How does the lac operon control gene expression?

The lac operon controls the enzymes that metabolise lactose in bacteria. Without lactose, a regulatory gene makes a repressor protein that binds the operator region, blocking RNA polymerase from the promoter, so the structural genes are not transcribed. When lactose is present, it (as allolactose) binds the repressor and changes its shape so it can no longer bind the operator; RNA polymerase can then transcribe the structural genes, and the lactose-metabolising enzymes are made. This means the enzymes are produced only when their substrate is available, which is efficient.

How do you use the chi-squared test in genetics?

The chi-squared test compares observed offspring numbers with those expected from a genetic ratio, to decide whether any difference is significant or just due to chance. Calculate chi-squared as the sum of (observed minus expected) squared divided by expected. State a null hypothesis (no significant difference), find the degrees of freedom (number of categories minus one), and compare your value with the critical value at p equals 0.05. If chi-squared is less than the critical value, the difference is not significant and you accept the null hypothesis (the data fit the ratio); if it is equal to or greater, the difference is significant.

How is the Hardy-Weinberg principle used to calculate allele frequencies?

For two alleles of frequency p (dominant) and q (recessive), p plus q equals 1, and p squared plus 2pq plus q squared equals 1, where p squared is the homozygous dominant genotype frequency, 2pq the heterozygotes and q squared the homozygous recessive. Because the recessive phenotype shows only in q squared, you usually start there: take the recessive phenotype frequency as q squared, take the square root to find q, then find p as 1 minus q, and finally calculate 2pq for the carriers. The principle assumes no mutation, no selection, no migration, random mating and a large population.

What are the three temperature stages of PCR?

The polymerase chain reaction cycles through three temperatures to amplify DNA. Denaturation at about 95 degrees Celsius breaks the hydrogen bonds and separates the two DNA strands. Annealing at about 55 degrees Celsius lets the primers bind to their complementary sequences flanking the target region. Extension at about 72 degrees Celsius is when heat-stable Taq polymerase adds complementary nucleotides from the primers to build new strands. Each cycle doubles the number of DNA copies, so repeating it gives exponential amplification from a tiny starting sample.

Why is only about 10 percent of energy transferred between trophic levels?

Energy is lost at each transfer in a food chain. Not all of an organism is eaten, and not all of what is eaten is digested, so energy is lost in faeces (egestion). A large proportion of the energy taken in is lost as heat through respiration, and some is lost in excretion. As a result only around 10 percent of the energy at one trophic level is incorporated into the next. This is why biomass and energy decrease up the chain, food chains are short, and there are few top predators.

EnglandBiology

OCR A-Level Biology Module 6 Genetics, evolution and ecosystems: gene control, inheritance, genomes, cloning and ecosystems

A deep-dive OCR A-Level Biology guide to Module 6 Genetics, evolution and ecosystems. Covers cellular control and gene expression, patterns of inheritance and the chi-squared test, populations and the Hardy-Weinberg principle, manipulating genomes, cloning and biotechnology, and ecosystems and sustainability, with the exam patterns OCR repeats.

Generated by Claude Opus 4.822 min readOCR-H420-Module-6Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Module 6 actually demands
Cellular control and gene expression
Patterns of inheritance
Populations and evolution
Manipulating genomes
Cloning and biotechnology
Ecosystems and sustainability
How Module 6 is examined
Check your knowledge

What Module 6 actually demands

Genetics, evolution and ecosystems is the synoptic capstone of OCR A-Level Biology A, assessed mainly in Paper 2 (and heavily in the Paper 3 unified paper). It runs from how genes are controlled and inherited, through how populations evolve and how we read and manipulate genomes, to how organisms are cloned and how ecosystems work. The examiners reward precise genetics vocabulary, confident calculation (chi-squared, Hardy-Weinberg, trophic efficiency), and the ability to link this module back to cells, molecules and the rest of the course.

This guide ties together the six dot-point pages for Module 6 and sets out the exam patterns OCR repeats.

Cellular control and gene expression

Gene mutations (substitutions, which may be silent because the code is degenerate, and damaging frameshift insertions or deletions) can change a protein's tertiary structure and function. Gene expression is controlled mainly at transcription: the lac operon uses a repressor that binds the operator unless lactose is present; transcription factors and epigenetic changes switch eukaryotic genes on or off. Homeobox (Hox) genes code for transcription factors that control the body plan and are highly conserved. Apoptosis is controlled programmed cell death.

Patterns of inheritance

Monohybrid crosses give a 3:1 ratio and dihybrid crosses a 9:3:3:1 ratio. You also need codominance (both alleles expressed, for example AB blood), multiple alleles (such as ABO), sex linkage (X-linked recessive conditions commoner in males, with female carriers) and epistasis (one gene affecting another). The chi-squared test, $\chi^2 = \sum (O-E)^2 / E$ , compares observed with expected results against a critical value at p = 0.05.

Populations and evolution

A gene pool is all the alleles in a population, and allele frequency is the proportion of an allele. The Hardy-Weinberg principle ( $p + q = 1$ ; $p^2 + 2pq + q^2 = 1$ ) predicts constant frequencies given no mutation, no selection, no migration, random mating and a large population. Allele frequencies change by natural selection, genetic drift, the founder effect and migration. Speciation requires reproductive isolation: allopatric (geographical) or sympatric (within the same area, for example by polyploidy).

Manipulating genomes

DNA sequencing reads the base order; PCR amplifies DNA through denaturation, annealing and extension; gel electrophoresis separates fragments by size (DNA is negative and moves to the positive electrode). Genetic engineering uses the same restriction enzyme to cut a gene and a plasmid into complementary sticky ends, joined by ligase to make recombinant DNA. Gene editing (CRISPR) makes precise changes. DNA profiling amplifies and separates variable repeat regions to identify individuals.

Cloning and biotechnology

Plants are cloned naturally and by micropropagation (an explant grown aseptically on a hormone medium, forming a callus that becomes identical plantlets); animals by embryo splitting and somatic cell nuclear transfer. Microorganisms are grown in industrial fermenters (controlled temperature, pH, oxygen and stirring, kept sterile), and their batch growth shows lag, log, stationary and death phases. Immobilised enzymes are easily recovered and reused, do not contaminate the product, and are more stable.

Ecosystems and sustainability

Energy flows from producers (GPP, then NPP = GPP minus respiration) along trophic levels, with only about 10 percent transferred each step (trophic efficiency). The nitrogen cycle (fixation, ammonification, nitrification, denitrification) and the carbon cycle recycle nutrients via microorganisms and photosynthesis and respiration. Succession runs from pioneer species to a climax community (primary, or faster secondary). Sustainable management and conservation use and protect ecosystems to maintain biodiversity.

How Module 6 is examined

A typical OCR profile for Genetics, evolution and ecosystems:

Multiple choice and short answer. Identifying a type of mutation, naming a stage of PCR, matching a bacterium to a nitrogen-cycle process.
Maths. Genetic crosses and ratios, the chi-squared test, Hardy-Weinberg calculations, and trophic efficiency and NPP calculations.
Applied and data questions. Reading an electrophoresis gel, predicting the effect of a mutation, and interpreting an energy-flow diagram.
Level-of-Response and synoptic answers. The lac operon, recombinant DNA production, speciation and the nitrogen cycle are predictable, and Paper 3 links them to earlier modules.

OCR Module 6 Genetics, evolution and ecosystems covers cellular control (mutations, the lac operon, transcription factors, Hox genes and apoptosis), patterns of inheritance (monohybrid, dihybrid, codominance, sex linkage, epistasis and the chi-squared test), populations and evolution (gene pools, the Hardy-Weinberg principle, genetic drift, the founder effect, migration and allopatric and sympatric speciation), manipulating genomes (sequencing, PCR, electrophoresis, recombinant DNA with restriction enzymes and ligase, gene editing and DNA profiling), cloning and biotechnology (micropropagation, somatic cell nuclear transfer, fermenters, growth curves and immobilised enzymes), and ecosystems and sustainability (energy flow with GPP, NPP and trophic efficiency, the nitrogen and carbon cycles, succession and conservation). Master the genetics calculations and link the module synoptically to the rest of the course.

Check your knowledge

A mix of recall and application questions covering the whole of Module 6. Attempt them under timed conditions, then check against the solutions.

Describe how the lac operon is switched on when lactose is present. (3 marks)
Explain why a frameshift mutation usually has a greater effect than a substitution. (3 marks)
A 3:1 cross gives 78 dominant and 22 recessive offspring. Calculate chi-squared (expected 75 and 25). (3 marks)
In a population, 16 percent show the recessive phenotype. Find the frequency of the dominant allele. (3 marks)
Describe the three temperature stages of PCR. (3 marks)
Explain how recombinant DNA is made using restriction enzymes and ligase. (4 marks)
Describe how micropropagation produces identical plants. (4 marks)
Explain the roles of microorganisms in the nitrogen cycle. (4 marks)

Solutions

Q1: When lactose is present, it (as allolactose) binds the repressor protein and changes its shape so it can no longer bind the operator. RNA polymerase can then bind the promoter and transcribe the structural genes, so the lactose-metabolising enzymes are made.
Q2: A frameshift (insertion or deletion) shifts the reading frame, so every codon after the mutation is changed; almost all the amino acids change, so the protein's structure and function are usually destroyed. A substitution changes only one codon, so at most one amino acid changes (and it may be silent because the code is degenerate), so the effect is usually much smaller.
Q3: $\chi^2 = \dfrac{(78-75)^2}{75} + \dfrac{(22-25)^2}{25} = \dfrac{9}{75} + \dfrac{9}{25} = 0.12 + 0.36 = 0.48$ . (With 1 degree of freedom this is below the critical value 3.84, so the difference is not significant.)
Q4: The recessive phenotype is $q^2 = 0.16$ , so $q = \sqrt{0.16} = 0.4$ . Then $p = 1 - q = 1 - 0.4 = 0.6$ , so the dominant allele frequency is 0.6.
Q5: Denaturation at about 95 degrees Celsius breaks the hydrogen bonds and separates the strands. Annealing at about 55 degrees Celsius lets the primers bind to the complementary sequences. Extension at about 72 degrees Celsius is when Taq polymerase adds nucleotides to build new strands; each cycle doubles the DNA.
Q6: The same restriction enzyme cuts the desired gene and the plasmid vector at its recognition sequence, leaving complementary sticky ends. The gene and cut plasmid are mixed; their complementary sticky ends base-pair by hydrogen bonding, and DNA ligase joins the sugar-phosphate backbones, forming a recombinant plasmid containing the gene.
Q7: A small explant (often a meristem) is taken from the parent and grown under aseptic conditions on a sterile nutrient medium containing plant hormones. The totipotent cells divide to form a callus, which is induced to differentiate into plantlets with roots and shoots; these are grown on and transferred to soil. All are genetically identical clones of the parent.
Q8: Nitrogen-fixing bacteria convert nitrogen gas to ammonium; decomposers convert proteins and urea to ammonium (ammonification); nitrifying bacteria oxidise ammonium to nitrite then nitrate (nitrification), which plants absorb; and denitrifying bacteria convert nitrate back to nitrogen gas (denitrification) in anaerobic soils.