3.2Reproduction & Inheritance

Inheritance — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions

10 min read 8 parts Lesson 2 of 4 1 quiz

📚 Edexcel International GCSE Biology 4BI1 Hub / Topical Revision

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Lesson summary

Inheritance Edexcel IGCSE Biology 4BI1 notes: Describe the structure of DNA and RNA and how genes are located on chromosomes.Human body cells have 46 chromosomes (23 pairs); gametes have 23.Base pairing in DNAA with T, C with G.

ABO blood groups
The genome and genes
DNA structure
RNA structure
Stages of protein synthesis
Key terms in genetics

Part 1 of 8Key ideas

Key ideas

Human body cells have 46 chromosomes (23 pairs); gametes have 23.

Base pairing in DNA

A with T, C with G. In RNA, A pairs with U.

A DNA nucleotide = phosphate + deoxyribose sugar + base.

Three bases (a triplet / codon) code for one amino acid.

Heterozygous × heterozygous for a dominant/recessive trait gives a 3 : 1 phenotype ratio.

A test cross with homozygous recessive reveals an unknown genotype.

ABO blood groups

I^A and I^B are codominant; I^O is recessive.

Part 2 of 8Learning objectives

Describe the structure of DNA and RNA and how genes are located on chromosomes.

Outline transcription and translation in protein synthesis.

Use genetic key terms and Punnett squares to predict offspring ratios.

Explain codominance and polygenic inheritance with examples.

Part 3 of 8Essential knowledge

The genome and genes

The genome and genes the genome is all the DNA in an organism. DNA is packaged into chromosomes in the nucleus; a gene is a length of DNA on a chromosome that codes for a particular protein, and the base sequence determines the amino acid sequence.

DNA structure

DNA is a polymer of nucleotides. Two strands wind into a double helix. The sugar-phosphate backbones are on the outside; bases on the inside pair by hydrogen bonds: adenine with thymine and cytosine with guanine (complementary base pairing).

RNA structure

RNA is single-stranded, contains the sugar ribose and the base uracil in place of thymine. Messenger RNA (mRNA) carries a copy of a gene to the ribosome; transfer RNA (tRNA) carries amino acids to the ribosome.

Stages of protein synthesis

Stages of protein synthesis in transcription a gene is copied into mRNA using one DNA strand as a template. In translation the ribosome reads the mRNA codons; tRNA molecules with complementary anticodons deliver the correct amino acids, which are joined into a polypeptide.

Alleles

Alleles genes exist in different versions called alleles. Body cells have two copies of each gene, one from each parent, so an individual can be homozygous or heterozygous.

Key terms in genetics

Key terms in genetics a dominant allele (capital letter) masks a recessive allele (lower case).

Genotype describes the alleles (e.g. Tt), phenotype the characteristic (e.g. tall).
Punnett squares predict the probability of offspring genotypes;
- family pedigrees can show how a condition such as cystic fibrosis is inherited.

Codominance

Codominance both alleles are expressed. In ABO blood groups, I^AI^B gives group AB. In some flowers, red and white alleles are codominant and heterozygotes show both colours (e.g. red and white patches); a blended pink is strictly incomplete dominance.

Polygenic inheritance

Polygenic inheritance many characteristics, such as human height, are controlled by several genes, each with alleles adding a small effect. This produces continuous variation, shown by a normal distribution curve, and the environment also has a strong influence.

Sex determination

Females are XX and males are XY. Every egg carries an X chromosome, but half of sperm carry X and half carry Y, so the sperm decides the sex. A Punnett square of XX × XY gives a 1 : 1 ratio, a 50% chance of either sex.

Mitosis

Mitosis is nuclear division producing two genetically identical diploid cells. Each chromosome is copied, the copies line up across the middle of the cell and are pulled apart, then the cell divides. It is used for growth, repair, replacing cells and asexual reproduction.

Meiosis, diploid and haploid

Meiosis involves two divisions and produces four genetically different haploid gametes. Diploid cells (2n) have two sets of chromosomes (46 in humans); haploid gametes (n) have one set (23). Halving the number means fertilisation restores the diploid number.

Sources of genetic variation

Meiosis gives every gamete a different combination of alleles, and random fertilisation means any sperm can fuse with any egg, so each zygote is unique. Mutations add new alleles to the population.

Genetic and environmental variation

Discontinuous variation (e.g. blood group) is usually caused by genes alone. Continuous variation (e.g. height, body mass) results from several genes plus the environment, such as diet. Purely environmental differences include scars and the language a person speaks.

Mutation

A mutation is a rare, random change in the base sequence of DNA. Most have no effect on the protein. Some change the amino acid sequence and so the protein's shape, e.g. an enzyme's active site, which can be harmful, or occasionally gives a new advantageous allele.

Types of gene mutation

A substitution swaps one base, so at most one triplet (and one amino acid) changes. An insertion or deletion shifts the reading frame, changing every triplet after it, so it usually has a much larger effect on the protein.

Causes of mutation

Mutations occur spontaneously, but the rate is increased by ionising radiation (gamma rays, X-rays, ultraviolet) and chemical mutagens such as tar in tobacco smoke. A higher mutation rate increases the risk of cancer.

Evolution by natural selection

Individuals in a population vary because of different alleles. Those with features better suited to the environment are more likely to survive and reproduce, passing on their alleles. Over many generations the advantageous alleles become more common in the population.

Antibiotic resistance

A random mutation gives a bacterium an allele for resistance. When an antibiotic is used, non-resistant bacteria die but resistant ones survive and reproduce, passing on the allele. The resistant strain becomes common, so a different antibiotic is needed.

Part 4 of 8Process & sequence

- In the nucleus, the DNA double helix unzips at the gene as hydrogen bonds break.
- Free RNA nucleotides pair with the template strand to build mRNA (transcription).
- mRNA leaves the nucleus through a nuclear pore and attaches to a ribosome.
- The ribosome reads mRNA in triplets called codons.
- tRNA molecules carrying specific amino acids bring them to matching codons (anticodons pair).
- Amino acids are joined by peptide bonds in the order set by the codons.
- The polypeptide chain folds into a functional protein.
Gene to proteinwrite these steps in this order; each step is a marking point.
1. DNA unzips at gene
2. mRNA made by transcription
3. mRNA moves to ribosome
4. Codons read in triplets
5. tRNA brings amino acids
6. Polypeptide folds into protein

Part 5 of 8Worked examples

Worked examples

Two heterozygous tall pea plants (Tt) are crossed. Predict the ratio of tall to short offspring.

GametesT or t from each parent.
- Punnett square gives TT, Tt, Tt, tt.
- TT and Tt are tall (3), tt is short (1).
- Ratio 3 tall : 1 short; probability of short = 0.25 (25%).

A mother is blood group A (I^AI^O) and the father is group B (I^BI^O). What blood groups could the children have?

Gametes
- I^A or I^O;
  - I^B or I^O.
- Offspring: I^AI^B (AB), I^AI^O (A), I^BI^O (B), I^OI^O (O).
- Each group has a 1 in 4 chance.

Part 6 of 8Summary

- The genome is all the DNA; a gene codes for one protein.
- DNA is a double helix with complementary base pairs A-T and C-G.
- Protein synthesis = transcription in the nucleus + translation at ribosomes.
- Alleles can be dominant, recessive or codominant.
- Many genes together give polygenic, continuous variation.

Part 7 of 8Keep going

Inheritance Edexcel IGCSE Biology 4BI₁: what to do next
These Inheritance Edexcel IGCSE Biology 4BI₁ notes covered the key ideas, worked examples and exam technique for this topic. Check your Inheritance Edexcel IGCSE Biology 4BI₁ understanding with the practice questions below, then reread any Inheritance Edexcel IGCSE Biology 4BI₁ point you were unsure about before moving on.
Inheritance Edexcel IGCSE Biology 4BI₁: revision checklist
- Revise the main Inheritance Edexcel IGCSE Biology 4BI₁ definitions until you can write them from memory.
- Practise Inheritance Edexcel IGCSE Biology 4BI₁ exam questions under timed conditions.
- Link each Inheritance Edexcel IGCSE Biology 4BI₁ idea to a diagram or a worked example.
- List the common Inheritance Edexcel IGCSE Biology 4BI₁ mistakes and how to avoid them.
- Practice: Inheritance Edexcel IGCSE Biology 4BI1 practice questions
- Previous lesson: 3.1 Reproduction — Notes
- Next lesson: 4.1 The Organism in the Environment — Notes
- Course: Edexcel International Gcse Biology 4BI1 — all units
- Official syllabus: Pearson Edexcel International GCSEs

Part 8 of 8Compare: Compare

DNA compared with RNA
Feature	DNA	mRNA
Number of strands	Two (double helix)	One
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G
Where found	Nucleus (chromosomes)	Made in nucleus, moves to ribosomes
Length	Very long	Short copy of one gene

Single-gene vs polygenic inheritance
Feature	Single gene	Polygenic
Genes involved	One gene	Many genes
Variation	Discontinuous (distinct groups)	Continuous (range of values)
Examples	ABO blood group, cystic fibrosis	Human height, skin colour
Environmental effect	Usually little	Often large

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Genome · Gene · Allele · Dominant · Recessive · Homozygous · Heterozygous · Genotype · Phenotype · Codominance

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. eukaryotic/prokaryotic, enzyme–substrate complex, active site, complementary).

Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

Avoid vague words

“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.

Do not extrapolate a graph line beyond the data or to zero unless asked.

Six-mark extended responses are marked on content AND a logical, linked line of reasoning.

Last updated: 14 October 2026