Edexcel IGCSE Biology: Double Science 复习笔记 3.1.1 Types of Reproduction

• Sexual reproduction is:

The process involving the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other

• Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring

Gametes and zygotes

• A gamete is a sex cell (in animals: sperm and ovum; in plants: pollen nucleus and ovum)
• Gametes differ from normal cells as they contain half the number of chromosomes found in other body cells - we say they have a haploid nucleus
• This is because they only contain one copy of each chromosome, rather than the two copies found in other body cells
• In human beings, a normal body cell contains 46 chromosomes but each gamete contains 23 chromosomes
• When the male and female gametes fuse, they become a zygote (fertilised egg cell)
• This contains the full 46 chromosomes, half of which came from the father and half from the mother - we say the zygote has a diploid nucleus

Advantages and disadvantages of sexual reproduction

• Although there are some clear advantages of sexual reproduction (in comparison to asexual reproduction), there are also a few disadvantages of sexual reproduction

Advantages & Disadvantages of Sexual Reproduction Table

• Asexual reproduction is:

The process resulting in genetically identical offspring being produced from one parent

• Asexual reproduction does not involve gametes or fertilisation
• Only one parent is required so there is no fusion of gametes and no mixing of genetic information
• As a result, the offspring are genetically identical to the parent and to each other (they are clones)
• Many plants reproduce via asexual reproduction
• Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission

Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission

Advantages and disadvantages of asexual reproduction

• Whilst asexual reproduction has several advantages over sexual reproduction, it also has a few disadvantages

Advantages & Disadvantages of Asexual Reproduction Table

• The key differences between sexual and asexual reproduction include:
  • The number of parent organisms
  • How offspring are produced (the type of cell division required)
  • The level of genetic similarity between offspring
  • The possible sources of genetic variation in offspring
  • The number of offspring produced
  • The time taken to produce offspring

   

Comparing Sexual & Asexual Reproduction Table

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