IB Biology genetics explained clearly comes down to a small set of connected ideas: DNA stores information, genes are expressed as proteins, alleles are inherited through meiosis and fertilization, and mutations create new variation. Exams test whether you can apply these ideas to genetic crosses, pedigrees, molecular diagrams, unfamiliar data, and explanations of inheritance.
The current course, first assessed in 2025, organizes much of this material within Theme D, Continuity and change, including DNA replication, protein synthesis, mutation, cell division, gene expression, and inheritance. The official IB Biology course overview and Biology guide roadmap should be used to confirm which understandings apply to your level.
The genetic vocabulary you must use precisely
Genetics questions often award marks for distinctions that disappear if terminology is used loosely.
| Term | Exam-ready meaning |
|---|---|
| Gene | A heritable factor consisting of a length of DNA that influences a characteristic or produces a functional RNA or polypeptide |
| Allele | One alternative form of a gene |
| Locus | The position of a gene on a chromosome |
| Genotype | The alleles possessed by an organism, usually for a specified gene or genes |
| Phenotype | The observable or measurable characteristics produced by genotype and environmental influence |
| Homozygous | Having two identical alleles at a locus |
| Heterozygous | Having two different alleles at a locus |
| Dominant allele | An allele expressed in the phenotype of a heterozygote |
| Recessive allele | An allele expressed only when no dominant allele is present |
| Codominant alleles | Alleles that are both expressed in a heterozygote |
Do not write that a dominant allele is stronger, more common, or automatically beneficial. Dominance describes expression in a heterozygote, not frequency or biological value. Similarly, an organism does not have a dominant genotype; it possesses alleles that produce a genotype.
From DNA to phenotype
A gene affects phenotype because its base sequence can determine the amino acid sequence of a polypeptide. The connection is summarized as DNA to RNA to polypeptide, followed by folding and possible modification into a functional protein.
During transcription, RNA polymerase uses one DNA strand as a template to produce complementary RNA. During translation, a ribosome reads mRNA codons, while tRNA molecules carrying amino acids pair their anticodons with complementary codons. Peptide bonds join the amino acids into a polypeptide.
For sequence questions, identify which strand is provided:
- The mRNA sequence is complementary to the template DNA strand.
- It matches the coding DNA strand except that RNA contains uracil instead of thymine.
- A codon is a three-base sequence on mRNA, not DNA or tRNA.
- An anticodon is found on tRNA.
- Translation proceeds along the mRNA in the 5′ to 3′ direction.
A useful causal explanation is: changed DNA base sequence leads to a changed mRNA codon, which may change an amino acid, which may alter folding or an active site, potentially changing the phenotype. The word may matters because the genetic code is degenerate, so some substitutions are silent.
Use the RevisionDojo molecular biology questionbank to practise converting diagrams and sequences into markscheme language.
DNA replication and mutation
Before cell or nuclear division, DNA is replicated semi-conservatively. Helicase separates the strands by breaking hydrogen bonds between complementary bases, and each original strand acts as a template. DNA polymerase adds nucleotides according to complementary base pairing, producing molecules containing one original strand and one newly synthesized strand.
DNA polymerase synthesizes new DNA in the 5′ to 3′ direction. At the more detailed level, this produces continuous synthesis on the leading strand and discontinuous synthesis through fragments on the lagging strand. Answer only to the depth required by the question and your SL or HL syllabus content.
A mutation is a change in genetic material. A base substitution may be silent, missense, or nonsense, while an insertion or deletion can cause a frameshift if the number of bases changed is not a multiple of three. Mutations in body cells are not normally inherited by offspring, whereas mutations affecting a germ-line cell may enter a gamete and be inherited.
Avoid claiming that every mutation is harmful. Its effect can be harmful, neutral, or beneficial depending on the sequence affected and the environment. Targeted practice is available in the mutations and gene editing questionbank.
Meiosis links chromosomes to inheritance
Meiosis converts one diploid nucleus into haploid nuclei, allowing fertilization to restore the diploid chromosome number. Homologous chromosomes separate during meiosis I; sister chromatids separate during meiosis II. Confusing these events is one of the most common genetics errors.
Meiosis generates variation through:
- Crossing over between non-sister chromatids of homologous chromosomes
- Independent orientation of homologous pairs at metaphase I
- The resulting independent assortment of alleles on different chromosomes
- Random fertilization of genetically different gametes
Genes on the same chromosome are linked and therefore do not assort independently in the same way as unlinked genes. Crossing over can nevertheless create recombinant chromosome combinations. The probability of recombination generally increases when loci are farther apart because a crossover between them is more likely.
Review chromosome structure through the chromosomes questionbank and connect it to the stages of meiosis using the cell and nuclear division questionbank.
How to solve a genetic cross
Never jump directly to a phenotype ratio. A clear method earns marks and reduces avoidable mistakes.
Suppose A is dominant and a is recessive. A heterozygous individual is crossed with a homozygous recessive individual.
- State the parental genotypes: Aa × aa.
- Identify gametes: the first parent produces A or a; the second produces a only.
- Combine gametes in a Punnett grid.
- Report genotype probabilities: 1/2 Aa and 1/2 aa.
- Report phenotype probabilities: 1/2 dominant phenotype and 1/2 recessive phenotype.
| a | a | |
|---|---|---|
| A | Aa | Aa |
| a | aa | aa |
Punnett grids show probabilities, not a guaranteed composition of a small family. Each fertilization is a separate event, so four offspring do not have to match a predicted 3:1 ratio.
For codominance, use clear allele notation such as Iᴬ, Iᴮ, and i for ABO blood groups. For sex-linked inheritance, show the allele on the relevant sex chromosome, such as XᴺXⁿ, rather than writing an unattached N or n.
Pedigrees and probability questions
Begin a pedigree by locating affected individuals born to unaffected parents. This supports recessive inheritance because both unaffected parents may be heterozygous carriers. If an affected father passes a condition to his son, the pattern cannot be X-linked through that father because a son receives his Y chromosome from him.
Treat pedigree conclusions as deductions from the complete evidence. One family pattern may be consistent with more than one inheritance model, especially when the pedigree is small.
In probability questions, use the multiplication rule for independent events. If two carrier parents have a 1/4 probability of an affected child, the probability that two specified children are both affected is 1/4 × 1/4 = 1/16. Use addition when alternative, mutually exclusive outcomes can produce the requested result.
How genetics appears in IB examinations
Under the current assessment model, Biology includes multiple-choice, data-based, short-answer, and extended-response work. The official Biology subject brief outlines the assessment structure, while the IB provides official sample examination papers for the revised course.
| Question wording | What your response should do |
|---|---|
| State | Give a concise answer without explanation |
| Determine | Reach an answer from information supplied, showing relevant steps |
| Calculate | Substitute correctly, show working, and give an appropriate unit or probability |
| Describe | Report the pattern, stages, or observed features |
| Explain | Link causes and consequences using biological reasoning |
| Compare and contrast | Give both similarities and differences using paired points |
| Predict | Use the genetic model or data to state an expected outcome |
For a data-based question, quote values when describing a trend and then use genetic principles to explain it. For an extended response, organize the answer as a causal sequence rather than listing isolated terms. The IB's Biology curriculum update confirms the emphasis on data analysis and application across the revised papers.
Common mistakes that lose marks
- Giving phenotype ratios when the question asks for genotypes
- Omitting parental genotypes or gametes from a genetic cross
- Calling homologous chromosomes identical
- Saying meiosis produces genetically identical cells
- Treating dominant as meaning common or advantageous
- Confusing DNA triplets, mRNA codons, and tRNA anticodons
- Claiming that every base substitution changes an amino acid
- Describing crossing over as occurring between sister chromatids
- Reporting probability as a guaranteed number of offspring
- Ignoring the command term or the number of marks available
Practise one weakness at a time in the IB Biology questionbank. The dedicated genetics questionbank is particularly useful because its per-question worked video solutions show how a diagram, cross, or explanation is converted into marks.
A practical genetics revision method
First, learn the vocabulary and reconstruct each process from memory. Next, complete genetic crosses without notes and check notation, gametes, probabilities, and phenotype labels separately. Finally, answer mixed data-based questions so that genetics is tested in unfamiliar contexts rather than only through rehearsed Punnett grids.
When reviewing an error, classify it as a knowledge gap, notation error, misread command term, or probability mistake. Jojo AI can help explain the underlying concept, but you should then answer a fresh question without assistance. Worked past-paper video solutions are most valuable after a genuine attempt because you can compare both the biological reasoning and the structure of the response.
Conclusion
IB Biology genetics is most manageable when DNA, protein synthesis, mutation, meiosis, and inheritance are understood as one connected system. Accurate terminology and visible working are essential, especially in Punnett grids, pedigrees, sequence questions, and probability calculations.
RevisionDojo can support this process through focused Questionbank practice, Jojo AI explanations, and per-question past-paper video solutions. Start with the genetics Questionbank, watch the worked solution after attempting each question, and record the precise reason for every lost mark.
Sources and referenced URLs
- Official IB Biology course overview
- Official DP Biology guide roadmap
- Official IB Biology subject brief
- Official IB sample examination papers
- Official IB Biology curriculum update
- RevisionDojo IB Biology Questionbank
- RevisionDojo Genetics Questionbank and worked solutions
- RevisionDojo Molecular Biology Questionbank
- RevisionDojo Chromosomes Questionbank
- RevisionDojo Cell and Nuclear Division Questionbank
- RevisionDojo Mutations and Gene Editing Questionbank