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Every kind of living thing has to produce a new generation, or its species simply disappears. Yet nature does not do this in a single way. Some organisms make copies of themselves, so that each new individual is an exact genetic duplicate of the one parent that produced it. Others combine genetic material from two parents, so that the offspring resemble both parents but are never quite the same as either. This first lesson of Topic B5 (Genes, inheritance and selection) of your OCR Gateway Combined Science course sorts reproduction into these two contrasting strategies — sexual and asexual — and introduces the special kind of cell division, meiosis, that makes sexual reproduction possible. Fixing these ideas firmly now will make sense of everything that follows in the topic: inheritance, variation, natural selection and evolution.
By the end of this lesson you should be able to describe sexual and asexual reproduction, explain the role of meiosis in producing gametes, compare mitosis and meiosis, and give examples of organisms that use each strategy.
This lesson develops AO1 (recall and understanding of reproduction and meiosis) and AO2 (applying these ideas to unfamiliar organisms and to comparisons between mitosis and meiosis).
There are two fundamentally different ways in which living things reproduce, and telling them apart is the foundation of this whole topic.
If you hold on to just one idea from this lesson, make it this one: sexual reproduction generates variation, while asexual reproduction produces genetically identical clones. Nearly every other difference between the two strategies is a consequence of that single contrast.
Exam Tip: A common exam question describes how a particular organism reproduces and asks you to decide whether it is sexual or asexual. The decisive clue is whether two parents and gametes are involved (sexual) or whether one parent simply copies itself (asexual). If the offspring are described as "identical" or "clones", the answer is asexual.
In sexual reproduction, two parents each contribute a gamete — a sex cell that carries only half the usual number of chromosomes. In animals the gametes are the sperm (from the male) and the egg cell or ovum (from the female). In flowering plants they are the pollen (male) and the egg cell inside the ovule (female).
The two gametes join in a process called fertilisation. When they fuse, their chromosomes combine, so the new cell — the zygote — has the full number of chromosomes once more, half from each parent. The zygote then divides repeatedly by mitosis to grow into a new individual.
flowchart LR
A["Male parent"] --> B["Male gamete<br/>(half the chromosomes)"]
C["Female parent"] --> D["Female gamete<br/>(half the chromosomes)"]
B --> E["Fertilisation<br/>(gametes fuse)"]
D --> E
E --> F["Zygote<br/>(full chromosome number)"]
F --> G["New individual<br/>grows by mitosis"]
Because each offspring receives a mixture of genes from two different parents, every offspring is genetically different from its parents and from its brothers and sisters. That is why a litter of kittens, or the children in one family, are clearly related and yet all individuals. Two important terms describe the chromosome numbers involved:
Exam Tip: Be exact with "diploid" and "haploid". A diploid cell has chromosomes in pairs (the full number); a haploid gamete has one of each pair (half the number). Fertilisation restores the diploid number by bringing together two haploid gametes.
In asexual reproduction there is only one parent and there are no gametes. The parent cell or organism divides by mitosis, so that the offspring cells contain exactly the same genes as the parent. The offspring are therefore genetically identical to the parent and to one another — they are clones.
Because there is no mixing of genetic material, asexual reproduction produces no genetic variation among the offspring (any tiny differences come only from the environment or from rare mutations). Many organisms reproduce in this way:
Asexual reproduction has clear advantages: it is fast, it needs only one parent (so no time or energy is spent finding a mate), and many offspring can be produced quickly when conditions are favourable. Its major disadvantage is the absence of variation — if the environment changes or a new disease appears, a whole population of genetically identical clones can be wiped out, because none of them carries the variation that might let some survive.
| Sexual reproduction | Asexual reproduction | |
|---|---|---|
| Number of parents | Two | One |
| Gametes | Yes — they fuse at fertilisation | No |
| Cell division involved | Meiosis (to make gametes) and mitosis (to grow) | Mitosis only |
| Offspring | Genetically different (variation) | Genetically identical (clones) |
| Advantage | Variation aids survival if conditions change | Fast; only one parent needed |
| Disadvantage | Slower; needs two parents | No variation — vulnerable to change |
Sexual reproduction depends on gametes, and gametes are made by a special type of cell division called meiosis. Meiosis takes place only in the reproductive organs — in humans, the testes (making sperm) and the ovaries (making egg cells).
Meiosis does two essential jobs:
The key facts about meiosis you must know are:
flowchart TD
A["One diploid parent cell<br/>(chromosomes in pairs)"] --> B["Chromosomes copied"]
B --> C["First division"]
C --> D["Second division"]
D --> E["FOUR haploid gametes<br/>(half the chromosomes each)"]
E --> F["All four are<br/>genetically different"]
You do not need the fine detail of how the chromosomes move, but you do need the headline result: one diploid cell produces four genetically different haploid gametes. The shuffling of genes during meiosis, together with the random way two gametes meet at fertilisation, is the main source of the genetic variation seen in sexually reproducing species — an idea you will return to when you study variation and natural selection later in this topic.
Exam Tip: Two numbers earn marks for meiosis: it produces four gametes, and each has half the chromosome number. A very common misconception is that meiosis makes two cells — but that describes mitosis. Meiosis makes four, genetically different, haploid gametes.
Both mitosis and meiosis are types of cell division, and it is easy to confuse them. The table below places them side by side. (You met mitosis in Topic B1/B2 as the way body cells divide for growth and repair; here it is contrasted with meiosis.)
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where it happens | All body cells (growth, repair, asexual reproduction) | Reproductive organs only (making gametes) |
| Number of divisions | One | Two |
| Number of cells produced | Two | Four |
| Chromosome number | Stays the same (diploid → diploid) | Halved (diploid → haploid) |
| Are the cells identical? | Yes — genetically identical to the parent | No — all four are genetically different |
| Purpose | Growth, repair, asexual reproduction | Producing gametes for sexual reproduction |
A neat way to remember the contrast: mitosis keeps things the same (two identical diploid cells), while meiosis makes things different and halves the chromosomes (four different haploid gametes). The "mei-" at the start of meiosis can remind you that it makes the chromosome number smaller.
Exam Tip: If a question asks which type of division produces gametes, the answer is always meiosis. If it asks which produces genetically identical cells for growth and repair, the answer is mitosis. Never write that mitosis makes gametes or that meiosis is used for growth.
Many organisms are not tied to a single strategy — they can switch between sexual and asexual reproduction depending on conditions, gaining the best of both.
This flexibility makes good biological sense: asexual reproduction lets an organism multiply quickly when conditions are good, while sexual reproduction generates the variation that helps a population cope when conditions become harder.
The link between reproduction and survival is worth making explicit, because it comes up throughout the rest of the topic. Imagine a field of genetically identical clones produced asexually. Because every plant carries the same alleles, they all have the same strengths and the same weaknesses. If a new disease arrives that one plant cannot resist, then none of them can resist it, and the whole population can be lost at once. Now imagine a population produced sexually. Its members are all slightly different, so when the same disease arrives, a few individuals may, by chance, carry alleles that let them survive. Those survivors reproduce, and the population continues.
This is the great long-term advantage of sexual reproduction: the variation it produces is a kind of insurance against a changing world. It is also the reason variation is such an important idea in biology — it is the raw material that lets populations adapt and, over very long timescales, evolve. You will return to exactly this point when you study natural selection later in Topic B5.
A gardener notices that a single spider plant has produced several small "plantlets" on long stems. Each plantlet, when potted, grows into a plant identical to the original. (a) Name the type of reproduction. (b) State the type of cell division involved. (c) Give one advantage and one disadvantage of this method.
Step 1 — read the clues. There is one parent, no other plant or gamete involved, and the offspring are identical to the parent.
Step 2 — identify the strategy. One parent producing identical offspring means asexual reproduction.
Step 3 — the cell division. Asexual reproduction uses mitosis only.
Step 4 — advantage and disadvantage. An advantage is that it is fast and needs only one parent; a disadvantage is that the offspring show no genetic variation, so the whole group is vulnerable if conditions change or a disease strikes.
Answer: (a) asexual reproduction; (b) mitosis; (c) advantage — fast, only one parent needed; disadvantage — no variation, so vulnerable to a change in the environment.
Common error: writing that the plantlets are made by meiosis. Meiosis only makes gametes for sexual reproduction; asexual reproduction uses mitosis.
Question (6 marks): Compare sexual and asexual reproduction, including the role of meiosis and the effect on the offspring.
Mid-band response: "Sexual reproduction uses two parents and asexual uses one parent. Sexual reproduction makes offspring that are all different, but asexual reproduction makes offspring that are identical to the parent. Meiosis makes the gametes for sexual reproduction."
Examiner-style commentary: This earns marks for the number of parents, the variation difference, and identifying meiosis as making gametes. It is held back by vagueness — it does not say that gametes fuse at fertilisation, that asexual offspring are clones, or that meiosis halves the chromosome number. Adding these details would move it up a band.
Stronger response: "In sexual reproduction two parents each produce gametes, which fuse at fertilisation, so the offspring get a mixture of genes from both parents and are all genetically different. In asexual reproduction there is one parent that divides by mitosis, so the offspring are genetically identical clones. Meiosis makes the gametes and halves the chromosome number, so that when two gametes join the offspring has the full number again."
Examiner-style commentary: A clear, well-organised comparison that brings in fertilisation, clones, mitosis and the halving role of meiosis. To reach the top band, use the terms diploid and haploid, note that meiosis produces four genetically different gametes, and add an advantage of each strategy.
Top-band response: "In sexual reproduction, two parents each produce gametes by meiosis. Meiosis halves the chromosome number, turning a diploid cell into haploid gametes, and produces four gametes that are all genetically different. At fertilisation a gamete from each parent fuses, restoring the diploid number, so the offspring inherit a mixture of genes from both parents and show genetic variation — an advantage if the environment changes. In asexual reproduction there is only one parent, which divides by mitosis with no gametes, so the offspring are genetically identical clones. This is fast and needs only one parent, but the lack of variation makes the population vulnerable to disease or a change in conditions. The central difference is therefore that sexual reproduction generates variation, whereas asexual reproduction produces identical offspring."
Examiner-style commentary: Full marks. The answer contrasts the two strategies point by point, uses the correct terms (gametes, meiosis, fertilisation, diploid, haploid, clones), states that meiosis makes four genetically different haploid gametes, and weighs an advantage of each — exactly the balanced, precise comparison examiners reward.
This content is aligned with OCR Gateway Combined Science A (J250), Topic B5 Genes, inheritance and selection. Refer to the official OCR specification for exact wording.