Pearson Edexcel IGCSE Biology: Reproduction and Inheritance — Revision Notes (4BI1)

Related: Topic 3 study guide.

Condensed, exam-focused notes for Topic 3 of Pearson Edexcel International GCSE Biology (4BI1), Issue 3.

(a) Reproduction

Sexual vs asexual: sexual reproduction fuses a male and female gamete (fertilisation) to form a zygote; asexual reproduction does not involve gamete fusion. In flowering plants, know how insect-pollinated and wind-pollinated flowers are each structurally adapted for their pollination method, and the sequence pollen tube growth → fertilisation → seed and fruit formation → germination (a required practical). Plants also reproduce asexually by natural methods (runners) and artificial methods (cuttings).

Human reproduction: structures of the male and female reproductive systems suited to their functions; the menstrual cycle explained via oestrogen and progesterone (FSH and LH are Paper 2 content); the placenta nourishes the developing embryo; amniotic fluid protects it; oestrogen and testosterone drive secondary sexual characteristics.

(b) Inheritance – core vocabulary

• Genome: an organism’s entire DNA.
• Gene: a section of DNA coding for a specific protein, located on a chromosome.
• Allele, dominant, recessive, homozygous, heterozygous, phenotype, genotype – learn these as a fixed set; monohybrid inheritance questions assume fluent use of all seven terms.

Paper 2 adds DNA structure (double helix, two strands, paired bases – adenine/thymine, cytosine/guanine) and RNA (single-stranded, uses uracil instead of thymine), plus protein synthesis (transcription and translation; mRNA, ribosomes, tRNA, codons, anticodons).

Monohybrid inheritance

Represent every cross with a full genetic diagram (parental genotypes → gametes → offspring genotypes and phenotypes with ratio) – marks are awarded for the working, not just the final probability. Most phenotypic features actually arise from polygenic inheritance (many genes together), not a single gene – the specification explicitly warns against treating single-gene inheritance as the default case. Sex determination: one chromosome pair (XX female, XY male), shown via its own genetic diagram.

Mitosis vs meiosis

	Mitosis	Meiosis
Cells produced	2, genetically identical	4, genetically different
Chromosome number	Diploid → diploid	Diploid → haploid (halved)
Occurs in	Growth, repair, cloning, asexual reproduction	Gamete formation

Human diploid number = 46; haploid = 23. Random fertilisation of genetically different gametes is what produces genetic variation in offspring – this is the direct link between meiosis and variation.

Variation, mutation and natural selection

Variation can be genetic, environmental, or both. Mutation: a rare, random, inheritable change in genetic material. Paper 2 extends this to how a mutation can alter a protein’s amino acid sequence and therefore phenotype (usually no effect, occasionally small, rarely significant), and how ionising radiation and certain chemicals increase mutation rates. Natural selection (Darwin): applied concretely to antibiotic resistance increasing within bacterial populations, making infections harder to treat.

Worked example: a monohybrid cross

Two heterozygous pea plants (Tt, tall dominant over short) are crossed.

Parents:    Tt x Tt
Gametes:    T, t  x  T, t
Offspring:  TT, Tt, Tt, tt
Genotype ratio: 1 TT : 2 Tt : 1 tt
Phenotype ratio: 3 tall : 1 short

Always show the full diagram – gametes listed separately before combining them – since the working, not just the 3:1 ratio, is what earns most of the marks.

The required practical: seed germination conditions

The germination practical asks candidates to identify which conditions (water, oxygen, a suitable temperature) a seed actually needs to germinate, typically by setting up several test tubes or dishes that each vary one condition while keeping the others constant – a control comparison. Be ready to explain why a seed placed in a sealed, waterlogged tube fails to germinate (insufficient oxygen for respiration) as distinct from one kept dry at room temperature (insufficient water), since exam questions often present a results table and ask which specific condition was missing in a given tube, not just “why didn’t this one germinate” in general terms.

Common mistakes

• Treating single-gene inheritance as the norm when most traits are polygenic.
• Confusing mitosis and meiosis’s outcomes (identical vs genetically different cells; diploid vs haploid).
• Describing a mutation’s effect as always harmful, when the specification states most have no effect.
• Forgetting that reduced enzyme/protein function from a mutated gene traces back to an altered amino acid sequence, not a vaguely “changed” gene.

How to approach it

Keep Reproduction (largely descriptive: structures, processes, one required practical) mentally separate from Inheritance (largely applied: genetic diagrams, probability, pedigrees), even though they share one topic number. Learn mitosis and meiosis side by side, since exam questions frequently ask which process is occurring and why, based on the number and genetic identity of resulting cells.

Self-test

1. Distinguish sexual from asexual reproduction in terms of gamete fusion.
2. Define genome and gene.
3. Why does the specification warn against treating single-gene inheritance as the default case?
4. Give two differences between mitosis and meiosis.
5. What effect does a mutation usually have on phenotype?

Answers: 1. Sexual reproduction involves fusion of a male and female gamete; asexual reproduction does not involve gamete fusion. 2. Genome: an organism’s entire DNA. Gene: a section of DNA coding for a specific protein. 3. Because most phenotypic features actually arise from polygenic inheritance (many genes acting together), not a single gene. 4. Any two of: mitosis produces 2 genetically identical (diploid) cells, meiosis produces 4 genetically different haploid cells; mitosis occurs in growth/repair/cloning/asexual reproduction, meiosis occurs in gamete formation. 5. Usually no effect on phenotype, occasionally a small effect, rarely a significant one.

Official syllabus

Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3 (September 2024) – the same source cited by the study guide.