IGCSE Biology: Ecology, Inheritance and Genetic Engineering

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IGCSE Biology: Ecology, Inheritance and Genetic Engineering

TL;DR

Ecology is about how organisms interact with their environment and each other; inheritance explains how traits pass between generations through DNA; genetic engineering lets us deliberately change an organism's genes. These three topics connect: understanding inheritance helps you understand population genetics in ecology, and genetic engineering applies inheritance principles in new ways. You need to know the definitions, the mechanisms, and real examples for each.

1. The Mental Model

Think of ecology as who lives where and why—populations compete, predators hunt prey, nutrients cycle. Inheritance is how instructions pass down—DNA holds the code, and we inherit it through meiosis and fertilisation. Genetic engineering is we now rewrite those instructions—we copy useful genes from one organism into another, or edit genes directly in the lab. The core idea tying them together: populations evolve because genes change and spread; we can now change genes deliberately instead of waiting for random mutation.

2. The Core Material

2.1 Ecology: Populations and Communities

In ecology you're studying populations (all the oak trees in a wood) and how they interact. A population's size depends on birth rate, death rate, immigration and emigration. When resources are unlimited, a population grows exponentially; when space or food becomes scarce, growth slows—this is called the carrying capacity of the environment.

Organisms also live in communities—different species sharing a habitat. They compete for resources (two plants fighting for light), or have predator-prey relationships (wolves hunting deer). Energy flows in one direction: producers (plants) → primary consumers (herbivores) → secondary consumers (carnivores). At each step, roughly 90% of energy is lost as heat, so food chains rarely have more than four or five links.

Nutrient cycling closes the loop: carbon, nitrogen and water move between organisms and the abiotic (non-living) environment. Decomposers—bacteria and fungi—break down dead material and return nutrients to soil. This matters in exams: you need to explain why nutrient cycles must be closed (or the ecosystem dies), and be able to sketch a simple carbon or nitrogen cycle showing at least three processes (photosynthesis, respiration, decomposition).

Biodiversity—the number of different species in an ecosystem—makes ecosystems more stable. If you lose one food source, herbivores can switch to another; if you lose one predator, another can fill that role. Monocultures (just one crop species) are fragile and often need heavy pesticide use.

2.2 Inheritance: DNA, Meiosis and Genetic Crosses

DNA is a double helix with four bases: A, T, G, C. Base pairing is strict: A always pairs with T, G with C. A gene is a short stretch of DNA coding for one protein (or trait).

Humans have 23 pairs of chromosomes; most are autosomes, one pair are sex chromosomes (XX for female, XY for male). During meiosis, a cell divides twice, halving the chromosome number. This produces gametes (sperm or eggs) with just one copy of each chromosome. When sperm and egg fuse at fertilisation, the chromosome number is restored.

This is why inheritance follows patterns. If you cross a homozygous dominant parent (AA) with a homozygous recessive parent (aa), all offspring are heterozygous (Aa) and show the dominant phenotype. A cross between two heterozygotes (Aa × Aa) produces a 3:1 phenotypic ratio—three dominant to one recessive. These ratios only appear if you have large sample sizes; with small families, you often see deviation from the expected ratio by chance.

Sex-linked inheritance complicates things. The X chromosome is larger and carries more genes than the Y. A recessive allele on the X chromosome will show in males (who have only one X) but be masked in heterozygous females (who have two X copies). Colour blindness and haemophilia follow this pattern: affected mothers are rare; affected sons have an affected or carrier mother.

Worked cross (autosomal):
Trait: pea plant height. T (tall, dominant) vs t (short, recessive).

Cross: Tt × Tt
Gametes from each parent: T or t

Punnett square:
       T      t
T    TT     Tt
t    Tt     tt

Result: 1 TT : 2 Tt : 1 tt (genotypic)
        3 tall : 1 short (phenotypic)

Monohybrid crosses track one trait; dihybrid crosses track two. With two genes (each with two alleles), you get a 9:3:3:1 ratio in the F₂ generation if both genes assort independently.

2.3 Genetic Engineering: Techniques and Applications

Genetic engineering means deliberately inserting or editing DNA in an organism. The most common technique uses restriction enzymes (molecular scissors) to cut DNA at specific sequences. You isolate a useful gene—say the insulin gene from humans—cut it out, and insert it into a vector (usually a plasmid from bacteria). The plasmid now enters a bacterial cell, where it replicates. Every time the bacterium divides, the plasmid (and the insulin gene) is copied, so you get millions of identical copies—cloning at the molecular level.

CRISPR-Cas9 is newer and more precise. It's a natural bacterial immune system; you program it with a guide RNA to find one exact DNA sequence, and the Cas9 enzyme cuts there. You can delete the sequence, or insert a correct version if the gene is faulty.

Genetic engineering has huge medical applications: making insulin for diabetics, correcting cystic fibrosis (in theory; trials are ongoing), producing enzymes to break down toxins. In agriculture, genetically modified crops are engineered for drought resistance, pest resistance or higher yield. Golden Rice is a famous example: scientists inserted genes for beta-carotene (vitamin A precursor) so it could help prevent blindness in developing countries.

The science is powerful, but there are ethical concerns: should we edit human germline (reproductive) cells, passing changes to all descendants? Could GMO crops harm ecosystems if they interbreed with wild relatives? These questions don't have single "right" answers; exams expect you to present both pros and cons, not just cheerleading.

graph TD
    A["Natural Mutation<br/>(random, slow)"] -->|Selection over<br/>generations| B["Population adapts<br/>(evolution)"]
    C["Genetic Engineering<br/>(deliberate, fast)"] -->|Insert/edit specific<br/>genes| D["Organism has new trait<br/>(immediate)"]

    B -->|Inheritance passes<br/>genes forward| E["Genetic diversity<br/>in population"]
    D -->|Trait depends on<br/>gene function| F["Phenotype matches<br/>design"]

    E -.->|Studied in| G["Ecology:<br/>Population genetics"]
    F -.->|Applied in| G

3. Worked Example

A population of moths in a forest. The forest has peppered trees (light grey bark). Most moths are pale (blending in). A factory moves in, soot blackens the trees. Now pale moths stand out to birds and get eaten more; darker moths survive. Over 50 years, the population shifts from 95% pale to 95% dark. The allele frequency changed because selection pressure was intense.

Now imagine a scientist isolates the dark pigment gene and uses CRISPR to insert it into laboratory flies. The flies now all produce dark pigment even in the lab, where there's no predation pressure. The inserted gene doesn't help them survive—it just proves the engineering worked.

In the moth population, inheritance of the dark allele is random (meiosis shuffles it); which individuals breed is not random (predation selects survivors). In the engineered flies, the gene is in every organism by design, not by chance.

Why this matters for exams: you must explain that natural selection acts on variation created by genetic recombination and mutation, whereas genetic engineering creates variation on demand. Also note that ecological change (soot) created selection pressure—without it, the dark allele would stay rare. Environment shapes which genes spread.

4. Examiner's Breakdown

4.1 What Examiners Actually Reward

1. "Explain why nutrient cycles must be closed." Top answers state: "If nitrogen/carbon leaves the ecosystem as gas or in harvested crop, organisms cannot replace lost nutrients, so growth stops and the ecosystem cannot sustain its population."

2. "A 3:1 ratio appears in an F₂ monohybrid cross because heterozygotes produce gametes with both alleles in equal proportions; random fusion of these gametes gives the 3:1 phenotypic ratio." Must mention both equal proportion of gametes and random fusion.

3. "Sex-linked traits appear more often in males than females because males have only one X chromosome, so a recessive allele on the X is always expressed; females need two recessive alleles to show the phenotype." Examiners want you to explicitly link having one copy (males) to always expressed.

4. "Genetic engineering allows us to insert a desired allele rapidly into an organism without waiting for natural mutation and selection." Contrast with natural processes to show you grasp the difference.

5. "Carrying capacity is the maximum population size an environment can sustain because resources (food, space) become limiting, so birth rate falls or death rate rises until population growth stops." Must link limiting resource to growth stopping.

6. "CRISPR works by using a guide RNA to direct Cas9 protein to a specific DNA sequence, where Cas9 cuts the DNA, allowing precise deletion or insertion of genes." Specificity matters—say precise, not vague "it edits genes".

4.2 Trapdoor Mistakes

4.3 Score-Boosting Comparisons

5. Now Try It

Exam-style question (20 minutes):

A scientist discovers a gene in arctic fish that produces an antifreeze protein. She isolates this gene and inserts it into tomato plants using a bacterial plasmid vector, intending to create frost-resistant crops.

(a) Describe how a plasmid vector is used to insert the fish gene into tomato cells. (4 marks)

(b) Explain why the resulting tomato plant could grow in colder climates. (2 marks)

(c) Discuss the potential benefits and risks of releasingthis genetically modified tomato into farms in a cold climate. (6 marks)

Top-band answer structure (12 marks total):

(a) Must include: restriction enzyme cuts plasmid and fish gene at specific sequences; same sticky ends allow ligation (enzyme seals the joints); plasmid enters tomato cell (via Agrobacterium or electroporation); plasmid replicates inside cell; fish gene is now copied in every daughter cell.

(b) Must state: the antifreeze protein prevents ice crystals forming in cell sap / lowers freezing point; tomato cells tolerate sub-zero temperatures that would normally kill them.

(c) Benefits: increased yield in cold regions; food security in marginal climates; reduced need for heated greenhouses. Risks: unknown allergenicity if fish protein is toxic to humans; gene might transfer to wild tomato relatives via pollination, creating invasive species; ecosystem disruption if non-target insects are affected; farmers become dependent on one seed supplier (monopoly concern).

Success looks like: you describe the full molecular pathway (restriction → ligation → insertion), explain the mechanism behind the new trait (antifreeze protein function), and present both pros and risks with specific examples, not just generic "it's good/bad".

Mini-task (15 minutes): Predict a monohybrid cross

In guinea pigs, black fur (B) is dominant to white (b). A breeder crosses two black guinea pigs and gets offspring: 24 black, 8 white.

1. What are the genotypes of the parents? (Show your reasoning.)
2. Write out the Punnett square.
3. What does the 3:1 ratio tell you about meiosis?

Success looks like: you identify both parents as Bb (because white offspring appeared, so each parent must carry a recessive b); you draw a correct 2×2 Punnett square; you explain that the 3:1 ratio proves each parent made gametes with B and b in equal proportions (50:50), and random fusion of these gametes produces the ratio.

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