Cambridge O Level Biology 5090: Inheritance (5090) – Revision Notes

These revision notes cover Topic 17, Inheritance, of the Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4): sections 17.1 Variation, 17.2 DNA, 17.3 Inheritance and 17.4 Selection. The syllabus is not tiered, so all of it applies to every candidate. For full explanations and worked examples, use the study guide.

When you are ready, test yourself with the practice questions. The course hub lists every topic, the printable checklist lists every outcome, and the free 10-minute diagnostics show where to focus. Meiosis and chromosome numbers are in Development of organisms and continuity of life.

17.1 Variation

• Variation: differences between individuals of the same species.

	Continuous	Discontinuous
Phenotypes	A range between two extremes	A limited number, no intermediates
Examples	Body length, body mass	ABO blood groups; seed shape and seed colour in peas
Cause	Genes and environment	Usually genes only
Graph	Histogram (bars touch)	Bar chart (bars do not touch)

Method in steps: investigating variation (17.1.5)

1. Pick one species and one feature.
2. Continuous feature (e.g. leaf length): measure many individuals with the same instrument and units; group into equal classes; draw a histogram.
3. Discontinuous feature (e.g. pea seed shape): count the number in each category; draw a bar chart.
4. Use a large sample so the pattern is clear.

17.2 DNA

Structure (17.2.1)

• Two strands coiled into a double helix.
• Each strand is a chain of nucleotides.
• Each nucleotide contains a base: A, T, C or G.
• Bonds between pairs of bases hold the strands together.
• Pairing rule: A with T, C with G. Always.

From gene to protein (17.2.2–17.2.5)

• Gene: a length of DNA that codes for a protein.
• DNA controls cell function by controlling the production of proteins, including enzymes.
• Base sequence in a gene → amino acid sequence of a specific protein.
• Different amino acid sequences → different protein shapes → different functions.

Worked reminder. 18% of the bases in a DNA molecule are G. So C = 18%. That leaves 100 − 36 = 64% for A + T, so A = T = 32%.

17.3 Inheritance

Definitions

• Inheritance: the transmission of genetic information from generation to generation.
• Allele: an alternative form of a gene.
• Genotype: the alleles present (e.g. Tt). Phenotype: the observable features (e.g. tall).
• Homozygous: two identical alleles (TT or tt). Heterozygous: two different alleles (Tt).
• Dominant: expressed when one copy is present. Recessive: expressed only when no dominant allele is present.
• Pure-breeding: two identical homozygous individuals that breed together will be pure-breeding.
• Codominance: both alleles in a heterozygote are expressed in the phenotype.

Method in steps: any monohybrid cross (17.3.4)

1. Key: capital letter = dominant allele, same small letter = recessive allele.
2. Parental phenotypes, then parental genotypes.
3. Gametes: one allele each, circled or listed.
4. Punnett square.
5. Offspring genotypes → phenotypes → ratio.

The two ratios you need

Cross	Offspring genotypes	Phenotypic ratio
Heterozygous × heterozygous (Tt × Tt)	1 TT : 2 Tt : 1 tt	3 dominant : 1 recessive
Heterozygous × homozygous recessive (Tt × tt)	1 Tt : 1 tt	1 dominant : 1 recessive
Homozygous × same homozygous (TT × TT or tt × tt)	All the same	Pure-breeding

Worked reminder. Tt × Tt gives 240 seeds. Expected dwarf = 240 × 1/4 = 60.

Observed v expected (17.3.5). Fertilisation is random, so the ratio is a probability. Small numbers of offspring often give ratios far from 3 : 1 or 1 : 1. Larger samples come closer.

Worked reminder. Tt × tt gives 100 plants: 52 tall and 48 dwarf. Observed ratio = 52 ÷ 48 = 1.08 : 1. Expected 1 : 1 (50 : 50). The small difference is due to chance.

Spot the cross from the offspring

• All offspring show the dominant phenotype, and one parent is homozygous recessive → the other parent is almost certainly homozygous dominant.
• Some offspring show the recessive phenotype → both parents must carry at least one recessive allele.
• About half dominant, half recessive → one parent heterozygous, the other homozygous recessive.

ABO blood groups (17.3.7)

Genotype	Phenotype
IᴬIᴬ, IᴬIᵒ	A
IᴮIᴮ, IᴮIᵒ	B
IᴬIᴮ	AB
IᵒIᵒ	O

Iᴬ and Iᴮ are codominant; Iᵒ is recessive to both. Two parents of group A and group B can have a child of group O only if both are heterozygous (IᴬIᵒ and IᴮIᵒ).

Sex determination (17.3.8)

• Female XX, male XY.
• Eggs: all X. Sperm: half X, half Y.
• XX × XY → 1 female : 1 male. Each child has an equal chance of either sex, whatever the earlier children were.

Mutation and variation (17.3.9–17.3.12)

	Gene mutation	Chromosome mutation
Definition	Random change in the base sequence of DNA	Change in chromosome number or structure
Example	Sickle cell anaemia	Down’s syndrome: 47 chromosomes instead of 46

• Sickle cell anaemia: changed base sequence in the gene for haemoglobin → changed amino acid sequence → haemoglobin of a different shape → red blood cells can become sickle-shaped.
• Sources of genetic variation: mutation, meiosis, random mating, random fertilisation.
• Ionising radiation and some chemicals increase the rate of mutation.

17.4 Selection

Natural selection: five points (17.4.1)

1. Variation within the population.
2. Many offspring produced.
3. Struggle for survival, including competition for resources.
4. Individuals better adapted to the environment survive and reproduce.
5. They pass on their alleles to the next generation.

Over many generations the inherited features of the population change: it evolves (17.4.2).

Antibiotic-resistant bacteria (17.4.3)

Mutation gives a few bacteria resistance → antibiotic kills non-resistant bacteria → resistant ones survive and reproduce → resistance allele passed on → resistant strain, e.g. MRSA. Link: Disease and immunity.

Artificial selection: four points (17.4.4)

1. Humans select individuals with desirable features.
2. Cross them.
3. Select offspring with the desirable features.
4. Repeat over many generations.

Role (17.4.5): producing economically important plants and animals, such as high-yield or disease-resistant crops and cattle with high milk yield.

Must-know distinctions

• Gene v allele: a gene codes for a protein; an allele is one form of that gene.
• Genotype v phenotype: letters v what you see.
• Homozygous v heterozygous: same pair v different pair.
• Dominance v codominance: one allele hidden v both alleles expressed.
• Continuous v discontinuous: range v categories; genes and environment v genes only.
• Gene mutation v chromosome mutation: base sequence v chromosome number or structure.
• Natural v artificial selection: environment selects v humans select.

Quick self-test

1. Define variation.
2. Is body mass an example of continuous or discontinuous variation? What causes it?
3. One DNA strand reads TTACGC. Write the complementary strand.
4. In a DNA molecule, 18% of the bases are G. What percentage are A?
5. Define a gene.
6. Explain why changing the base sequence of a gene can stop an enzyme working.
7. A plant of genotype Tt is crossed with a plant of genotype tt. State the expected phenotypic ratio. (T = tall, t = dwarf.)
8. A cross between two Tt plants gives 300 tall and 98 dwarf offspring. Calculate the observed ratio of tall to dwarf.
9. Give the genotype of a person with blood group AB, and name the type of inheritance shown.
10. How many chromosomes are present in the body cells of a person with Down’s syndrome?
11. State four sources of genetic variation in populations.
12. State the four steps of artificial selection.

Answers

1. Differences between individuals of the same species.
2. Continuous; caused by genes and the environment.
3. AATGCG.
4. C = 18%, so A + T = 64%; A = 32%.
5. A length of DNA that codes for a protein.
6. The base sequence sets the amino acid sequence; a different sequence gives a different shape, so the active site may no longer fit the substrate.
7. 1 tall : 1 dwarf.
8. 300 ÷ 98 = 3.06 : 1 (close to the expected 3 : 1).
9. IᴬIᴮ; codominance.
10. 47.
11. Mutation, meiosis, random mating, random fertilisation.
12. Select individuals with desirable features; cross them; select offspring with the desirable features; repeat over many generations.

Where marks are usually lost

• Giving gametes with two alleles, or forgetting to show gametes at all.
• Stating a genotypic ratio (1 : 2 : 1) when the question asks for a phenotypic ratio (3 : 1).
• Using two different letters for the alleles of one gene (e.g. T and d).
• Writing blood group alleles as A, B and O instead of Iᴬ, Iᴮ and Iᵒ.
• Saying a small family “should” show an exact 3 : 1 ratio.
• Listing natural selection points but leaving out “pass on their alleles”.
• Saying bacteria “become resistant” because they are exposed to the antibiotic, rather than resistant ones being selected.
• Mixing up artificial selection (humans choose) with natural selection (the environment chooses).
• Describing Down’s syndrome as a gene mutation.

Official syllabus

Cambridge O Level Biology 5090 syllabus for examination in 2026, 2027 and 2028 (version 4), Topic 17: Inheritance (17.1 Variation; 17.2 DNA; 17.3 Inheritance; 17.4 Selection), published by Cambridge Assessment International Education.