What is the difference between genotype and phenotype?

Genotype refers to the genetic makeup of an organism, specifically the alleles it carries for a particular gene (e.g., AA, Aa, or aa). Phenotype is the observable expression of that genotype, influenced by both genetics and the environment (e.g., eye colour or height). For example, a plant with the genotype for tallness may be short if grown in poor soil.

How do you calculate allele frequencies using the Hardy-Weinberg equation?

First, identify the frequency of the recessive phenotype (q²) from the population data. Take the square root to find q (recessive allele frequency). Then use p + q = 1 to find p (dominant allele frequency). You can then calculate the frequency of heterozygotes (2pq) and homozygous dominants (p²). Always check that p² + 2pq + q² = 1.

What is the difference between allopatric and sympatric speciation?

Allopatric speciation occurs when a population is geographically separated (e.g., by a mountain range), leading to reproductive isolation and divergence. Sympatric speciation happens within the same area, often due to behavioural or ecological factors (e.g., different mating seasons or host plant preferences), without physical separation.

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

Energy is lost at each trophic level primarily through respiration (heat), uneaten parts (e.g., bones, faeces), and excretion. Only the energy stored in biomass that is consumed and assimilated by the next level is transferred. This inefficiency limits the number of trophic levels in an ecosystem.

What is the role of decomposers in the nitrogen cycle?

Decomposers (bacteria and fungi) break down dead organic matter and waste, releasing ammonia (NH₃) into the soil. This ammonia is then converted to nitrites and nitrates by nitrifying bacteria, which plants can absorb. Without decomposers, nitrogen would remain locked in organic compounds and unavailable to plants.

How does antibiotic resistance evolve in bacteria?

Antibiotic resistance arises from random mutations in bacterial DNA. When antibiotics are used, susceptible bacteria die, but resistant mutants survive and reproduce. Over time, the resistant allele becomes more common in the population due to natural selection. This is an example of directional selection.

Module 6: Genetics, evolution and ecosystems

OCR

A-Level

Module 1 focuses on the development of essential practical skills in biology, which are integrated throughout the theoretical modules. It covers the planning, implementation, analysis, and evaluation of experimental work, alongside the requirement for students to demonstrate competence in specific apparatus and techniques through a minimum of 12 assessed practical activities.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

Topic Overview

Module 6 of OCR A-Level Biology delves into the intricate mechanisms of genetics, evolution, and ecosystems. You'll explore how genetic information is stored, expressed, and inherited, from the molecular level of DNA replication and protein synthesis to the population-level dynamics of natural selection and speciation. This module also covers the flow of energy and nutrients through ecosystems, including the roles of producers, consumers, and decomposers, as well as the impact of human activities on biodiversity and conservation. Understanding these concepts is crucial for grasping how life evolves and sustains itself, forming the foundation for fields like medicine, agriculture, and environmental science.

This module builds on your knowledge from earlier topics, such as cell biology and basic genetics, and connects to real-world issues like antibiotic resistance, genetic engineering, and climate change. You'll learn to apply mathematical skills to calculate allele frequencies and ecological indices, and develop practical skills through investigations like sampling populations or analysing DNA profiles. Mastery of Module 6 is essential for the A-Level exams, as it appears in both the multiple-choice and long-answer sections, often requiring you to synthesise information across different areas.

By the end of this module, you should be able to explain how genetic variation arises through mutations and meiosis, how natural selection drives evolution, and how ecosystems maintain stability through nutrient cycles and energy transfer. You'll also appreciate the ethical and practical implications of biotechnologies like gene therapy and selective breeding. This knowledge not only prepares you for exams but also equips you to think critically about biological issues in everyday life.

Key Concepts

Core ideas you must understand for this topic

→The central dogma of molecular biology: DNA → RNA → protein, including the processes of transcription and translation, and the role of codons and anticodons.
→The Hardy-Weinberg principle: a mathematical model that predicts allele frequencies in a non-evolving population, with the equation p² + 2pq + q² = 1, and the five conditions required for equilibrium.
→Natural selection and speciation: how variation, selection pressures, and reproductive isolation lead to the formation of new species, including allopatric and sympatric speciation.
→Energy transfer through ecosystems: the 10% rule of energy transfer between trophic levels, and the roles of gross primary production (GPP) and net primary production (NPP).
→Nutrient cycles: the carbon and nitrogen cycles, including the processes of decomposition, nitrification, and denitrification, and the importance of microorganisms.

What You Need to Demonstrate

Key skills and knowledge for this topic

Correct application of experimental design and identification of control variables.
Accurate use of scientific apparatus and techniques as specified in the Practical Activity Groups (PAGs).
Correct use of units, significant figures, and appropriate data presentation formats.
Ability to process, analyze, and interpret qualitative and quantitative results.
Evaluation of experimental methods, including identification of anomalies, limitations, and sources of error.
Demonstration of competence in the Common Practical Assessment Criteria (CPAC) across 12 assessed practicals.

Marking Points

Key points examiners look for in your answers

Correct application of experimental design and identification of control variables.
Accurate use of scientific apparatus and techniques as specified in the Practical Activity Groups (PAGs).
Correct use of units, significant figures, and appropriate data presentation formats.
Ability to process, analyze, and interpret qualitative and quantitative results.
Evaluation of experimental methods, including identification of anomalies, limitations, and sources of error.
Demonstration of competence in the Common Practical Assessment Criteria (CPAC) across 12 assessed practicals.

Examiner Tips

Expert advice for maximising your marks

💡Ensure you can link practical techniques to the theoretical content of Modules 2-6.
💡Practice calculating and interpreting gradients and intercepts from experimental graphs.
💡Be prepared to suggest improvements to experimental designs to increase accuracy or precision.
💡Familiarize yourself with the specific apparatus and techniques listed in Section 1.2.2.
💡Always include units in your measurements and final answers.
💡When answering questions on the Hardy-Weinberg principle, always state the assumptions (e.g., no mutation, random mating) and show your working step-by-step. Many marks are awarded for correct substitution into the equation, even if the final answer is wrong.
💡For evolution questions, use specific terminology like 'selection pressure', 'differential survival', and 'reproductive success'. Avoid vague phrases like 'the fittest survive' – explain what 'fittest' means in context.
💡In ecology questions, remember to include units for energy values (e.g., kJ m⁻² year⁻¹) and use the correct formula for calculating efficiency (e.g., (energy transferred to next trophic level / energy available) × 100).

Common Mistakes

Pitfalls to avoid in your exam answers

Failure to use an appropriate number of significant figures in calculations.
Inaccurate labeling of graph axes or failure to use appropriate scales.
Inability to correctly identify and control all relevant variables in an investigation.
Poor evaluation of experimental limitations or failure to suggest meaningful improvements.
Incorrect calculation or interpretation of percentage error and uncertainty.
Misconception: Mutations are always harmful. Correction: Most mutations are neutral, and some can be beneficial, providing the raw material for evolution. Only a small fraction cause genetic disorders.
Misconception: Natural selection acts on individuals. Correction: Natural selection acts on populations over generations. Individuals do not evolve; they either survive and reproduce or not, based on their traits.
Misconception: Ecosystems are closed systems. Correction: Ecosystems are open systems that exchange energy and matter with their surroundings. For example, sunlight enters and heat is lost, and nutrients can be imported or exported.

Frequently Asked Questions

Common questions students ask about this topic

Before You Start

Prior knowledge that will help with this topic

•Basic genetics: understanding of DNA structure, genes, alleles, and inheritance patterns (monohybrid crosses) from Module 2.
•Cell biology: knowledge of cell organelles (ribosomes, nucleus) and the cell cycle, including mitosis and meiosis, from Module 2.
•Ecology basics: familiarity with food chains, food webs, and the terms 'producer', 'consumer', and 'decomposer' from GCSE or earlier A-Level topics.

Likely Command Words

How questions on this topic are typically asked

Plan

Evaluate

Analyze

Interpret

Describe

Explain

Calculate

Ready to test yourself?

Practice questions tailored to this topic

Module 6: Genetics, evolution and ecosystems

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

How to Revise Module 6: Genetics, evolution and ecosystems — OCR A-Level Biology

Examiner Tips for Module 6: Genetics, evolution and ecosystems

Common Mistakes in Module 6: Genetics, evolution and ecosystems

Key Marking Points

Module 6: Genetics, evolution and ecosystems

Topic Overview

Key Concepts

What You Need to Demonstrate

Marking Points

Examiner Tips

Common Mistakes

Frequently Asked Questions

What is the difference between genotype and phenotype?

How do you calculate allele frequencies using the Hardy-Weinberg equation?

What is the difference between allopatric and sympatric speciation?

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

What is the role of decomposers in the nitrogen cycle?

How does antibiotic resistance evolve in bacteria?

Before You Start

Likely Command Words

Ready to test yourself?

Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules