Sexual Reproduction (Cambridge (CIE) O Level Biology): Revision Note

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

Gametes

• A gamete is a sex cell (in animals: sperm and ovum; in plants pollen nucleus and ovum)

• 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 zygote

• 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

• 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 and Disadvantages of Sexual Reproduction

• 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