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
| Advantages | Disadvantages |
| Increases genetic variation |
It takes time and energy to find mates |
| The species can adapt to new environments due to variation giving them a survival advantage | Difficult for isolated members of the species to reproduce |
| Disease is less likely to affect population (due to variation) |
- 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
