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Every species has to make a new generation, or it dies out. But there is more than one way to do it. Some organisms simply copy themselves, producing offspring that are exact genetic copies of the single parent. Others mix the genetic material of two parents, producing offspring that are similar to — but never identical to — either of them. This opening lesson of Topic B5 (Genes, inheritance and selection) of OCR Gateway Science A sorts reproduction into these two strategies, sexual and asexual, and introduces the special kind of cell division, meiosis, that makes sexual reproduction possible. Getting the difference between these two strategies, and between meiosis and mitosis, secure now sets up the whole of the rest of 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 making gametes, compare mitosis and meiosis, and give examples of organisms that use each strategy.
There are two fundamentally different ways that living things reproduce:
The single most important idea to hold onto is this: sexual reproduction produces variation; asexual reproduction produces genetically identical clones. Almost every other difference between the two strategies follows from that one point.
Exam Tip: A favourite OCR question gives you a short description of how an organism reproduces and asks you to decide whether it is sexual or asexual. The deciding 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", it is asexual.
In sexual reproduction, two parents each contribute a gamete — a sex cell that carries only half the normal 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 in the ovule (female).
The two gametes join together in a process called fertilisation. When they fuse, their chromosomes combine, so the new cell — the zygote — has the full number of chromosomes again, half from each parent. The zygote then divides 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 the offspring receive a mixture of genes from two different parents, every offspring is genetically different from its parents and from its brothers and sisters. This is why a litter of kittens, or the children in a human family, are all clearly related yet all individuals. The chromosome numbers are worth pinning down with two key terms:
Exam Tip: Be precise with "diploid" and "haploid". A diploid cell has chromosomes in pairs (full number); a haploid gamete has one of each pair (half the number). Fertilisation restores the diploid number by combining two haploid gametes.
In asexual reproduction, there is only one parent and there are no gametes. The parent cell or organism divides by mitosis, producing offspring whose cells contain exactly the same genes as the parent. The offspring are therefore genetically identical to the parent and to each other — they are clones.
Because there is no mixing of genetic material, asexual reproduction produces no genetic variation among the offspring (any tiny differences are caused only by the environment, or by rare mutations). Many organisms reproduce asexually:
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 good. Its great disadvantage is the lack of variation — if the environment changes or a new disease appears, a whole population of genetically identical clones may be wiped out, because none of them has 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 helps 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 happens 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 → four genetically different haploid gametes. The shuffling of genes during meiosis, combined with the random way two gametes meet at fertilisation, is the main source of the genetic variation you see 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 error is to say meiosis makes two cells (that is mitosis). Meiosis = four, genetically different, haploid gametes.
Both mitosis and meiosis are types of cell division, and students often muddle them. The table below sets them side by side. (You met mitosis in Topic 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 difference: mitosis keeps things the same (two identical diploid cells), while meiosis makes things different and halves the chromosomes (four different haploid gametes). The "mei-" in 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 limited to one strategy — they can switch between sexual and asexual reproduction depending on conditions, getting 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 get harder.
A gardener notices that a single spider plant has produced several small "plantlets" on long stems. Each plantlet, when potted, grows into a plant that is 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 = 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.
| Misconception | The correct idea |
|---|---|
| "Meiosis produces two cells" | Meiosis produces four cells; mitosis produces two |
| "Gametes are diploid" | Gametes are haploid — they have half the chromosome number |
| "Asexual offspring can be quite varied" | Asexual offspring are genetically identical clones; the only variation is environmental or from rare mutation |
| "Sexual reproduction always needs two separate organisms" | It needs two gametes that fuse; some organisms (e.g. some plants) can self-pollinate, but male and female gametes still fuse |
| "Mitosis and meiosis are the same thing" | Mitosis keeps the chromosome number the same (identical cells); meiosis halves it (different gametes) |
| "Fertilisation is the same as meiosis" | Meiosis makes the gametes; fertilisation is the later fusion of two gametes |
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 gametes fuse at fertilisation, that the offspring are clones in asexual reproduction, or that meiosis halves the chromosome number. To climb a band, add these details.
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 which 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 and 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. So the central difference is 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 Science A GCSE Biology (J247), Topic B5 Genes, inheritance and selection. Refer to the official OCR specification document for the exact wording.