4.6.1Additional lessons
Sexual and Asexual Reproduction — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 116 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5Asexual Reproduction
Asexual reproduction is quick and needs no mate, but it makes no new combinations of alleles.
One parent
- One parent A single parent produces offspring without gametes or fertilisation, so no partner has to be found.
Mitosis
- Mitosis Offspring form by mitotic cell division, so each receives a complete copy of the parent's DNA.
Clones
- Clones Offspring are genetically identical to the parent and to one another, unless a rare mutation occurs.
Examples
- Examples Binary fission in bacteria, budding in yeast, and vegetative propagation by runners or tubers in plants such as strawberry and potato.
Part 2 of 5Sexual Reproduction
Two gametes
- Two gametes Haploid sex cells from usually two parents fuse at fertilisation to form a diploid zygote.
Meiosis
- Meiosis Gametes are made by meiosis, which halves the chromosome number so the species number is restored in the zygote.
Variation
- Variation Each offspring has a new combination of alleles, so offspring differ from each other and from the parents.
Human example
- Human example Each human gamete contains 23 chromosomes and the zygote contains 46, arranged as 23 pairs.
Part 3 of 5Comparing the Two
Comparing the Two
Number of parents
- Number of parents Asexual reproduction needs one parent;
- sexual reproduction normally needs two, though some organisms can self-fertilise.
Genetic similarity
- Genetic similarity Asexual offspring are clones;
- sexual offspring are genetically different from one another and from their parents.
Sources of variation
- Sources of variation In sexual reproduction variation comes from crossing over, random orientation in meiosis, random fertilisation and mutation.
Speed and numbers
- Speed and numbers Asexual reproduction is usually faster and can produce many offspring quickly;
- sexual reproduction is slower and often yields fewer.
Energy cost
- Energy cost Sexual reproduction needs gamete production, mate finding and courtship, so it costs more energy.
| Feature | Asexual | Sexual |
|---|---|---|
| Parents | One | Usually two |
| Genetic variation | None (clones) | High |
| Time taken | Short | Longer |
| Offspring number | Often many | Often fewer |
| Advantage | Rapid colonisation of a suitable habitat | Adaptation if the environment changes |
Part 4 of 5Evaluating Each Strategy
1
In a stable environment, an asexually produced clone of a well-adapted parent is likely to be well-adapted as well.
2
Asexual populations can grow very quickly, so a species can spread through an empty habitat before competitors arrive.
3
If conditions change, a population of clones may all be vulnerable to the same disease or stress, and could be wiped out.
4
Sexual reproduction creates variation, so some individuals may carry alleles that survive the new conditions.
5
Natural selection can then act on this variation, which is why sexual reproduction supports long-term adaptation.
Part 5 of 5Exam Checklist & Tips
4.6.1 – Key Points for the Exam
- Asexual reproduction Production of genetically identical offspring from one parent without fertilisation.
- Sexual reproduction Fusion of two haploid gametes to produce genetically varied offspring.
- Clone An organism genetically identical to its parent.
- Haploid A cell with one set of chromosomes, n.
- Diploid A cell with two sets of chromosomes, 2n.
- Binary fission Splitting of a prokaryotic cell into two daughter cells after DNA replication.
Common Mistakes
- Stating that asexual offspring can never vary; mutation can still change DNA, although it is rare.
- Saying sexual reproduction always involves two parents; self-fertilisation in some plants uses one.
- Writing that sexual reproduction is better in all circumstances;
- asexual reproduction is more efficient in stable habitats.
- Confusing the number of chromosomes in gametes and in the zygote when describing humans.
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