Singapore-Cambridge GCE O-Level Combined Science, Biology: Genetics and Ecology, from DNA, chromosomes and cell division through monohybrid inheritance to food chains, the carbon cycle and human impact on the environment

What this module is about

Genetics and Ecology takes Combined Science Biology from the molecule that stores information, DNA, out to the ecosystems and global cycles that information shapes. The first half is about inheritance: how DNA is packaged, how cells divide to grow and to make gametes, and how characteristics are passed on and predicted. The second half is about interactions: how energy flows through food chains, how carbon is recycled, and how human activity disturbs both. The connecting idea is that variation, created and passed on at the level of genes, plays out at the level of whole populations and environments.

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.

DNA, genes, chromosomes and cell division

The starting point is cell division and DNA. DNA is the molecule that carries the genetic code; it is packaged into chromosomes in the nucleus, and a gene is a length of DNA coding for one characteristic. Cells divide in two ways. Mitosis produces two genetically identical diploid cells for growth and repair. Meiosis produces four genetically different haploid gametes and halves the chromosome number so that fertilisation can restore the full set. Keeping those two straight, identical versus varied, diploid versus haploid, is worth a steady stream of marks.

Predicting inheritance

With genes and alleles defined, inheritance and genetics shows how to predict offspring. You use the terms gene, allele, dominant, recessive, genotype and phenotype, and draw a genetic diagram (a Punnett square) to work out the ratio of offspring. A dominant allele is expressed whenever it is present; a recessive allele is only expressed when two copies are inherited. Homozygous means two identical alleles, heterozygous means two different alleles. The classic monohybrid cross of two heterozygotes gives the expected 3 : 1 ratio of dominant to recessive phenotypes.

Energy flow and the carbon cycle

Moving outward, ecology and food chains describes how energy enters an ecosystem through producers (plants capturing light) and passes along food chains and webs to consumers. Energy is lost at every transfer, mostly through respiration and heat, which is why food chains are short and why there are far fewer top predators than producers. The same dot point covers the carbon cycle: carbon dioxide is removed from the air by photosynthesis, returned by respiration, combustion and decomposition, and locked up for long periods in fossil fuels until they are burned.

Human impact on the environment

The final dot point, humans and the environment, applies the cycle to human activity. Burning fossil fuels and deforestation raise atmospheric carbon dioxide, strengthening the greenhouse effect and driving global warming and climate change. Pollution of air and water damages habitats and health, and clearing forests removes a carbon store and destroys biodiversity. The flip side is mitigation: conserving forests, switching to renewable energy, reducing and recycling waste, and managing pollution all reduce the impact.

How this module is examined

• Use the genetics vocabulary precisely. Marks are lost when genotype and phenotype, or dominant and recessive, are muddled. Always state which allele is dominant before reading off a cross.
• Draw clean genetic diagrams. Show parent genotypes, gametes, the Punnett square and the offspring ratio. A neat diagram earns method marks even if the final ratio is misread.
• Explain energy loss and the carbon cycle as processes. Name the process that moves carbon (photosynthesis, respiration, combustion, decomposition) and the form energy is lost in (mostly respiration and heat).

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.

1. Define the terms gene and allele. (2 marks)
2. State two differences between mitosis and meiosis. (2 marks)
3. In a cross between two heterozygous plants (Bb x Bb) where B is dominant, state the expected phenotype ratio. (1 mark)
4. Explain why food chains rarely have more than four or five links. (2 marks)
5. Name two processes that return carbon dioxide to the atmosphere. (2 marks)
6. Explain how deforestation contributes to the rise in atmospheric carbon dioxide. (2 marks)