CCEA A-Level Biology Genetics, Evolution and Ecosystems: a complete overview of inheritance, gene technology, evolution, ecology and immunity

What this module demands

Genetics, Evolution and Ecosystems is the synoptic module that ties the qualification together: how characteristics are inherited, how DNA is manipulated, how populations change and form species, how ecosystems function, and how the body defends itself. CCEA tests confident genetic and statistical calculation alongside clear, ordered explanation.

This guide walks through the five dot points of the module, then sets out the exam patterns CCEA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Genetics and inheritance

A gene codes for a polypeptide and its versions are alleles; the genotype is the alleles present and the phenotype the observed result. Monohybrid crosses of heterozygotes give a 3 to 1 ratio and dihybrid crosses give 9 to 3 to 3 to 1. Sex-linked recessive conditions such as haemophilia are commoner in males, codominance expresses both alleles, and the chi-squared test checks whether observed ratios fit the expected.

Gene technology

Recombinant DNA technology transfers a gene using restriction enzymes to cut, ligase to join it into a vector such as a plasmid, and a host cell to express it. PCR amplifies DNA through cycles of heating and cooling with primers and Taq polymerase. Gel electrophoresis separates fragments by size, and DNA profiling compares the patterns for identification. The applications raise ethical and safety questions.

Populations and evolution

Variation arises from mutation, meiosis and random fertilisation; natural selection then makes advantageous alleles more common over generations. Directional selection shifts the mean and stabilising selection favours it. The Hardy-Weinberg principle (p plus q equals 1; p squared plus 2pq plus q squared equals 1) predicts allele frequencies in a stable population. Speciation follows reproductive isolation, often allopatric (geographical), until populations can no longer interbreed.

Ecosystems and immunity

In an ecosystem, energy flows from producers along food chains, with only about 10 percent passing to each trophic level, and nutrients are recycled through the carbon and nitrogen cycles. Succession leads to a climax community, and conservation maintains biodiversity through sustainable management. In immunity, the cellular response uses T cells and the humoral response uses B cells and antibodies, with memory cells giving a fast secondary response; vaccination produces immunity without illness.

How this module is examined

A typical CCEA profile for Genetics, Evolution and Ecosystems:

• Genetic diagrams. Monohybrid, dihybrid and sex-linked crosses with parental genotypes, gametes and offspring ratios.
• Calculations. Chi-squared tests and Hardy-Weinberg allele and genotype frequencies.
• Process recall. Gene technology steps, the nitrogen cycle, and the immune responses.
• Explanation and ethics. Natural selection and speciation, energy loss between trophic levels, and the ethics of gene technology and conservation.

Check your knowledge

A mix of recall, calculation and explanation questions covering the module. Attempt them under timed conditions, then check against the solutions.

1. Two heterozygous tall pea plants (Tt) are crossed. State the expected phenotype ratio. (1 mark)
2. Explain why haemophilia is more common in males than females. (2 marks)
3. State the roles of restriction enzymes and DNA ligase in genetic engineering. (2 marks)
4. Explain how gel electrophoresis separates DNA fragments. (2 marks)
5. In a population the recessive phenotype frequency (q squared) is 0.16. Calculate q. (1 mark)
6. Explain how a geographical barrier can lead to speciation. (3 marks)
7. State the role of nitrogen-fixing and denitrifying bacteria in the nitrogen cycle. (2 marks)
8. State one difference between active and passive immunity. (1 mark)