Sexual reproduction
The process follows this logical sequence:
- Gamete Formation: Two parents produce gametes (sex cells). In animals, these are sperm and egg cells; in plants, pollen and ovules.
- Fertilisation: The nuclei of two gametes fuse together. This specific event is called fertilisation.
- Zygote Formation: The fusion creates a single cell called a zygote.
- Offspring Development: The zygote divides and develops into an embryo, then a new organism.
Building on the concept of cell division, sexual reproduction relies on meiosis (a type of reduction division) to create gametes. This ensures that when two gametes fuse, the resulting offspring has the correct number of chromosomes.
Understanding the chromosome count is essential for Paper 4 (Supplement content).
- Haploid (n): Cells containing one set of chromosomes. Gametes are always haploid. This ensures that when two gametes fuse, the offspring does not end up with double the required number of chromosomes.
- Diploid (2n): Cells containing two complete sets of chromosomes (one from each parent). The nucleus of a zygote is diploid. Most somatic (body) cells in animals and plants are also diploid.
| Chromosome Set Count |
|---|
| One set (n) |
| Two sets (2n) |
| Two sets (2n) |
Offspring are genetically different from each other and their parents because they inherit a random mix of genetic material from two different parents.
For example, in humans:
- The mother contributes 23 chromosomes via her egg.
- The father contributes 23 chromosomes via his sperm.
- The zygote has 46 chromosomes. Because the specific combination of chromosomes from each parent is random (due to meiosis), no two siblings (except identical twins) have the exact same genetic makeup.
Correction: You must specify that the nuclei of the gametes fuse. The cytoplasm may also merge, but the critical genetic event is the fusion of the haploid nuclei to form a diploid zygote nucleus. Examiners look for the word nuclei.
Correction: Gametes are haploid. If gametes were diploid, the zygote would have four sets of chromosomes (4n), which is usually lethal or abnormal. Meiosis reduces the chromosome number by half specifically to prevent this.
When to use: When asked to discuss the advantages of sexual reproduction for a species in the wild or its population.
Examiner Expectation: Examiners want you to link genetic variation to survival and adaptation. Do not just say 'it creates variety.' Explain why that variety is useful.
Correct Phrasing Strategy:
- State that sexual reproduction produces offspring with genetic variation (or diversity).
- Explain that this means some individuals may possess alleles (gene variants) that allow them to survive new challenges.
- Use the phrase: 'Increased genetic variation allows the population to adapt to changing environments or resist diseases/pests.'
- Contrast with asexual reproduction implicitly: If a disease kills one individual, it is likely to kill all clones. In a sexually reproducing population, some individuals may be resistant.
When to use: When asked to discuss disadvantages of sexual reproduction for a species in the wild.
Examiner Expectation: Focus on energy expenditure, reproductive efficiency, and risk compared to asexual reproduction.
Correct Phrasing Strategy:
- State that producing gametes, finding mates, and courtship behaviors require significant energy/time.
- Explain that only half the population (females) typically produces offspring, whereas in asexual reproduction all individuals can reproduce.
- Use the phrase: 'Sexual reproduction is energetically costly and slower than asexual reproduction because it requires finding a mate.'
- Mention risk: 'Gametes and offspring may be vulnerable to predation or environmental hazards before fertilisation occurs.'
When to use: When asked to discuss advantages of sexual reproduction (e.g., seed production) for agriculture or crop farming.
Examiner Expectation: Focus on disease resistance and genetic improvement.
Correct Phrasing Strategy:
- State that sexual reproduction creates genetic variation in the offspring.
- Explain that this allows breeders to select plants with desirable traits (e.g., higher yield, drought tolerance).
- Use the phrase: 'Genetic variation provides a source of new alleles that can be selected for disease resistance or improved crop quality.'
When to use: When asked to discuss disadvantages of sexual reproduction for crops.
Examiner Expectation: Focus on loss of desirable traits and time/energy costs.
Correct Phrasing Strategy:
- State that offspring are genetically different from the parent, so they may not have the same desirable qualities (e.g., taste, size).
- Use the phrase: 'Offspring may not retain the specific desirable characteristics of the parent plant.'
- Mention cost: 'Sexual reproduction often requires more time and energy (e.g., producing flowers, fruit) than vegetative propagation.'