offsprings for such animals. 
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