Two ways of making a new organism, and the difference between them comes down to one thing: variation. One strategy produces offspring that are all slightly different; the other produces perfect copies. Neither is better — each wins in different conditions, and that trade-off is what exam questions are really testing.
📚 What you need to know
Sexual reproduction involves two parents and the fusion of the nuclei of two gametes to form a zygote. The offspring are genetically different from each other.
A gamete is a sex cell: sperm and ovum in animals, pollen nucleus and ovum in plants.
Gametes have a haploid nucleus — one copy of each chromosome, half the number in a body cell. In humans that is 23 instead of 46.
The zygote has a diploid nucleus with the full 46 chromosomes, half from each parent.
Asexual reproduction needs only one parent, uses mitosis, and involves no gametes and no fertilisation.
Asexual offspring are genetically identical to the parent and to each other — they are clones. Bacteria do this by binary fission.
The only source of variation in asexual reproduction is mutation.
Sexual reproduction: two parents, one zygote
Sexual reproduction is the fusion of two gamete nuclei. Because gametes are haploid, fusing two of them restores the full diploid number without doubling it every generation. That is the whole point of having a halved nucleus in the first place.
Work through the numbers for a human. A body cell has 46 chromosomes, arranged as 23 pairs. A gamete carries one chromosome from each pair, so 23 in total. Sperm plus ovum gives 23 + 23 = 46, and every one of those 46 has a partner again.
The arithmetic of fertilisation
23 (haploid sperm) + 23 (haploid ovum) = 46 (diploid zygote)
The left-hand offspring are all different because meiosis and fertilisation shuffle alleles. The right-hand ones came from copying, so there is nothing to shuffle.
Advantages and disadvantages of sexual reproduction
Advantages
Disadvantages
Increases genetic variation in the population
Takes time and energy to find a mate
Variation lets the species adapt to new environments, giving a survival advantage
Difficult for isolated individuals to reproduce at all
Disease is less likely to wipe out the whole population, because individuals differ
Usually produces fewer offspring, more slowly
Asexual reproduction: one parent, exact copies
Asexual reproduction involves no gametes and no fertilisation. A single parent divides by mitosis, so there is no fusion and no mixing of genetic information. Every offspring is genetically identical to the parent and to its siblings — a clone.
Many plants reproduce this way, and bacteria do it constantly by binary fission, producing exact genetic copies of themselves.
Advantages
Disadvantages
Population can grow rapidly when conditions are right
Limited genetic variation, since offspring are identical to the parent
A suitable environment can be exploited quickly
Population is vulnerable to a change in conditions and may suit only one habitat
More time and energy efficient, with no mate to find
A disease is likely to affect the entire population, as there is no variation to protect it
Notice the mirror image. Every advantage of one method is a disadvantage of the other, and it always traces back to variation. Say that explicitly in an answer and you will pick up the reasoning mark.
Comparing the two directly
Six features separate them, and questions almost always pick from this list.
Feature
Asexual reproduction
Sexual reproduction
Number of parents
One
Two
Type of cell division
Offspring produced by mitosis
Offspring produced by the fusion of two haploid gametes made by meiosis
Genetic similarity
Offspring identical to each other and to the parent
Offspring genetically unique and different from both parents
Sources of variation
One only: mutation
Three: 50% of the DNA from each parent, crossing over and independent assortment during meiosis, and random fertilisation
Number of offspring
Usually large numbers
Usually relatively few
Time taken
Usually fast
Usually slow
Learn the “sources of variation” row properly. It is the one that separates a top answer from an average one, and it links straight into meiosis on the next page.
Worked examples
WORKED EXAMPLE
Explain why a population of bacteria reproducing asexually is vulnerable to a new disease. [3]
Point 1 — how they reproduce
Binary fission produces offspring by mitosis, so all the bacteria are genetically identical clones.
Point 2 — the consequence
There is almost no genetic variation in the population, so no individuals carry alleles giving resistance.
Point 3 — the outcome
If one individual is susceptible, all of them are, so the disease can affect the entire population.
3 marksMutation is the one exception, and mentioning it shows real understanding.
WORKED EXAMPLE
A human body cell contains 46 chromosomes. State the number of chromosomes in a sperm cell and in a zygote, and explain why they differ. [3]
Step 1 — the gametesperm = 23 chromosomes (haploid)
A gamete carries only one copy of each chromosome instead of two.
Step 2 — the zygotezygote = 23 + 23 = 46 chromosomes (diploid)Step 3 — why
Halving the number in the gametes means that fertilisation restores the diploid number rather than doubling it each generation.
3 marksUse the words haploid and diploid — the numbers alone rarely get full credit.
💡 Exam tip
Anchor every comparison on variation. It explains the advantages, the disadvantages and the disease point in one go.
Say fusion of nuclei, not just “the gametes join”. The nuclei are what combine.
Match the division to the method: asexual uses mitosis, sexual gametes come from meiosis.
“Clone” means genetically identical, not physically identical. Environment can still make clones look different.
If a question gives a chromosome number for a species, halve it for the gamete and keep it for the zygote.
Both strategies are successful. Never write that one is simply “better” — say when each one wins.
⚠ Common mix-up
Haploid and diploid get swapped. Haploid is the gamete, diploid is the body cell and the zygote.
Asexual reproduction is not variation-free. Mutation still occurs, just rarely.
Two parents does not always mean two sexes. Some organisms have both sets of parts, as in many plants.
Binary fission is asexual. It is not a form of sexual reproduction just because it makes lots of offspring.
Do not say meiosis produces the offspring. Meiosis produces the gametes; fertilisation produces the zygote.
Fast is not the same as successful. Rapid asexual growth is a disadvantage when conditions change.
Up next: Sexual Reproduction — how meiosis makes gametes, the three sources of genetic variation, and why sperm and eggs look nothing like each other.
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