Sexual Reproduction (CIE IGCSE Biology)

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

Gametes

• 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

• 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

• 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