Q: What does H2 Biology: Genetic Basis of Disease - Notes & Key Concepts (2026) cover? A: Comprehensive H2 Biology notes on genetic diseases - inheritance patterns, pedigree analysis, genetic testing, gene therapy, and exam question patterns for 9477.
TL;DR This guide covers every examinable angle of genetic diseases for H2 Biology: inheritance patterns (autosomal dominant/recessive, sex-linked, codominance), pedigree analysis techniques, genetic crosses and chi-squared testing, mutation types, genetic testing and counselling ethics, and gene therapy. Use it alongside the H2 Biology notes hub to build a complete revision map.
Pedigree evidence checkpoint
Before assigning genotypes, test the inheritance pattern against the pedigree evidence in a fixed order.
Evidence to check
What it supports
What would contradict it
Affected child born to two unaffected parents
Autosomal recessive or X-linked recessive
Autosomal dominant, unless a new mutation is stated
Affected father and unaffected son
Not father-to-son transmission
X-linked inheritance is still possible because fathers pass Y chromosomes to sons
Many more affected males than females
X-linked recessive
Equal male and female frequency makes autosomal inheritance more likely
Trait appears in every generation
Autosomal dominant
Skipped generations make recessive inheritance more likely
Misconception check: do not choose the inheritance pattern from one clue only. State the strongest supporting evidence, then state which competing pattern it rules out.
Understanding how gene mutations cause disease is one of the most integrated topics in the H2 Biology syllabus. It draws on Core Idea 2 (Genetics and Inheritance) and connects to molecular biology, evolution, and biotechnology. Questions on genetic diseases appear across Papers 2, 3, and 4, testing your ability to interpret pedigrees, predict offspring ratios, evaluate ethical dilemmas in genetic testing, and explain how gene therapy works at the molecular level.
Status: SEAB's current H2 Biology (9477) syllabus PDF is labelled for 2026 and identifies 2026 as the first year of examination. This page covers inheritance patterns, pedigree analysis, mutations, genetic testing and counselling, and gene therapy as examined under Core Idea 2 and Core Idea 3. [1]
Quick revision box
What this topic tests: Inheritance patterns of genetic diseases, pedigree analysis, genetic crosses with expected ratios, mutation types, genetic testing ethics, and gene therapy mechanisms.
Top mistakes to avoid: Forgetting to consider carrier status in autosomal recessive conditions; misidentifying sex-linked inheritance as autosomal; omitting the chi-squared test when question data provides observed and expected ratios; confusing somatic and germline gene therapy in ethics questions.
20-minute sprint plan: 5 min key definitions and inheritance pattern summary table; 10 min pedigree analysis worked example; 5 min gene therapy mechanism and limitations.
1 Syllabus Context
The genetic basis of disease sits primarily within Core Idea 2 (Genetics and Inheritance) of the SEAB 9477 syllabus. It connects to several learning outcomes: [1]
Inheritance patterns: Monohybrid and dihybrid crosses, dominance relationships, sex linkage.
Mutations: Gene mutations and their phenotypic consequences, including human genetic disorders.
Biotechnology applications: Genetic testing, counselling, and gene therapy.
This topic integrates with Core Idea 1 (Cell and Biomolecules) through protein structure and function, and with Core Idea 4 (Biological Evolution) through natural selection acting on allele frequencies in populations (for example, heterozygote advantage in sickle cell trait).
2 Key Definitions
Before tackling inheritance patterns and disease mechanisms, ensure these terms are precise in your answers. Examiners penalise vague or circular definitions.
Term
Definition
Gene
A heritable unit consisting of a sequence of DNA nucleotides that codes for a polypeptide or functional RNA.
Allele
An alternative form of a gene, arising from mutation, that occupies the same locus on homologous chromosomes.
Locus
The specific position of a gene on a chromosome.
Genotype
The combination of alleles an organism possesses for a particular gene or set of genes.
Phenotype
The observable characteristics of an organism, resulting from the interaction of genotype and environment.
Dominant
An allele whose phenotypic effect is expressed in both homozygous and heterozygous states.
Recessive
An allele whose phenotypic effect is only expressed in the homozygous state.
Codominant
A pair of alleles that are both fully expressed in the heterozygote, producing a distinct phenotype.
Sex-linked
A gene located on one of the sex chromosomes (usually the X chromosome in humans).
Carrier
A heterozygous individual who carries one copy of a recessive disease allele without showing the disease phenotype.
3 Inheritance Patterns of Genetic Diseases
3.1 Autosomal dominant
The disease allele is on an autosome and only one copy is needed for the disease to manifest.
Example - Huntington disease:
Caused by an expansion of CAG trinucleotide repeats in the huntingtin gene on chromosome 4.
Affected individuals are typically heterozygous (Hh); homozygous dominant (HH) is extremely rare.
Late onset (symptoms usually appear after age 35), which means the allele can be passed to offspring before diagnosis.
If one parent is affected (Hh) and the other is unaffected (hh), the expected ratio is 1 affected : 1 unaffected among offspring.
Key pedigree features: The trait appears in every generation; affected individuals have at least one affected parent; males and females are equally affected.
3.2 Autosomal recessive
Two copies of the disease allele are required for the disease phenotype. Carriers (heterozygotes) are phenotypically normal.
Example - Sickle cell anaemia:
A point mutation (A→T) in the beta-globin gene on chromosome 11 causes a glutamic acid to valine substitution at position 6 of the beta-globin polypeptide.
The altered haemoglobin (HbS) polymerises under low oxygen tension, distorting red blood cells into a sickle shape.
Homozygous individuals (HbSHbS) have sickle cell anaemia; heterozygous individuals (HbAHbS) have sickle cell trait and are largely asymptomatic.
Example - Cystic fibrosis:
Caused by mutations in the CFTR gene on chromosome 7, most commonly a deletion of three nucleotides resulting in loss of phenylalanine at position 508 (ΔF508).
The defective CFTR protein fails to function as a chloride ion channel, leading to thick, sticky mucus in the lungs, pancreas, and other organs.
Two carrier parents (Cc×Cc) produce offspring in the expected ratio 1 CC : 2 Cc : 1 cc, so one in four offspring is predicted to be affected.
Key pedigree features: The trait may skip generations; affected individuals often have unaffected parents (both carriers); consanguinity increases the probability of affected offspring.
3.3 Sex-linked recessive
The disease allele is on the X chromosome. Males (XY) are hemizygous and express the disease if they inherit one copy. Females require two copies to be affected.
Example - Haemophilia A:
A mutation in the F8 gene on the X chromosome leads to deficiency of clotting factor VIII.
Males are affected (XhY); females are carriers (XHXh) or, rarely, affected (XhXh).
A carrier mother (XHXh) and unaffected father (XHY) produce: half of sons affected, half of daughters carriers.
Example - Red-green colour blindness:
Mutations in the OPN1LW or OPN1MW genes on the X chromosome affect red or green cone photopigments.
Inheritance pattern is identical to haemophilia: more common in males; affected fathers cannot pass the condition to sons.
Key pedigree features: More males affected than females; no male-to-male transmission; affected males inherit the allele from carrier mothers.
3.4 Codominance
Both alleles contribute to the phenotype in the heterozygote. Neither is dominant over the other.
Example - Sickle cell trait (heterozygote):
Individuals with genotype HbAHbS produce both normal haemoglobin (HbA) and sickle haemoglobin (HbS).
Under normal oxygen levels, red blood cells function adequately. Under severe low-oxygen stress, some sickling may occur.
This is a case of codominance at the molecular level: both types of haemoglobin are synthesised. At the whole-organism level, the individual is largely unaffected, which is sometimes described as incomplete dominance depending on the level of analysis.
Heterozygote advantage: in malaria-endemic regions, carriers have a survival advantage because the malarial parasite reproduces less efficiently in cells containing HbS.
4 Pedigree Analysis Technique
Pedigree analysis is a high-frequency exam question type. Use this systematic approach:
Step 1 - Determine the mode of inheritance.
If the trait appears in every generation, consider autosomal dominant.
If the trait skips generations, consider autosomal recessive.
If significantly more males are affected, consider X-linked recessive.
If an affected father has all affected daughters and no affected sons, consider X-linked dominant (rare in the syllabus).
Step 2 - Assign alleles.
Choose a clear symbol convention (e.g., A for the dominant allele, a for the recessive allele; or XH and Xh for sex-linked traits).
State your convention explicitly in the answer. Examiners expect this.
Step 3 - Determine genotypes of key individuals.
Start with affected individuals (their genotype is usually unambiguous).
Work outward to parents and siblings using the rules of inheritance.
Use a Punnett square to verify expected offspring ratios.
Step 4 - Calculate probabilities if asked.
Express probabilities as fractions or percentages.
For multi-step probability questions, multiply independent probabilities along a branch and add across branches (product rule and sum rule).
Common examiner traps:
Assuming a condition is autosomal when the data is also consistent with X-linked inheritance. Always test both hypotheses against the pedigree.
Forgetting that unaffected individuals in autosomal recessive pedigrees may be carriers (Aa) or homozygous normal (AA). If a question asks for a probability, consider both possibilities and use conditional probability where appropriate.
5 Genetic Crosses
5.1 Monohybrid cross
A cross involving one gene with two alleles. For autosomal recessive diseases, crossing two carriers:
Cc×Cc→1,CC:2,Cc:1,cc
Phenotypic ratio: 3 unaffected : 1 affected.
Carrier-probability checkpoint
Autosomal recessive probability questions often change depending on whether the question asks about any child, an unaffected child, or a future child after one child is known to be affected. Write the sample space before choosing the fraction.
Question wording
Sample space to use
Probability move
Common trap
"What is the probability that a child is affected?"
All possible offspring from the cross.
For Cc×Cc, use cc=1/4.
Forgetting that carrier parents can have an affected child.
"What is the probability that a child is a carrier?"
All possible offspring from the cross.
For Cc×Cc, use Cc=2/4=1/2.
Treating carriers as affected individuals.
"Given that the child is unaffected, what is the probability that the child is a carrier?"
Only unaffected offspring.
Remove cc, then compare Cc with CC: 2/3.
Using 1/2 even though affected offspring are no longer in the sample space.
"The first child is affected. What is the probability the next child is affected?"
The next pregnancy, unless the question states otherwise.
Use the same parental cross again: 1/4.
Thinking the first child's genotype changes the next child's Mendelian probability.
Worked check: for two cystic fibrosis carriers, the possible genotypes are CC, Cc, Cc, and cc. If a child is known to be unaffected, cc is excluded, leaving CC, Cc, and Cc. Therefore the probability that the unaffected child is a carrier is 2/3, not 1/2.
Misconception check: "unaffected" is a condition on the sample space. Once the question gives that condition, update the denominator before writing the final probability.
5.2 Dihybrid cross
A cross involving two independently assorting genes. If both parents are heterozygous for two traits:
AaBb × AaBb → 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb
The 9:3:3:1 ratio assumes independent assortment (genes on different chromosomes or far apart on the same chromosome).
5.3 Test cross
Crossing an individual of unknown genotype with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous.
5.4 Chi-squared test
Used to determine whether observed offspring ratios deviate significantly from expected Mendelian ratios.
Chi-squared = sum of [(Observed − Expected)² / Expected]
Where O = observed frequency, E = expected frequency.
Degrees of freedom = number of phenotypic classes minus 1.
Compare the calculated χ2 value against the critical value at the 0.05 significance level.
If χ2 exceeds the critical value, reject the null hypothesis (the deviation is statistically significant).
Exam tip: Always state the null hypothesis explicitly (e.g., "There is no significant difference between observed and expected ratios"). Show all working, including expected values.
Chi-squared expected-count checkpoint
Before calculating χ2, turn the genetic ratio into expected counts. The test compares counts, not just ratio labels.
Step
What to write
Common trap
State the null hypothesis
There is no significant difference between the observed and expected numbers of offspring.
Saying only "the ratio is 3:1" without a statistical claim.
Convert the ratio to expected counts
Multiply the total offspring number by each ratio fraction.
Comparing observed counts directly with 3 and 1.
Calculate each contribution
Use (O−E)2/E for every phenotype class.
Rounding expected values too early.
Make the decision
Compare the final χ2 value with the critical value for the correct degrees of freedom.
Rejecting the null hypothesis just because the observed numbers are not identical to the expected numbers.
Worked check: suppose two carrier parents are expected to produce a 3 unaffected : 1 affected ratio, and a sample of 80 offspring gives 56 unaffected and 24 affected. The expected counts are 60 unaffected and 20 affected. Therefore χ2=(56−60)2/60+(24−20)2/20=16/60+16/20≈1.07. The calculation tests whether the deviation from the expected inheritance ratio is large enough to matter statistically.
Misconception check: the expected ratio tells you the model to test; the observed counts tell you what happened in the sample. Do not change the expected ratio to match the observed data.
6 Mutations and Genetic Diseases
6.1 Point mutations (gene mutations)
Substitution: One nucleotide is replaced by another. Can be silent (same amino acid due to degeneracy of the genetic code), missense (different amino acid, e.g., sickle cell anaemia), or nonsense (premature stop codon, producing a truncated polypeptide).
Insertion or deletion: Addition or removal of one or a few nucleotides causes a frameshift, altering every codon downstream. Frameshift mutations usually produce non-functional proteins.
6.2 Trinucleotide repeat expansions
In Huntington disease, the CAG repeat in the huntingtin gene expands beyond the normal range (36 or more repeats). The expanded polyglutamine tract causes the protein to misfold and aggregate, leading to neuronal death.
The number of repeats can increase across generations (anticipation), resulting in earlier onset in successive generations.
6.3 Chromosomal mutations
Aneuploidy: Gain or loss of individual chromosomes, usually due to non-disjunction during meiosis. Example: Down syndrome (trisomy 21).
Structural changes: Deletions, duplications, inversions, and translocations. Example: Cri-du-chat syndrome results from a deletion on chromosome 5.
6.4 Linking mutations to protein function
Examiners expect you to trace the pathway from DNA change to altered protein to disease phenotype. Practice writing this chain explicitly:
Mutation changes the nucleotide sequence of the gene.
Altered mRNA is transcribed.
Altered polypeptide is translated (or no polypeptide if a nonsense mutation occurs).
Protein structure and function are affected (e.g., enzyme activity reduced, structural protein weakened, channel protein non-functional).
Cellular and physiological consequences produce the disease phenotype.
7 Genetic Testing and Counselling
7.1 Types of genetic testing
Carrier testing: Identifies heterozygous carriers of autosomal recessive conditions (e.g., cystic fibrosis screening before marriage or pregnancy).
Prenatal testing: Amniocentesis (sampling amniotic fluid) or chorionic villus sampling (CVS) can detect chromosomal abnormalities and specific gene mutations in the foetus.
Newborn screening: Routine tests at birth for treatable conditions (e.g., phenylketonuria).
Predictive testing: Identifies individuals at risk of late-onset conditions (e.g., Huntington disease) before symptoms appear.
7.2 Genetic counselling
Genetic counsellors help individuals and families understand test results, inheritance risks, and reproductive options. They provide non-directive advice, meaning they present information without recommending a specific course of action.
7.3 Ethical considerations (frequently examined)
Exam questions often require balanced discussion of benefits and concerns. Structure your answer around these points:
Benefits of genetic testing:
Allows informed reproductive decisions.
Early detection enables preventive treatment or lifestyle adjustments.
Carrier screening reduces the incidence of genetic disorders in populations.
Prenatal diagnosis provides parents with information to prepare for the care of an affected child.
Ethical concerns:
Psychological impact: A positive result for a late-onset untreatable condition (e.g., Huntington disease) can cause severe anxiety and depression.
Privacy and discrimination: Genetic information could be used by employers or insurers to discriminate against individuals.
Informed consent: Testing should only be carried out with full, voluntary, informed consent.
Reproductive autonomy: Prenatal testing raises questions about termination of pregnancy, which involves deeply personal and cultural values.
Eugenics concerns: Population-level screening could lead to societal pressure to eliminate certain genotypes, raising questions about disability rights and human diversity.
Exam tip: When asked to "discuss" ethical issues, present at least two arguments on each side and link them back to the biological context. Avoid one-sided answers.
8 Gene Therapy
8.1 Principle
Gene therapy aims to treat genetic diseases by introducing a functional copy of a defective gene into the patient's cells. The functional gene may restore normal protein production.
8.2 Somatic gene therapy vs germline gene therapy
Feature
Somatic gene therapy
Germline gene therapy
Target cells
Body (somatic) cells of the patient
Gametes or early embryo
Heritability
Changes are not passed to offspring
Changes are inherited by future generations
Current status
Approved and in clinical use for some conditions
Banned in most countries for clinical use
Ethical acceptance
Generally considered more acceptable
Raises significant ethical concerns about consent of future generations
8.3 Vectors for gene delivery
Viral vectors: Modified retroviruses, adenoviruses, or adeno-associated viruses (AAVs) are used to deliver the functional gene. The virus is engineered to be replication-deficient so it cannot cause disease.
Non-viral methods: Liposomes (lipid vesicles) can carry DNA into cells. Less efficient than viral vectors but carry fewer safety risks.
8.4 Limitations and challenges
Targeting: Ensuring the gene is delivered to the correct cell type and integrates into a safe location in the genome.
Expression level: The introduced gene must be expressed at appropriate levels; too much or too little protein can be harmful.
Immune response: The patient's immune system may mount a response against the viral vector, reducing effectiveness and causing side effects.
Insertional mutagenesis: If the gene integrates near an oncogene or disrupts a tumour suppressor gene, it could trigger cancer. This occurred in early gene therapy trials for X-linked SCID.
Duration of effect: In some cases, the therapeutic effect is temporary, and repeated treatments are needed.
Only suitable for recessive conditions: Gene therapy works by adding a functional gene. For dominant conditions where the disease allele produces a toxic product (e.g., Huntington disease), simply adding a normal copy does not silence the mutant allele.
9 How This Topic Appears in Exams
Paper 2 (Structured Questions)
Pedigree analysis: Determine inheritance pattern, assign genotypes, calculate probability of affected offspring.
Genetic crosses: Monohybrid or dihybrid crosses with expected ratios, sometimes including chi-squared calculations.
Mutation-to-phenotype chains: Explain how a specific mutation leads to an altered protein and disease symptoms.
Paper 3 (Free-Response / Essay)
Discuss the ethical implications of genetic testing and screening - a very common essay question. Structure your response with clear biological context (what the test detects, how it works) before moving into ethical arguments.
Compare somatic and germline gene therapy - requires precise definitions, examples, and balanced ethical discussion.
Explain how mutations cause genetic diseases - link point mutations, frameshifts, and chromosomal abnormalities to specific examples.
General exam strategy
Always define key terms before using them in extended responses.
Use named examples (sickle cell anaemia, cystic fibrosis, Huntington disease, haemophilia) to anchor abstract concepts.
In pedigree questions, state your reasoning explicitly at each step. Do not skip logical links.
For ethics questions, use the "benefit then concern" structure with at least two points on each side.
Where can I find the full H2 Biology Notes series? Start at the H2 Biology Notes hub, then follow the Core Ideas and Extension Topics in order.
Is this topic tested in Paper 1 (MCQ)? Yes. Paper 1 includes MCQs on inheritance patterns, pedigree interpretation, and mutation types. However, the more demanding application questions (pedigree analysis with probability calculations, ethics discussions) appear in Papers 2 and 3.
How do I distinguish between autosomal recessive and X-linked recessive in a pedigree? Check whether significantly more males are affected. In X-linked recessive conditions, affected fathers cannot pass the trait to sons (no male-to-male transmission). If the pedigree shows an affected father with an affected son, the trait is autosomal, not X-linked.
Do I need to memorise the chromosomal locations of specific disease genes? The syllabus does not require you to memorise chromosome numbers, but knowing the key examples strengthens your answers. For instance, knowing that cystic fibrosis involves a gene on chromosome 7 and haemophilia involves a gene on the X chromosome helps you explain why these conditions have different inheritance patterns. [1]
What is the most common gene therapy exam question? The most frequently examined angle is comparing somatic and germline gene therapy with reference to ethical issues. You should be able to explain why germline therapy is banned in most countries (the changes affect future generations who cannot consent) and why somatic therapy, while more accepted, still has limitations such as immune responses and insertional mutagenesis.
