AQA A-Level Biology: DNA, Genes and Inheritance — Complete Revision Guide (7402)
AQA A-Level Biology: DNA, Genes and Inheritance
DNA, genes and inheritance is the fourth course on the AQA A-Level Biology (7402) path and the pivot point on which the rest of the specification turns. Every later topic — gene expression, the regulation of cell division, recombinant DNA biotechnology, the evolution of populations, the speciation events that drive ecology — depends on a working command of the molecular machinery of inheritance and the genetic logic of variation. Why does a single base substitution sometimes have no phenotypic effect and sometimes destroy a protein? Why are sex-linked traits expressed differently in male and female offspring? Why do two genes co-segregate when linked and assort independently when not? Why does a single locus epistatically mask the effects of another? Every answer routes back through the molecular and Mendelian vocabulary built here.
Course 4 of 11 on the LearningBro AQA A-Level Biology learning path deliberately co-hosts AQA 7402 Section 3.4 (DNA, replication, the genetic code, protein synthesis, mutations and meiosis) and Section 3.7.1 (inheritance — Mendelian, linkage, sex linkage and epistasis), even though the specification splits them. The pedagogical reason is that the teaching flow from DNA structure through protein synthesis through variation through inheritance is more coherent than the spec's split would suggest. Mendel's laws are most meaningfully introduced once the student understands that genes are sequences of nucleotides, that alleles differ in those sequences, and that meiotic recombination is what allows independent assortment in the first place. Hosting the two sections together is an intentional architectural decision documented in the Phase 2 architecture map.
Guide Overview
The DNA, Genes and Inheritance course is structured as ten lessons that move from molecular structure through the central dogma of protein synthesis to mutations and meiotic variation, then through Mendelian crosses to dihybrid inheritance, linkage and epistasis.
- DNA structure and replication
- Genes and the genetic code
- Transcription
- Translation
- Gene mutations
- Meiosis and genetic variation
- Mendelian genetics
- Dihybrid inheritance
- Sex linkage and autosomal linkage
- Epistasis and polygenic inheritance
AQA 7402 Specification Coverage
This course covers AQA 7402 Section 3.4 in full plus Section 3.7.1 on inheritance. The specification splits these sections, but the LearningBro architecture deliberately co-hosts them because the teaching flow from molecular structure through protein synthesis through meiosis into inheritance is more pedagogically coherent than the spec's separation (refer to the official AQA specification document for exact wording).
| Sub-topic | Spec area | Primary lesson(s) |
|---|---|---|
| DNA replication | 3.4.1 | DNA structure and replication |
| Genetic diversity and gene structure | 3.4.2 / 3.4.3 | Genes and the genetic code |
| Protein synthesis (transcription) | 3.4.4 | Transcription |
| Protein synthesis (translation) | 3.4.4 | Translation |
| Gene mutations | 3.4.5 | Gene mutations |
| Meiosis and genetic variation | 3.4.6 | Meiosis and genetic variation |
| Mendelian inheritance | 3.7.1 | Mendelian genetics |
| Dihybrid inheritance and ratios | 3.7.1 | Dihybrid inheritance |
| Sex linkage and autosomal linkage | 3.7.1 | Sex linkage and autosomal linkage |
| Epistasis and polygenic inheritance | 3.7.1 | Epistasis and polygenic inheritance |
Section 3.4 is examined heavily on Paper 2 (in synoptic combination with respiration and photosynthesis content) and on Paper 3 in extended-response form. Section 3.7.1 inheritance items appear on Paper 2 and especially on Paper 3, where calculations of expected ratios, chi-squared analysis of observed-versus-expected data, and pedigree-chart interpretation are reliable fixtures. The chi-squared test for goodness of fit is the canonical Paper 3 inheritance calculation and is developed further in statistics and practical skills.
DNA Structure and Replication
The DNA structure and replication lesson builds on the molecular foundation laid in nucleic acids to develop semi-conservative replication in mechanistic detail. The helicase enzyme unwinds the parental duplex at a replication fork. DNA polymerase synthesises new strands using the parental strands as templates, with new nucleotides added 5' to 3'. Because the two parental strands are antiparallel, one daughter strand (the leading strand) is synthesised continuously while the other (the lagging strand) is synthesised discontinuously as Okazaki fragments later joined by DNA ligase. The Meselson-Stahl density-gradient experiment is the canonical evidence for semi-conservative replication and is examined as an AO3 evaluation item.
Each daughter duplex contains one parental and one newly synthesised strand. This semi-conservative mechanism ensures that the inheritance of genetic information across generations preserves the parental sequence with fidelity (typically one error per billion bases, after proofreading and mismatch repair). The fidelity of replication is the molecular basis of inheritance and is the necessary substrate for the Mendelian content later in the course.
Genes and the Genetic Code
The genes and the genetic code lesson develops the gene as a sequence of nucleotides coding for a polypeptide (or for a functional RNA), the triplet codon structure of the genetic code (three bases encoding one amino acid, giving 64 possible codons for 20 amino acids), and the code's three defining properties: it is degenerate (most amino acids are encoded by more than one codon), it is non-overlapping (each base is part of only one codon), and it is universal (the same codon table operates in essentially every cellular organism). Degeneracy explains why some single-base substitutions are silent (synonymous): the third base of a codon often varies without changing the encoded amino acid.
The lesson also develops the genome (the complete DNA complement) vs the proteome (the complete protein complement) distinction, and the existence of non-coding DNA — introns within genes (removed before translation), and regulatory and structural sequences between genes. Non-coding DNA is a substantial fraction of the eukaryotic genome and underwrites the regulatory biology developed in gene expression and biotechnology.
Transcription
The transcription lesson develops the synthesis of messenger RNA from a DNA template. RNA polymerase binds to a promoter sequence upstream of the gene, unwinds the local duplex, and uses one strand (the template strand) to direct the addition of complementary ribonucleotides 5' to 3'. The resulting pre-mRNA in eukaryotes undergoes post-transcriptional processing: a 5' cap is added, a 3' poly-A tail is added, and introns are spliced out by the spliceosome to leave only exons in the mature mRNA. Alternative splicing of the same primary transcript can yield multiple mRNA products and therefore multiple polypeptides from a single gene — a fact that re-enters in the proteome-vs-genome reasoning developed in gene expression and biotechnology.
Translation
The translation lesson develops the ribosomal synthesis of polypeptides from mRNA templates. The small ribosomal subunit binds the mRNA at the 5' cap and scans to the start codon AUG. Transfer RNA molecules with anticodons complementary to each codon deliver their cognate amino acids to the ribosomal A site. Peptide bonds form between successive amino acids in the P site, and the ribosome translocates one codon at a time toward the stop codon. The completed polypeptide is released and folds into its tertiary structure under the rules developed in protein structure and function.
The central dogma (DNA → RNA → protein) is the closing summary, with the explicit caveat that the directional flow has documented exceptions (reverse transcription in retroviruses, RNA editing, prion replication) that are not examined at A-Level but that examiners increasingly acknowledge.
Gene Mutations
The gene mutations lesson develops the four principal categories of point mutation — substitution (one base replaced by another), insertion, deletion and duplication — and their phenotypic consequences. A substitution may be silent (no amino-acid change because of code degeneracy), missense (one amino acid changed, with variable functional impact depending on residue and position) or nonsense (a premature stop codon truncating the protein). Insertions and deletions of one or two bases shift the reading frame downstream, with typically devastating consequences for the protein product. Sickle-cell anaemia (a single A-to-T substitution in the beta-globin gene producing a glutamate-to-valine missense at residue 6) is the canonical worked example, returned to in the haemoglobin content of exchange and transport and in the inheritance lessons later in this course.
Mutagens (ionising and ultraviolet radiation, intercalating agents, alkylating agents, deaminating agents) increase the basal mutation rate. The relevance to oncogenesis — somatic mutations in proto-oncogenes or tumour-suppressor genes driving uncontrolled cell-cycle progression — is developed in gene expression and biotechnology.
Meiosis and Genetic Variation
The meiosis and genetic variation lesson develops the two-division reductive cell cycle that produces haploid gametes from diploid germ cells. Meiosis I separates homologous chromosomes (reducing chromosome number from 2n to n); meiosis II separates sister chromatids (analogous to mitosis but acting on haploid cells). Three mechanisms generate genetic variation: independent assortment of homologous chromosomes at metaphase I (2^n combinations for n chromosome pairs, giving over 8 million combinations in humans), crossing over between non-sister chromatids at prophase I (recombining alleles within chromosomes), and random fusion of gametes at fertilisation (squaring the gametic variation in the zygotic population).
Meiotic crossing over is the mechanism that underwrites linkage breakage and the recombination frequencies central to the sex linkage and autosomal linkage lesson later in the course. The cytological observation of bivalents at prophase I — and chiasma formation — is a microscopy and AO3 evaluation staple. The mitotic-index practical from cell cycle and mitosis is the natural cross-reference: meiotic and mitotic figures look similar at low magnification, and the structural distinctions (bivalents, chiasmata, two divisions vs one) must be drilled.
Mendelian Genetics
The Mendelian genetics lesson develops Mendel's first law (the law of segregation: each gamete carries one allele of each gene chosen at random from the two parental alleles) and the monohybrid cross. The vocabulary — gene, allele, locus, homozygous, heterozygous, dominant, recessive, genotype, phenotype, F1, F2 — is introduced with care. Punnett squares give the expected genotypic and phenotypic ratios for monohybrid crosses; the 3:1 phenotypic ratio of the F2 generation from a homozygous-dominant × homozygous-recessive parental cross is the canonical worked example. Codominance and multiple alleles (the ABO blood group system) are included; incomplete dominance (the snapdragon flower-colour example) is included for AO2 application.
Dihybrid Inheritance
The dihybrid inheritance lesson develops Mendel's second law (the law of independent assortment: alleles of different genes are inherited independently of each other) and the dihybrid cross. A dihybrid cross between two heterozygotes at both loci produces a 9:3:3:1 phenotypic ratio in the F2 generation when both loci show complete dominance. Punnett squares with the sixteen gametic combinations are drilled here; pedigree-chart interpretation is introduced; the chi-squared test for goodness of fit is the canonical statistical analysis (developed fully in statistics and practical skills).
The pedagogical importance of the 9:3:3:1 ratio is that deviations from it diagnose either linkage (in which the genes do not assort independently) or epistasis (in which one gene's product masks another's), each developed in the next two lessons.
Sex Linkage and Autosomal Linkage
The sex linkage and autosomal linkage lesson develops the consequences of sex-chromosome and autosomal linkage on inheritance ratios. Sex-linked genes on the X chromosome show different inheritance patterns in male and female offspring: X-linked recessive traits (haemophilia, red-green colour blindness, Duchenne muscular dystrophy) appear more frequently in males because males have only one X and therefore no second allele to mask a recessive phenotype. Pedigree charts of sex-linked inheritance show the characteristic skipping-of-generations pattern and the male-affected, female-carrier transmission route.
Autosomal linkage — two genes on the same autosome — causes co-segregation of alleles unless meiotic crossing over separates them. The recombination frequency between two linked loci is the basis of genetic mapping: closely linked loci recombine rarely (low recombination frequency, expressed as a percentage), more distant loci recombine more frequently, and the recombination frequency in centimorgans approximates physical distance for closely linked genes. The deviation from the dihybrid 9:3:3:1 ratio toward parental-type excess and recombinant deficit is the diagnostic signature.
Epistasis and Polygenic Inheritance
The epistasis and polygenic inheritance lesson closes the course with the two remaining departures from simple Mendelian expectation. Epistasis occurs when one gene's product masks the phenotypic effect of another gene at a different locus. Recessive epistasis (the classic 9:3:4 ratio in the F2 generation of a coat-colour cross) and dominant epistasis (the 12:3:1 ratio in poultry plumage) are the two AQA-examined patterns. Each is a logical consequence of one gene encoding an enzyme on a metabolic pathway whose product is the substrate for a second gene's enzyme.
Polygenic inheritance — the contribution of many genes of small individual effect to a single continuous trait (height, skin pigmentation, blood pressure) — generates the bell-shaped distributions seen for quantitative traits in populations. The contrast with discontinuous, single-locus Mendelian traits is examined as an AO2 reasoning item. The genetic-and-environmental interactive variance underwrites the heritability and selection content developed in populations and ecosystems and in the Hardy-Weinberg material in statistics and practical skills.
Synoptic Links Across the Specification
DNA, genes and inheritance is the molecular and Mendelian pivot of AQA 7402. The nucleotide chemistry developed in nucleic acids is the structural prerequisite for replication; the protein-folding rules developed in protein structure and function are the structural consequence of translation; the cell-cycle vocabulary from cell cycle and mitosis is the immediate prerequisite for meiosis. Downstream, every content area of gene expression and biotechnology — gene regulation, epigenetics, polymerase chain reaction, gel electrophoresis, recombinant DNA, gene therapy — is a direct application of the molecular machinery built here. The population genetics, Hardy-Weinberg and speciation content in populations and ecosystems extends the Mendelian framework from individual crosses to allele frequencies in populations.
Required Practical Anchors
This course does not own a dedicated AQA required practical; the mitotic-index practical (RP2) in cell cycle and mitosis is the natural microscopy cross-reference, with the explicit caveat that meiotic stages must be distinguished from mitotic stages by their structural signatures (bivalents, chiasmata, two reductive divisions). The chi-squared analysis of observed-versus-expected inheritance ratios is the statistical anchor for this course and is developed in full in statistics and practical skills.
Revision Strategy
Inheritance is the highest-density calculation-style content on the AQA 7402 path and rewards a heavily drilled question-answering revision habit. Build a Punnett-square workflow that takes you from parental genotypes to gametes to F1 to F2 phenotypic ratio in under two minutes. Drill it on monohybrid, dihybrid, codominant, sex-linked and epistatic problems until each ratio is automatic. Practise the chi-squared test calculation in batches of ten: degrees of freedom equals the number of categories minus one; the critical value at 5 percent significance for one degree of freedom is 3.84 and for three degrees of freedom is 7.82; chi-squared values above the critical value reject the null hypothesis that observed equals expected.
For the molecular content, sketch the replication fork from memory each week with helicase, DNA polymerase, leading and lagging strands, Okazaki fragments and DNA ligase labelled. Do the same for the ribosomal A and P sites, the spliceosome and the central-dogma flowchart. Interleave inheritance questions with population genetics questions, because Paper 3 will reliably combine the two through Hardy-Weinberg calculations.
Closing
DNA, genes and inheritance is the pivot of AQA A-Level Biology and the foundation on which gene expression, biotechnology, population genetics and evolutionary biology all rest. The course's deliberate co-hosting of specification Sections 3.4 and 3.7.1 reflects the pedagogical reality that Mendelian inheritance is most meaningfully understood once the molecular basis of alleles and meiotic recombination is in place. Start with the DNA, Genes and Inheritance course and work through all ten lessons in sequence; drill Punnett squares and chi-squared calculations until they are automatic; treat the central-dogma diagram and the meiotic-variation mechanisms as transferable schemas that recur across the rest of the path. The full AQA A-Level Biology learning path walks the whole sequence end-to-end.