CIE IGCSE Biology: 复习笔记:16.1.2 Sexual Reproduction

CIE IGCSE Biology: 复习笔记:16.1.2 Sexual Reproduction

Gametes & Zygotes

Sexual Reproduction


  • Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) 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


  • 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



Haploid & Diploid Cells: Extended

  • The nuclei of gametes are haploid
    • They contain half the number of chromosomes of a normal body cell
    • In humans, this is 23 chromosomes
  • The nucleus of a zygote is diploid
    • It contains the same number of chromosomes as a normal body cell
    • In humans, this is 23 pairs of chromosomes
    • The zygote continues to stay diploid as it grows into a fetus and embryo during pregnancy


Advantages & Disadvantages of Sexual Reproduction: Extended


  • Most crop plants reproduce sexually and this is an advantage as it means variation is increased and a genetic variant may be produced which is better able to cope with weather changes, or produces significantly higher yield
  • The disadvantage is that the variation may lead to offspring that are less successful than the parent plant at growing well or producing a good harvest