IB Biology genetics & evolution (HL) explained simply comes down to connecting inheritance at the molecular and organism level with changes in populations over time. For exams, you must be able to predict inheritance, interpret genetic data, explain natural selection and speciation, and apply tools such as chi-squared tests and the Hardy-Weinberg equations.
Under the current IB Biology course, first assessed in 2025, these ideas are distributed across several syllabus topics rather than contained in one official unit called “Genetics and Evolution.” The most relevant are D3.2 Inheritance, D4.1 Natural selection, A4.1 Evolution and speciation, and the HL content within these topics.
Where genetics and evolution appear in the current syllabus
The current course is organized into four themes and four levels of biological organization. Genetics and evolution therefore connect material from molecules, cells, organisms, and ecosystems.
| Syllabus topic | Central exam knowledge | Important HL emphasis |
|---|---|---|
| D3.2 Inheritance | Alleles, genotype, phenotype, pedigrees, blood groups, sex linkage and polygenic inheritance | Dihybrid crosses, independent assortment, linkage, recombinants and chi-squared testing |
| D4.1 Natural selection | Variation, selection pressures, differential reproduction and adaptation | Gene pools, allele frequencies and Hardy-Weinberg equilibrium |
| A4.1 Evolution and speciation | Evidence for evolution, reproductive isolation and speciation | Allopatric and sympatric speciation, adaptive radiation, hybrid barriers and polyploidy |
| A3.2 Classification and cladistics | Evolutionary relationships and common ancestry | Interpretation of cladograms and molecular evidence |
This organization matters because Paper 2 can require holistic understanding across topics. For example, a question may begin with a mutation, ask how meiosis transmits the resulting allele, and then ask how natural selection changes its frequency.
Inheritance: from alleles to phenotypes
A gene is a heritable factor consisting of a length of DNA that influences a characteristic. An allele is one of the alternative forms of a gene, while the locus is the gene's position on a chromosome. An organism's genotype is its combination of alleles, whereas its phenotype consists of its observable or measurable characteristics resulting from genotype and environmental influence.
Dominance describes the phenotype of a heterozygote. It does not mean that an allele is stronger, more common, or evolutionarily advantageous.
You should distinguish three common relationships:
- In complete dominance, the heterozygote has the same phenotype as the homozygous dominant individual.
- In incomplete dominance, the heterozygote has an intermediate phenotype.
- In codominance, both alleles affect the phenotype distinctly, as with Iᴬ and Iᴮ in the ABO blood-group system.
For a genetic cross, state parental genotypes, identify possible gametes, construct the Punnett grid, and give the requested probability or ratio. Never assume that a dominant phenotype identifies a homozygous genotype because it could also be heterozygous.
Meiosis, independent assortment and linkage
During meiosis, homologous chromosomes separate so the two alleles of a gene segregate into different gametes. Independently oriented homologous pairs can produce different combinations of maternal and paternal chromosomes. This explains independent assortment for genes on different chromosomes, although genes close together on the same chromosome are usually linked.
Crossing over can separate linked alleles and create recombinant combinations. Recombinants are generally less frequent than parental combinations when genes are linked. In an explanation question, connect crossing over to chiasmata between non-sister chromatids during prophase I rather than merely stating that alleles become mixed.
IB guidance specifies that linked alleles should be represented beside vertical lines showing homologous chromosomes. This makes the arrangement of alleles on chromosomes explicit and prevents the cross from being treated as ordinary independent assortment.
Chi-squared tests in dihybrid crosses
At HL, the chi-squared test can determine whether differences between observed and expected dihybrid results are likely to be due to chance:
χ² = Σ((O - E)² / E)
Use this method:
- Write a null hypothesis, such as: “There is no significant difference between the observed results and the ratio expected under independent assortment.”
- Calculate expected values from the predicted ratio and total sample size.
- Calculate each contribution to χ² and add them.
- Determine degrees of freedom: number of categories - 1.
- Compare χ² with the critical value at p = 0.05.
- If χ² exceeds the critical value, reject the null hypothesis; otherwise, fail to reject it.
Failing to reject the null hypothesis does not prove that genes are unlinked. It means the evidence does not demonstrate a statistically significant departure from the expected ratio at the chosen probability level.
Natural selection and changing allele frequencies
Evolution is change in the heritable characteristics of a population over generations. Individuals do not evolve, and physiological changes acquired during an individual's lifetime are not automatically inherited.
A strong natural-selection explanation follows a causal sequence:
- Individuals in a population show variation.
- Some variation is heritable and arises ultimately from mutation, with meiosis and sexual reproduction generating new combinations.
- A selection pressure causes differences in survival or reproductive success.
- Individuals with an advantageous phenotype contribute more alleles to the next generation.
- The frequency of the associated allele increases over generations.
Examiners reward links between these stages. Saying that an organism “adapted because it needed to survive” incorrectly suggests purposeful change. Populations become adapted over generations because selection acts on existing heritable variation.
Hardy-Weinberg equilibrium
The Hardy-Weinberg model predicts genotype frequencies in a non-evolving population with two alleles:
- p + q = 1
- p² + 2pq + q² = 1
Here, p and q are allele frequencies, while p², 2pq and q² are the frequencies of the two homozygous genotypes and the heterozygous genotype. The model assumes a large population, random mating, no mutation, no migration and no natural selection.
Suppose 9% of a population has a recessive phenotype. If the population meets the model's assumptions, q² = 0.09, so q = 0.30 and p = 0.70. The expected heterozygote frequency is 2pq = 2(0.70)(0.30) = 0.42, or 42%.
A frequent error is treating the recessive phenotype frequency as q rather than q². First decide whether the question gives an allele frequency, genotype frequency, or number of individuals.
Evidence, speciation and evolutionary relationships
Evidence for evolution includes molecular sequence comparisons, selective breeding, and homologous structures. Greater similarity in DNA, RNA, or amino-acid sequences generally supports a more recent common ancestor, although conclusions must be based on the supplied evidence rather than assumed from appearance.
Homologous structures share an underlying structural origin because of common ancestry, even if their functions differ. Analogous structures perform similar functions but arose independently through convergent evolution. Examiners often ask students to distinguish these terms using both ancestry and function.
Speciation is the splitting of a pre-existing species into new species. It requires reduced gene flow through reproductive isolation, followed by genetic divergence under different selection pressures, mutation and sometimes genetic drift.
| Process | Geographic separation? | Typical mechanism |
|---|---|---|
| Allopatric speciation | Yes | A physical barrier divides populations, preventing gene flow |
| Sympatric speciation | No | Reproductive isolation develops within the same geographic area |
| Polyploid speciation | Not necessarily | Chromosome duplication can cause abrupt reproductive isolation, especially in plants |
Prezygotic barriers prevent mating or fertilization, while postzygotic barriers include low hybrid viability or hybrid sterility. Adaptive radiation occurs when a common ancestor gives rise to multiple species adapted to different ecological niches.
How these ideas are assessed
For HL students, external assessment consists of Paper 1, lasting 2 hours and worth 36% of the final grade, and Paper 2, lasting 2 hours 30 minutes and worth 44%. The scientific investigation contributes the remaining 20%.
Genetics and evolution can appear in several formats:
- Paper 1A: selecting a genotype, ratio, evolutionary interpretation, or numerical result.
- Paper 1B: analysing data, graphs, experimental methods or statistical evidence.
- Paper 2 short answers: explaining mechanisms such as crossing over, selection or reproductive isolation.
- Paper 2 extended responses: integrating several ideas into a sustained biological argument.
Command terms define the required response. State needs a concise answer; describe gives features or patterns; explain gives causes and mechanisms; deduce requires a conclusion from supplied evidence; and evaluate requires strengths, limitations and a supported judgement.
When interpreting a graph, quote relevant values and units before explaining the biological cause. When asked to calculate, show substitution and working because a correct method may receive credit even if an arithmetic error follows.
An efficient exam-focused revision method
Revise this material as connected mechanisms rather than isolated definitions. A useful cycle is:
- Retrieve definitions and equations from memory.
- Draw one genetic cross or solve one Hardy-Weinberg calculation.
- Answer an explanation question without notes.
- Compare the response with a worked solution and identify the missing causal links.
- Repeat the question several days later.
Use the D3.2 Inheritance questionbank, A4.1 Evolution and speciation practice, and D4.1 Natural selection questionbank to separate conceptual gaps from exam-technique errors. The broader Genetics and evolution HL questionbank is useful once you are ready to mix concepts.
Most importantly, watch IB Biology past-paper video solutions after attempting questions yourself. Per-question worked videos show how biological knowledge is converted into mark-earning steps, including which data to cite and how much explanation a command term requires. Jojo AI can then help analyse an attempted response, but it should support active correction rather than replace the attempt.
Common mistakes that lose marks
- Confusing genes, alleles and chromosomes.
- Calling a dominant allele the most common or most beneficial allele.
- Writing that individuals evolve or mutate because they need to adapt.
- Omitting heritability or differential reproduction from natural selection.
- Assuming every gene pair assort independently despite possible linkage.
- Confusing q with q² in Hardy-Weinberg calculations.
- Claiming that a non-significant chi-squared result proves the null hypothesis.
- Describing allopatric speciation without explaining how reproductive isolation reduces gene flow.
Conclusion
IB Biology HL genetics and evolution becomes manageable when you connect four ideas: meiosis transmits and recombines alleles, inheritance determines genotype probabilities, natural selection changes allele frequencies, and reproductive isolation allows populations to become separate species. High-scoring answers use precise terminology, show calculations clearly, and explain each causal link rather than listing disconnected facts.
RevisionDojo's topic Questionbanks, Jojo AI feedback, and worked video solutions can help you practise this conversion from biological understanding to exam marks. Begin with targeted inheritance and evolution questions, then use the per-question video solutions to compare your reasoning with an efficient exam method.
Sources and referenced URLs
- Official IB Biology subject page
- Official IB Biology subject brief, first assessment 2025
- Official overview of IB Biology curriculum and assessment updates
- IB Biology guide for first assessment 2025
- RevisionDojo IB Biology resources
- RevisionDojo Genetics and Evolution HL questionbank
- RevisionDojo D3.2 Inheritance questionbank
- RevisionDojo A4.1 Evolution and Speciation questionbank
- RevisionDojo D4.1 Natural Selection questionbank
- RevisionDojo IB Biology videos and worked solutions