Topic 16.2 · Core and Supplement
Sexual Reproduction
Sexual reproduction is defined by a single precise event — the fusion of the nuclei of two gametes — and that precision matters more than it looks like it should, because a looser definition built around two parents falls apart the moment self-pollination enters an exam question.
Sexual reproduction and fertilisation
Sexual reproduction is a process involving the fusion of the nuclei of two gametes to form a zygote, producing offspring that are genetically different from each other and from either parent. A gamete is a reproductive cell involved in sexual reproduction — sperm and egg cells in humans, or the male and female reproductive nuclei involved in reproduction in flowering plants. Fertilisation itself is defined narrowly: it is the fusion of the nuclei of gametes, nothing broader.
Resist defining sexual reproduction as “reproduction involving two parents.” It’s a common shortcut, but it’s not the biological definition, and it breaks down for self-pollination, where both gametes come from the same plant and yet gamete nuclei still fuse — which makes it sexual reproduction regardless. The defining event is the fusion of nuclei, not the number of individuals involved.
It’s also worth keeping pollination and fertilisation firmly apart from the start, since the two get confused constantly once flowering plants enter the chapter. Pollination is the transfer of pollen from an anther to a stigma. Fertilisation is the fusion of gamete nuclei that happens afterwards, once a pollen tube has reached an ovule. They are two separate events, not two names for the same thing.
Haploid gametes and the diploid zygote — Supplement
Gamete nuclei are haploid, meaning each one contains one set of chromosomes. When two gamete nuclei fuse during fertilisation, their chromosome sets combine, so the zygote nucleus that forms is diploid, containing two sets of chromosomes. The relationship is simple arithmetic once you see it stated plainly: haploid gamete plus haploid gamete gives a diploid zygote.
Haploid doesn’t mean the chromosomes themselves have been physically cut in half — it means the nucleus holds one complete set rather than two. The process that actually produces haploid gametes from diploid parent cells, meiosis, belongs formally to inheritance further into the syllabus; here, the relationship you need is simply between the haploid gamete and the diploid zygote it helps form.
Sexual reproduction compared with asexual reproduction — Supplement
| Feature | Asexual reproduction | Sexual reproduction |
|---|---|---|
| Fusion of gamete nuclei | Does not occur | Occurs |
| Genetic similarity of offspring | Genetically identical to the parent | Genetically different from each other |
| Genetic variation produced | Very low | Greater |
| Speed and reliance on another individual | Can often occur rapidly without needing a mate | Requires gametes to meet and fuse |
In a wild population, the advantage of sexual reproduction is genetic variation. When conditions change, individuals within a genetically varied population don’t all respond identically — some may carry characteristics that give them a better chance of surviving and reproducing under the new conditions. That variation comes at a cost: gametes have to meet and their nuclei have to fuse, which can require more time and energy than reproducing alone.
In crop production the same trade-off plays out in reverse from the asexual case. Sexual reproduction is useful when a breeder wants new combinations of characteristics to select from, since variation is exactly what produces those new combinations. But it’s a poor tool for preserving one particular parent plant’s characteristics unchanged, because sexually produced offspring are not genetically identical to that parent — the useful combination isn’t copied, it’s reshuffled.
Where this leads next
The rest of this chapter is this same mechanism worked out in two organisms: flowering plants, starting with sexual reproduction in plants, and humans, starting with sexual reproduction in humans. The fusion-of-nuclei definition on this page is the one both pathways build on, so it’s worth having it exact before moving on.