Edexcel International GCSE Biology 4BI1: Inheritance – Revision Notes

For full explanations and worked examples, use the Inheritance study guide.

These notes cover sub-topic 3(b), Inheritance (specification points 3.14–3.39), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Points with a B reference (3.16B, 3.17B, 3.18B, 3.21B, 3.35B, 3.36B, 3.37B) are bold in the specification and examined on Paper 2 only; the rest can appear on either paper. They apply to the June and November series examined under Issue 3.

Practise with the Inheritance practice questions. For the whole of Topic 3, see the topic revision notes and topic practice questions. Course hub: Edexcel IGCSE Biology. Tick off points on the printable checklist, or find gaps with a free diagnostic.

3.14–3.15 Definitions you must write word-perfect

Term	Definition
Genome	The entire DNA of an organism
Gene	A section of a molecule of DNA that codes for a specific protein
Chromosome	A structure in the nucleus on which genes are located
Allele	An alternative form of a gene
Genotype	The alleles an organism has (e.g. Hh)
Phenotype	The characteristic shown (e.g. tall)
Dominant	Expressed even when only one copy is present
Recessive	Expressed only when no dominant allele is present (two copies)
Homozygous	Two identical alleles (HH or hh)
Heterozygous	Two different alleles (Hh)
Codominance (Paper 2 only)	Both alleles in a heterozygote are expressed in the phenotype
Mutation	A rare, random change in genetic material that can be inherited

3.16B–3.17B DNA and RNA – Paper 2 only

Feature	DNA	RNA
Strands	Two, coiled into a double helix	One
Bases	A, T, C, G	A, U, C, G
Pairing	A–T, C–G	A on DNA pairs with U on RNA

3.18B Protein synthesis – Paper 2 only

Method in steps

1. Transcription (nucleus): the gene’s DNA strands separate; a complementary mRNA strand is built on one strand (U instead of T).
2. mRNA leaves the nucleus and attaches to a ribosome.
3. Translation (cytoplasm): the ribosome reads the mRNA one codon (three bases) at a time.
4. Each tRNA carries a specific amino acid and has an anticodon complementary to one codon.
5. Anticodon pairs with codon; amino acids are joined in the order the codons set.
6. The finished chain folds into a protein.

Quick numbers: bases ÷ 3 = number of codons = number of amino acids coded. An mRNA of 18 bases has 6 codons and codes for 6 amino acids. The codon GCA on mRNA pairs with the anticodon CGU on tRNA.

3.19–3.22 Alleles, codominance and polygenic features

• Two alleles per gene in a body cell; one allele per gamete.
• Use one letter: capital for dominant, lower-case for recessive (H and h, never H and t).
• Codominance (Paper 2 only): give each allele its own label on the same gene letter (e.g. CR and CW). The heterozygote shows both phenotypes at once. Two heterozygotes give a 1 : 2 : 1 phenotype ratio.
• Polygenic (3.22): most features (height, skin colour) are controlled by several genes, so they show a range of values rather than a few distinct types.

3.23 and 3.25 Genetic diagrams and probability

Method in steps

1. Choose and state the symbols (e.g. H = tall, h = short).
2. Parents’ phenotypes, then parents’ genotypes.
3. Gametes – circle them, one allele each.
4. Punnett square.
5. Offspring genotypes → phenotypes → ratio or probability, matching what the question asks for.

Standard results to recognise

Cross	Genotypes	Phenotypes
HH × hh	all Hh	all dominant
Hh × Hh	1 HH : 2 Hh : 1 hh	3 dominant : 1 recessive
Hh × hh	1 Hh : 1 hh	1 dominant : 1 recessive
HH × Hh	1 HH : 1 Hh	all dominant
Codominant heterozygote × same (Paper 2)	1 : 2 : 1	1 : 2 : 1

• Probability of a recessive offspring from Hh × Hh = 1/4 (0.25, 25%).
• From Hh × hh = 1/2.
• Each fertilisation is independent. Earlier children never change the chance for the next one.
• Expected number = probability × total offspring. From 160 offspring of Hh × Hh, expect 160 × 1/4 = 40 recessive.

Finding an unknown genotype: cross an organism with the dominant phenotype with a homozygous recessive. Any recessive offspring shows the parent was heterozygous. All dominant offspring (in a large sample) suggests it is homozygous dominant.

Worked reminder. A purple-flowered plant (P, dominant to white p) is crossed with a white plant. The offspring are 46 purple and 44 white. The white offspring are pp, so each received a p from the purple parent. The purple parent must be Pp, and the close to 1 : 1 ratio fits Pp × pp.

3.24 Pedigree clues

• Square = male, circle = female, shaded = affected.
• Two unaffected parents, affected child → allele is recessive; both parents are heterozygous.
• Two affected parents, unaffected child → allele is dominant; both parents are heterozygous.
• Anyone showing a recessive condition must be homozygous recessive.
• An unaffected person with an affected parent (recessive condition) must be heterozygous.
• Work out the certain genotypes first, then use them to deduce the others.

3.26–3.27 Sex determination

• Female XX, male XY: one pair of chromosomes.
• Eggs: all X. Sperm: half X, half Y.
• The sperm decides the sex at fertilisation.
• Probability of a boy (or a girl) = 1/2 at every fertilisation.

3.28–3.32 Cell division in one glance

Question	Mitosis	Meiosis
Starting cell	Diploid	Diploid
Cells made	2	4
Chromosome number	Same as parent cell	Half (haploid)
Genetically	Identical sets	Different from each other
Used for	Growth, repair, cloning, asexual reproduction	Making gametes

• Humans: diploid 46, haploid 23. Sperm 23 + egg 23 → zygote 46.
• Random fertilisation: any gamete can fuse with any gamete of the other sex, giving genetic variation in offspring.

3.33–3.37B Variation and mutation

Cause	Example
Genetic only	Blood group
Environmental only	A scar; a language spoken
Both	Height; body mass

Paper 2 only:

• 3.35B Changed DNA base sequence → changed codon → possibly a different amino acid → protein with a different shape → may change the phenotype.
• 3.36B Most mutations: no effect. Some: small effect. Rarely: significant effect.
• 3.37B Mutation rate increased by ionising radiation (gamma rays, X-rays, ultraviolet rays) and chemical mutagens (e.g. chemicals in tobacco).

3.38–3.39 Natural selection

Method in steps – use these words in this order:

variation (from mutation) → competition / selection pressure → survival of the better-adapted → reproduction → alleles passed on → allele frequency increases over many generations.

Antibiotic resistance: a random mutation gives resistance → the antibiotic kills non-resistant bacteria → resistant bacteria survive and multiply → the resistant population grows → infections become difficult to control because the antibiotic no longer works against them.

Must-know distinctions

• Gene vs allele: a gene codes for a protein; an allele is one version of that gene.
• Genotype vs phenotype: letters vs the visible or measurable feature.
• Codon vs anticodon (Paper 2): codon is on mRNA; anticodon is on tRNA.
• Diploid vs haploid: two sets of chromosomes vs one set.
• Ratio vs probability: 3 : 1 is a ratio; 3/4 is a probability.

Quick self-test

1. Define the term genome.
2. (Paper 2) Give the complementary DNA strand to TGCA.
3. (Paper 2) A DNA template strand reads ACC GTA. Write the mRNA.
4. (Paper 2) How many codons are in an mRNA of 30 bases?
5. A condition is caused by a recessive allele f. Two carriers (Ff) have a child. What is the probability the child has the condition?
6. A heterozygous tall plant (Hh) is crossed with a short plant (hh). What fraction of offspring are short?
7. A species has a diploid number of 14. How many chromosomes are in its gametes?
8. A couple have two sons. What is the probability their next child is a daughter?
9. State two uses of mitosis.
10. (Paper 2) Name two types of ionising radiation that increase the mutation rate.
11. Give one example of variation that is caused by both genes and environment.

Answers

1. The entire DNA of an organism.
2. ACGT.
3. UGG CAU.
4. 30 ÷ 3 = 10 codons.
5. 1/4 (0.25 or 25%).
6. 1/2 (Hh × hh gives 1 Hh : 1 hh).
7. 14 ÷ 2 = 7.
8. 1/2 – each fertilisation is independent.
9. Any two of: growth, repair, cloning, asexual reproduction.
10. Any two of: gamma rays, X-rays, ultraviolet rays.
11. Height (or body mass).

Where marks are usually lost

• Gametes written as a pair (Hh) instead of single alleles, or not shown at all – the gamete step usually carries its own mark.
• No key to symbols, so the examiner cannot tell which phenotype each genotype means.
• Giving “3 : 1” when the question asks for a probability or percentage.
• Saying a probability changes because of previous children.
• Pedigree answers that state “recessive” without quoting the evidence (which individuals, which phenotypes).
• Saying the egg or the mother decides a baby’s sex.
• (Paper 2) Using T in mRNA, or placing transcription in the cytoplasm.
• (Paper 2) Calling codominance “blending” or saying neither allele is expressed.
• Writing that antibiotics cause bacteria to mutate or “become immune”.
• Swapping 46 and 23, or saying meiosis makes two cells.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3 (September 2024), Pearson Education Limited; first assessment June 2019. Topic 3, Reproduction and inheritance, sub-topic (b) Inheritance, points 3.14–3.39.