What is the difference between a dominant and a recessive allele?

An allele is a version of a gene. A dominant allele shows its effect whenever it is present, even if only one copy is inherited, so it is written with a capital letter. A recessive allele only shows its effect when both copies are recessive, so it is written with a small letter and is masked when a dominant allele is also present. For example, in a cross between two organisms carrying one dominant and one recessive allele each, three quarters of the offspring on average show the dominant feature and one quarter shows the recessive feature.

Why is energy lost along a food chain?

Energy enters a food chain when producers capture light in photosynthesis, and it passes along the chain as one organism eats another. At each step a large part of the energy is lost rather than passed on: it is used in respiration to power life processes, lost as heat, and lost in undigested waste and in parts that are not eaten. Because so much is lost at every transfer, less energy is available higher up the chain, which is why food chains are usually short and there are fewer organisms at the top.

How do human activities harm the environment, and how can the harm be reduced?

Human activities harm the environment mainly through pollution and the destruction of habitats. Burning fuels releases pollutant gases that cause air pollution and add to the greenhouse effect, and deforestation removes habitats and reduces biodiversity. The harm can be reduced by conserving resources and reducing, reusing and recycling materials, by replanting trees and protecting habitats, and by switching to cleaner energy sources, which together cut pollution and help preserve the variety of living things.

SingaporeCombined Science

Singapore-Cambridge N(A)-Level Combined Science, Biology: Genetics and Ecology, from DNA, chromosomes and inheritance to food chains, energy flow and human impact on the environment

Q: What does the Genetics and Ecology module of N(A)-Level Combined Science cover?

It links the inheritance of features to the wider living world. You begin with chromosomes, genes and DNA and how dominant and recessive alleles decide the features of offspring, using a simple genetic diagram to predict a cross. You then move outward to ecosystems: producers, consumers and decomposers, food chains and food webs, why energy is lost at each step, and finally how human activities such as pollution and deforestation harm the environment and what can be done to conserve resources.

An N(A)-Level Combined Science module overview for Biology: Genetics and Ecology (SEAB 5106/5107). How chromosomes, genes and alleles pass features from parents to offspring, how a genetic diagram predicts a cross, and how energy flows through food chains and human activity affects the environment, with links to every dot point.

Generated by Claude Opus 4.87 min readSEAB-5107Updated 2026-06-13

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

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What this module is about
Inheritance: passing on the information
Ecosystems: how energy flows
Humans and the environment
How this module is examined
Check your knowledge

What this module is about

The Genetics and Ecology module of N(A)-Level Combined Science Biology (SEAB 5106 and 5107) connects two scales of life. First it asks how features pass from one generation to the next, through chromosomes, genes and alleles. Then it widens out to whole communities, asking how energy flows through living things and how human activity changes the environment they live in. The shared idea is that living things do not exist alone: each organism carries inherited information and each depends on the others around it.

This overview pulls the threads together and links to every dot point page in the module, each with its own worked answers and practice questions.

Inheritance: passing on the information

The module opens with cell division and inheritance. You need to know that genetic information is stored in DNA, which is packaged into chromosomes inside the nucleus, and that a gene is a length of DNA controlling one feature. Most features are controlled by a pair of alleles, one inherited from each parent. A dominant allele (capital letter) shows whenever it is present; a recessive allele (small letter) only shows when both alleles are recessive. A simple genetic diagram, or Punnett square, lets you set out the alleles from each parent and predict the proportions of offspring.

Ecosystems: how energy flows

Next comes ecosystems and food chains. Producers (green plants) capture light energy by photosynthesis and make food; consumers eat producers or other consumers; and decomposers break down dead material and waste, returning nutrients to the soil. A food chain shows the feeding links in order, and a food web shows how several chains connect. Energy is lost at every step, through respiration, heat and waste, so less is available higher up, which keeps chains short and limits the number of top predators.

Humans and the environment

The module closes with humans and the environment. Human activities such as pollution and deforestation damage habitats and reduce biodiversity, the variety of living things. Burning fuels pollutes the air and adds to the greenhouse effect, and clearing forests removes habitats and the trees that take in carbon dioxide. The harm can be reduced by conserving resources, by reducing, reusing and recycling, by replanting and protecting habitats, and by moving to cleaner energy.

How this module is examined

Use a clear genetic diagram every time. Define the alleles, give the parents' genotypes and gametes, draw the Punnett square, and state the ratio. Method marks reward the working.
Explain energy loss with reasons. Do not just say energy is lost; say it is used in respiration, lost as heat, and lost in waste and uneaten parts.
Give specific causes and solutions. Name the activity (burning fuels, deforestation), the effect (air pollution, lost habitats, lower biodiversity), and a matched solution.

Check your knowledge

A mix of recall and application questions covering the module. Attempt them under timed conditions, then check against the solutions, and use the dot point pages for fuller practice.

State where in a cell the chromosomes are found, and what a gene is. (2 marks)
Explain the difference between a dominant and a recessive allele. (2 marks)
In rabbits, black fur (B) is dominant to white (b). A Bb rabbit is crossed with a bb rabbit. Give the ratio of black to white offspring. (2 marks)
State the roles of producers, consumers and decomposers in a food chain. (3 marks)
Give two reasons why energy is lost along a food chain. (2 marks)
Describe one way human activity reduces biodiversity, and one way the harm can be reduced. (2 marks)

Solutions

Q1: Chromosomes are found in the nucleus of the cell. A gene is a length of DNA that controls one inherited feature.
Q2: A dominant allele shows its effect whenever it is present, even with one copy. A recessive allele shows its effect only when both alleles are recessive, because a dominant allele masks it.
Q3: The gametes are $B$ or $b$ from the $Bb$ parent and only $b$ from the $bb$ parent, giving $Bb$ and $bb$ offspring. The ratio of black to white is $1 : 1$ .
Q4: Producers (green plants) capture light energy and make food by photosynthesis; consumers obtain energy by eating producers or other consumers; decomposers break down dead organisms and waste, returning nutrients to the soil.
Q5: Any two of: energy used in respiration to power life processes; energy lost as heat to the surroundings; energy lost in undigested waste and in parts that are not eaten.
Q6: For example, deforestation removes habitats and reduces the variety of living things; the harm can be reduced by replanting trees and protecting habitats.

Sources & how we know this

Singapore-Cambridge N(A)-Level Science (Chemistry, Biology) Syllabus 5107 — Singapore Examinations and Assessment Board (2026)
Singapore-Cambridge N(A)-Level Science (Physics, Biology) Syllabus 5106 — Singapore Examinations and Assessment Board (2026)