Edexcel IGCSE Biology · Spec 3.1
Sexual and Asexual Reproduction
The differences between sexual and asexual reproduction, and the advantages and disadvantages of each.
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Sexual and Asexual Reproduction
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Explained
Two ways of making offspring, and what each one costs
Sexual reproduction involves two parent cells. Gametes, which are sex cells with half the normal number of chromosomes, fuse at fertilisation to produce a zygote with the full number again. The offspring are genetically different from each other and from both parents.
Asexual reproduction involves one parent. There are no gametes and no fertilisation. The parent divides by mitosis and the offspring are genetically identical to it and to each other, which makes them clones.
Where the variation comes from
Sexual reproduction produces variation in two separate ways, and a question asking for the reason usually wants both.
Meiosis shuffles the chromosomes as the gametes are made, so no two gametes from the same parent carry the same combination of alleles.
Fertilisation is then random. Any one of a huge number of gametes from one parent can fuse with any one from the other, so the combination in the zygote has almost certainly never existed before.
Asexual reproduction has neither step. Mitosis copies the genetic material exactly, so unless a mutation happens the offspring are identical. That is the whole difference between the two, and every advantage and disadvantage below follows from it.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a question asking for two differences between sexual and asexual reproduction. It records the answers that worked: the production of genetic variation, the roles of mitosis and meiosis, and the need for the fusion of haploid gametes in sexual reproduction.
Then it names the common error, and it is a single word. Candidates referred to the need for two parents, and the report states that they should refer to two parent cells. It records a response gaining one mark for saying that asexual reproduction produces genetically identical offspring, and adds that two parents was insufficient because two parent cells was required.
That distinction is not pedantry. Plenty of organisms reproduce sexually without anything you would call two parents in the everyday sense. A flowering plant can pollinate itself, and a hermaphrodite is a single organism producing both kinds of gamete. What is always true is that two cells fuse, so cells is the level the biology is actually happening at.
The report closes that section with a general instruction worth carrying into every answer: always be clear about what you are referring to, and try not to say it or they.
In a question comparing two processes, the word it has two possible meanings in every sentence, and the examiner cannot award a mark for a statement that might be about either one. Name the process each time, however repetitive it feels.
The advantages, and when each matters
Sexual reproduction produces variation, so if the environment changes some individuals are likely to survive it. It is the raw material natural selection works on, and it is why a population reproducing sexually can adapt over generations while a population of clones cannot.
It is also slow. It requires a mate, which has to be found, and producing gametes and raising offspring takes time and energy.
Asexual reproduction is fast, needs no mate, and produces many offspring quickly. If the parent is well suited to its surroundings, so is every offspring. That makes it ideal for colonising a stable habitat rapidly.
Its weakness is the same fact seen from the other side. No variation means that a change in conditions, or a new disease, affects every individual identically. A population of clones has no individuals that happen to be resistant, because there is no happening to be anything.
Examples worth naming
Asexual reproduction in plants includes runners in strawberry plants, bulbs in daffodils and tubers in potatoes. Bacteria divide by binary fission. Fungi produce spores. Some animals, including certain insects and reptiles, reproduce asexually too.
Many organisms do both, and that is the arrangement worth understanding. A strawberry plant sends out runners in good conditions, spreading quickly, and flowers to reproduce sexually as well. It gets speed when conditions are favourable and variation as insurance against them changing.
Spec 3.1
What you need to know
- Describe the difference between sexual and asexual reproduction
- Recall that sexual reproduction needs two parents and sex cells called gametes
- Recall that asexual reproduction needs only one parent and produces identical clones
Active recall
Quick check
Answer each question before opening the answer.
Give two differences between sexual and asexual reproduction.
Sexual — two parents, gametes fuse, offspring vary genetically. Asexual — one parent, no gametes, offspring genetically identical (clones).
State one advantage of sexual and one of asexual reproduction.
Sexual — variation helps species adapt/survive. Asexual — fast and no mate needed.
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