Cambridge IGCSE0610

Asexual reproduction

Biology 0610 Chapter Notes

What this chapter covers

Asexual reproductionSexual reproductionSexual reproduction in plantsSexual reproduction in humansSex hormones in humansSexually transmitted infections
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In this chapter

  1. 1Understanding Asexual Reproduction
  2. 2Plant Clones: Vegetative Propagation
  3. 3Reproduction in Microbes and Fungi
  4. 4Asexual vs. Sexual: A Comparison

1. Understanding Asexual Reproduction

Asexual reproduction is a method of producing new organisms from a single parent. Unlike sexual reproduction, it does not involve the fusion of gametes (sex cells) from two different parents. The entire process relies on one type of cell division: mitosis. This means the offspring receive all their genetic information from the one parent, resulting in them being genetically identical to the parent and to each other. These genetically identical individuals are known as clones.

Key term

Clone: A group of genetically identical organisms or cells produced from a single parent by asexual reproduction.

Examiner insight

Examiners award marks for using the precise term 'genetically identical' when describing clones. Simply saying they are 'the same' or 'similar' may not be sufficient.

Common pitfall

Confusing asexual reproduction with self-pollination in plants. Self-pollination is a form of sexual reproduction because it still involves the fusion of male and female gametes, even though they come from the same plant.

Worked example 13 marks

Explain how asexual reproduction differs from sexual reproduction in terms of parents and offspring genetics. [3 marks]

  1. 1

    Asexual reproduction involves only one parent, whereas sexual reproduction requires two parents (a male and a female). [1 mark]

  2. 2

    In asexual reproduction, there is no fusion of gametes, and the cell division is mitosis. Sexual reproduction involves the fusion of male and female gametes. [1 mark]

  3. 3

    The offspring of asexual reproduction are genetically identical to the parent (clones), while the offspring of sexual reproduction show genetic variation as they inherit genes from both parents. [1 mark]

Recap

  • Asexual reproduction requires only one parent.
  • It does not involve gametes or fertilisation.
  • The type of cell division involved is mitosis.
  • Offspring are genetically identical to the parent and are called clones.
  • This method is common in simpler organisms and many plants.

Quick check

  1. What type of cell division is used in asexual reproduction?1 mark
  2. How many parents are involved in asexual reproduction?1 mark

2. Plant Clones: Vegetative Propagation

Many plants can reproduce asexually using a method called vegetative propagation. This is where a new plant grows from a part of the parent plant, such as the stem, root, or leaf, rather than from a seed. There are both natural and artificial methods. A common natural example is the strawberry plant, which sends out 'runners' (specialist stems) that grow along the ground. Where a runner touches the soil, a new, genetically identical plantlet develops. Other natural examples include potato tubers (underground stems) and daffodil bulbs. Humans use this principle in artificial propagation, for example, by taking cuttings from a plant with desirable features (like beautiful flowers or tasty fruit) and planting them to grow new, identical plants.

Key term

Vegetative Propagation: A form of asexual reproduction in plants, in which new individuals arise from any vegetative part of the parent plant rather than from seeds or spores.

Fun fact

The world's largest organism is a grove of quaking aspen trees in Utah, called 'Pando'. It is a single clone connected by a massive root system, covering over 100 acres and estimated to be thousands of years old.

Worked example 14 marks

A strawberry plant reproduces asexually using runners. A student measures the lengths of the first runner (X) and second runner (Y) from the parent plant. The mean length for runner X was 494 mm. The lengths for runner Y were: 200, 280, 260, 260, 340, 250, 240, 270, 250, 290 mm.(a) Calculate the mean length for runner Y. [2](b) Suggest one reason for the difference in mean length between runners X and Y. [2]

  1. 1

    (a) To calculate the mean, sum the values and divide by the number of values. Sum = 200 + 280 + 260 + 260 + 340 + 250 + 240 + 270 + 250 + 290 = 2640 mm. [1 mark]

  2. 2

    Mean = 2640 / 10 = 264 mm. [1 mark]

  3. 3

    (b) Runner X grows directly from the parent plant, receiving a large supply of nutrients and energy. Runner Y grows from the first new plant, which is smaller and has fewer resources. [1 mark]

  4. 4

    Therefore, runner Y receives fewer nutrients/less energy for growth compared to runner X, resulting in a shorter mean length. [1 mark]

Recap

  • Vegetative propagation is asexual reproduction in plants.
  • Natural methods include runners (strawberries), tubers (potatoes), and bulbs (daffodils).
  • Artificial propagation involves taking cuttings to produce clones.
  • This method is used in agriculture and horticulture to produce large numbers of identical plants quickly.

Quick check

  1. Name one plant that reproduces using runners.1 mark
  2. Why would a farmer want to grow crops using vegetative propagation?1 mark

3. Reproduction in Microbes and Fungi

Asexual reproduction is the primary method for many microorganisms. Bacteria, which are simple prokaryotic cells, reproduce by binary fission. In this process, the cell grows, duplicates its circular DNA, and then splits into two identical daughter cells. Yeast, a single-celled fungus, reproduces asexually by budding. A small bud grows out from the parent cell, receives a copy of the nucleus, and eventually detaches as a new, smaller, but genetically identical cell. Other fungi, like the mould *Mucor* found on bread, can reproduce asexually by producing spores. These are tiny, lightweight cells that are released into the air and can grow into new fungi if they land in a suitable, moist environment.

Key term

Binary Fission: The method of asexual reproduction in prokaryotes, like bacteria, where a parent cell divides into two genetically identical daughter cells.

Examiner insight

Be precise with your terminology. Binary fission is for bacteria, budding is for yeast. Using the correct term for the specific organism demonstrates clear understanding.

Worked example 13 marks

Describe the process of asexual reproduction in yeast. [3 marks]

  1. 1

    Yeast reproduces asexually by a process called budding. [1 mark]

  2. 2

    A small outgrowth or 'bud' begins to form on the surface of the parent yeast cell. [1 mark]

  3. 3

    The nucleus of the parent cell undergoes mitosis, and one of the new nuclei moves into the bud. The bud then grows and either separates to become a new individual cell or remains attached to form a colony. [1 mark]

Recap

  • Bacteria reproduce asexually by binary fission.
  • Yeast, a single-celled fungus, reproduces asexually by budding.
  • Many multicellular fungi reproduce asexually by producing spores.
  • In all these cases, reproduction is rapid and produces genetically identical offspring.

Quick check

  1. What is the name for asexual reproduction in bacteria?1 mark
  2. How do fungi like bread mould spread asexually?1 mark

4. Asexual vs. Sexual: A Comparison

Organisms can gain different advantages from sexual and asexual reproduction. The best strategy often depends on the environment. Asexual reproduction is advantageous when conditions are stable and favourable. Sexual reproduction introduces variation, which is beneficial in changing environments. Many organisms, especially plants and fungi, can do both.

Advantages of Asexual Reproduction:

  • Speed: Populations can increase very rapidly.
  • Efficiency: Only one parent is needed; no time or energy is wasted finding a mate.
  • Favourable Genes: If a parent is well-adapted to its environment, all its offspring will inherit those successful genes.

Disadvantages of Asexual Reproduction:

  • No Genetic Variation: The offspring are clones. This means a single disease, pest, or change in environmental conditions (like a drought) could wipe out the entire population.
  • Vulnerability: The classic example is the Irish Potato Famine of the 1840s. The potatoes grown were all genetically identical clones. When a fungal disease called potato blight struck, all the plants were susceptible, leading to catastrophic crop failure.

Key term

Genetic Variation: The differences in the genetic makeup (DNA) between individuals within a species.

Examiner insight

High-scoring answers explicitly link the lack of genetic variation in asexual reproduction to a specific consequence, such as increased susceptibility to a named disease or a change in climate.

Common pitfall

Stating that asexual reproduction produces 'no variation' without qualifying it. It is crucial to specify 'no *genetic* variation'. Organisms can still show slight differences due to environmental factors, but these are not heritable.

Worked example 16 marks

A farmer can grow potatoes from seeds (sexual reproduction) or by planting tubers (asexual reproduction). Evaluate the advantages and disadvantages of using tubers to grow the crop. [6 marks]

  1. 1

    Advantage 1: Speed. Using tubers is much faster. A new plant grows directly from the tuber, whereas growing from seed is a longer process. This leads to a quicker harvest. [1 mark]

  2. 2

    Advantage 2: Certainty of characteristics. Since planting tubers is a form of asexual reproduction, all the new plants will be genetically identical to the parent. This means desirable features like size, taste, and yield are guaranteed. [1 mark]

  3. 3

    Disadvantage 1: Lack of genetic variation. All the potato plants in the field will be clones. [1 mark]

  4. 4

    Disadvantage 2: Disease susceptibility. Because of the lack of genetic variation, if a disease like potato blight appears, it is likely to affect all plants in the crop equally, potentially destroying the entire harvest. [1 mark]

  5. 5

    Evaluation: While using tubers is good for producing a fast, uniform crop with known qualities [1 mark], it carries the significant risk of total crop failure due to disease, unlike growing from seeds which would produce a more genetically diverse and resilient crop [1 mark].

Recap

  • Asexual reproduction is fast and requires only one parent.
  • It produces genetically identical offspring with no variation.
  • It is advantageous in stable, favourable environments.
  • The lack of genetic variation makes the population vulnerable to disease and environmental change.

Quick check

  1. State one advantage of asexual reproduction over sexual reproduction.1 mark
  2. State one disadvantage of asexual reproduction.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'clone'.1 mark
  2. Name two natural methods of vegetative propagation in plants.2 marks
  3. Describe the process of binary fission in bacteria.3 marks
  4. Explain why offspring from asexual reproduction are genetically identical to the parent.3 marks
  5. A gardener wants to produce many plants with desirable red flowers quickly. Explain why they should use asexual reproduction (cuttings) rather than sexual reproduction (seeds).4 marks
  6. The Irish Potato Famine of the 1840s was made worse because potatoes were grown asexually from tubers. Explain this link.4 marks
  7. Compare asexual and sexual reproduction by completing a table with the headings: Number of parents, Type of cell division, Genetic variation in offspring.6 marks
  8. Yeast is a single-celled fungus that reproduces by budding. Describe this process and explain why it is a form of asexual reproduction.4 marks
  9. A species of grass lives in a very stable environment where there are few pests and diseases. Suggest and explain which method of reproduction, sexual or asexual, would be more advantageous for this species in this environment.5 marks
  10. A fruit grower discovered one strawberry plant in their field that was resistant to a new fungal disease. Explain how the grower could use this one plant to produce a large crop of disease-resistant strawberries, and outline one major risk associated with this strategy.6 marks

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