16.2 Sexual reproduction
- Syllabus
- 0610–2026–2027
- Topic
- 16.2
- Level
- —
Learning objectives
16.2.1Sexual reproduction as a process• Describe sexual reproduction as a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other16.2.2Fertilisation as the fusion• Describe fertilisation as the fusion of the nuclei of gametes16.2.3Nuclei of gametes are haploid and that• State that nuclei of gametes are haploid and that the nucleus of a zygote is diploid16.2.4Advantages and disadvantages of sexual• Discuss the advantages and disadvantages of sexual reproduction: (a) to a population of a species in the wild (b) to crop production
Define sexual reproduction
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different from each other.
Two gametes contribute genetic information → their nuclei fuse → a zygote forms → the offspring contain a new combination of genetic information.
Sexual reproduction is defined by fusion of gamete nuclei, not simply by having two organisms present. Self-pollination is still sexual because male and female gamete nuclei fuse.
Define fertilisation
Fertilisation is the fusion of the nuclei of gametes.
A male-gamete nucleus and a female-gamete nucleus unite, producing the nucleus of a zygote.
Pollination, mating and the meeting of whole gamete cells may bring gametes together, but fertilisation specifically names the fusion of their nuclei.
Relate haploid gametes to a diploid zygote
The nuclei of gametes are haploid: each contains one set of chromosomes. The nucleus of a zygote is diploid: it contains two sets.
| Stage | Chromosome status | Relationship |
|---|---|---|
| male gamete | haploid, n | one chromosome set |
| female gamete | haploid, n | one chromosome set |
| fertilisation | n + n | gamete nuclei fuse |
| zygote | diploid, 2n | two chromosome sets |
Haploid gametes restore the diploid number at fertilisation, so the chromosome number does not double in every generation. For a species with diploid number 46, each gamete has 23 and the zygote has 46.
A gamete is haploid; a zygote is diploid. Fertilisation doubles the number relative to one gamete, but restores—not exceeds—the normal diploid number.
Discuss advantages and disadvantages
Sexual reproduction creates genetic variation because offspring receive different combinations of genetic information from gametes. That variation has different consequences in wild populations and crop production.
| Context | Advantage | Disadvantage |
|---|---|---|
| population in the wild | variation increases the chance that some individuals survive a disease or environmental change and allows adaptation by natural selection | two gametes or parents must meet; reproduction can be slower and use more time or energy; fewer offspring may be produced |
| crop production | variation allows breeders to select new combinations, including disease resistance or improved yield | offspring do not all retain desired features; the crop may be non-uniform; breeding, pollination and seed production can take time |
A complete discussion links variation to the context: varied wild populations are less likely to share one vulnerability, while varied crop offspring can help breeding but make a uniform, predictable harvest harder to obtain.
Variation is not automatically beneficial in every context. It supports adaptation and breeding, but it also makes offspring less predictable than genetically identical clones.