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Unit 2 · Reproduction & Inheritance

Unit 2 — Reproduction and Inheritance — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions

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📚 Edexcel IGCSE Biology Modular - UNIT 2 Hub / Topical Revision
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Lesson summary

Reproduction and Inheritance: Overview — EDEXCEL MODULAR - OL notes: Summarise sexual and asexual reproduction, DNA and genes, monohybrid inheritance, mitosis, meiosis, variation and natural selection.AsexualSexualParentsOneTwoGametesNoneMale and female gametes fuse (fertilisatio…

  • Worked example 1
  • Worked example 2
  • Worked example 3
  • Worked example 4
  • Meiosis halves the chromosome number to make genetically different haploid gametes
  • Fertilisation restores the diploid number and random fertilisation adds variation

Section 1 of 6Sexual and asexual reproduction

AsexualSexual
ParentsOneTwo
GametesNoneMale and female gametes fuse (fertilisation)
Cell divisionMitosisMeiosis to make gametes, mitosis after fertilisation
OffspringGenetically identical clonesGenetically different - variation
AdvantageQuick, no mate neededVariation helps survival when the environment changes
  • A human zygote contains the full 46 chromosomes (23 pairs), one set from each parent.
  • In flowering plants, the anthers of wind-pollinated flowers hang outside the flower so pollen is easily blown away, and the stigmas are feathery and hang out to catch pollen.

Section 2 of 6DNA, genes and alleles

  • DNA is the genetic material.
  • Genes are short lengths of DNA that code for a protein; they are found on chromosomes in the nucleus.
  • Alleles are alternative forms (versions) of the same gene.
  • The observable characteristics of an organism are its phenotype;
    • the combination of alleles is its genotype.
  • Differences between RNA and DNA: RNA is single-stranded, has the sugar ribose and the base uracil instead of thymine.
  • The sequence of bases decides the order of amino acids in a protein.

Section 3 of 6Inheritance

  • The height of pea plants is controlled by a single gene with two alleles, tall (T, dominant) and short (t, recessive).
  • A Punnett square shows the possible combinations of alleles in offspring.
  • A cross of two heterozygous parents (Tt x Tt) gives TT, Tt, Tt and tt: a 3:1 ratio of tall to short.
  • Blood group inheritance is an example of codominance, in which both alleles are expressed.
  • Sex is determined by the sex chromosomes: XX for females and XY for males; the father's sperm determines the sex of the child,
    • so the probability of a boy or girl is 50%.

Section 4 of 6Cell division, variation and selection

  1. Mitosis produces two genetically identical diploid cells; it is used for growth, replacement of cells such as skin and red blood cells,
    • and asexual reproduction.
  2. Meiosis halves the chromosome number to make genetically different haploid gametes.
  3. Fertilisation restores the diploid number and random fertilisation adds variation.
  • All populations contain genetic variation, which may be caused by mutations.
  • Some variants are advantageous; individuals with them survive and reproduce, passing on the alleles,
    • so advantageous alleles become more common over generations: natural selection.
  • In the reproductive system, the foetus is protected by amniotic fluid and the pituitary gland releases FSH to develop an ovarian follicle at the start of the menstrual cycle.
  • Seeds can be dispersed by animals; some fruits have sticky hooks that catch in fur.
  • Gardeners take cuttings from good parent plants to get clones.

Section 5 of 6Worked genetics example and exam approach

  • Worked examplein humans the allele for free earlobes (E) is dominant to attached earlobes (e).
  • Two heterozygous parents (Ee x Ee) have a child.
  • Gametes: E or e from each.
  • Punnett square: EE, Ee, Ee, ee.
  • Probability of attached earlobes = 1/4 (25%).
  • Probability of a child with free earlobes = 3/4. Always label the parental genotypes, gametes, offspring genotypes and finally the phenotype ratio.
  • Remember also that the zygote divides by mitosis to form an embryo,
    • which is why every body cell has the same genes, and that meiosis in the ovaries and testes shuffles alleles so that gametes differ from each other.
  • In a six-mark question on evolution, give the stages in order: variation by mutation, competition, survival of the better adapted, reproduction, and the increase in frequency of the advantageous allele over generations.

Section 6 of 6Worked examples

  1. State two differences between sexual and asexual reproduction. [2]2 marks

  2. Two heterozygous tall pea plants are crossed. Give the ratio and the probability of a short plant. [3]3 marks

  3. Describe how sex is inherited in humans. [3]3 marks

  4. Explain why meiosis halves the chromosome number. [2]2 marks

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🌳 Student Search Tree & Frequently Asked Exam Questions: unit 2

Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).

Q1: How to solve unit 2 examination questions step-by-step?

1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.

Q2: What are the common mistakes and experimental sources of error for unit 2?

Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.

🔍 Related Student Searches & Problem Trees for unit 2:
Where can I find unit 2 past paper questions by topic with the mark scheme?

On DABOURHUB every unit 2 question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.

What is the most common unit 2 exam question and why do students lose marks on it?

The repeated pattern on Edexcel papers is a short stem that asks you to apply unit 2 and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each unit 2 item here is written as a full-mark answer.

Is there an alternative to Save My Exams or Physics and Maths Tutor for unit 2?

Yes: the unit 2 notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.

How do I revise unit 2 in two weeks before the exam?

Three passes: read the unit 2 notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.