AQA A-Level Biology: Inheritance, Evolution and Genetics in Depth

Inheritance, evolution and genetics form some of the most demanding -- and rewarding -- content in the AQA A-Level Biology specification. These topics require you to combine a solid understanding of biological mechanisms with confident mathematical skills and the ability to reason through unfamiliar scenarios. Examiners consistently report that students lose marks in this area not because the biology is too hard, but because they do not practise the calculations and extended reasoning enough.

This guide covers everything you need for Topic 7 (Genetics, Populations, Evolution and Ecosystems) on Paper 2, along with the deeper genetics content that can appear across all three papers. Whether you are revising from scratch or fine-tuning your understanding before the exam, this is the resource to work through carefully.

Where This Content Appears in the Exam

AQA A-Level Biology (specification 7402) is assessed through three papers:

• Paper 1 -- Topics 1 to 4. 2 hours, 91 marks, 35% of the A-Level.
• Paper 2 -- Topics 5 to 8. 2 hours, 91 marks, 35% of the A-Level.
• Paper 3 -- Any content from Topics 1 to 8. 2 hours, 78 marks, 30% of the A-Level.

The bulk of inheritance, evolution and population genetics falls within Topic 7 and is examined primarily on Paper 2. However, related genetics content -- such as gene mutations, meiosis, and the molecular basis of inheritance -- is also examined on Paper 1 (Topic 4) and can appear on the synoptic Paper 3. You should therefore treat genetics as a thread that runs through the entire course, not as an isolated block.

Inheritance

Monohybrid and Dihybrid Crosses

A monohybrid cross involves a single gene with two alleles. You must be able to construct genetic diagrams showing parental genotypes, gametes, a Punnett square, and the expected offspring ratios. The classic 3:1 ratio in a monohybrid cross between two heterozygous parents is a starting point, but the exam will push you further.

A dihybrid cross involves two genes, each with two alleles, inherited independently. In a cross between two individuals heterozygous for both genes (AaBb x AaBb), the expected phenotypic ratio is 9:3:3:1. You must be comfortable constructing 4x4 Punnett squares and interpreting the resulting ratios. Practise until this feels routine -- exam questions often layer additional complexity on top of the basic dihybrid framework.

Codominance and Multiple Alleles

Codominance occurs when both alleles in a heterozygous individual are expressed in the phenotype. The ABO blood group system is the most commonly examined example and combines codominance with multiple alleles. The I^A and I^B alleles are codominant with each other, while both are dominant over the i allele. You must be able to work through crosses involving all three alleles and determine possible blood groups of offspring.

When writing genetic diagrams for codominance, use superscript notation to distinguish alleles clearly. Examiners expect precise notation -- sloppy labelling of alleles is a common source of lost marks.

Sex Linkage

Sex-linked genes are located on the X chromosome. Because males have only one X chromosome, a single recessive allele on the X will be expressed in the phenotype. This explains why X-linked recessive conditions such as haemophilia and red-green colour blindness are far more common in males than in females.

In genetic diagrams, represent alleles as superscripts on the X chromosome (for example, X^H for the normal allele and X^h for the haemophilia allele). Females can be carriers (X^H X^h) without showing the condition. Always state the sex of the offspring in your answer -- this is a requirement that students frequently overlook.

Autosomal Linkage

When two genes are located on the same chromosome, they tend to be inherited together. This is autosomal linkage, and it produces offspring ratios that deviate from the expected 9:3:3:1 of independent assortment. Linked genes produce a higher proportion of parental phenotypes and fewer recombinant phenotypes.

Crossing over during meiosis can break linkage, producing recombinant offspring. The frequency of recombination is proportional to the distance between the two genes on the chromosome -- genes that are further apart recombine more frequently.

Epistasis

Epistasis occurs when the allele of one gene masks or modifies the expression of another gene. This produces modified dihybrid ratios. You should be familiar with several patterns:

• 9:3:4 -- recessive epistasis, where the homozygous recessive genotype at one locus masks the expression of the other gene.
• 12:3:1 -- dominant epistasis, where a dominant allele at one locus masks the expression of the other gene.
• 9:7 -- complementary gene interaction, where both genes must have at least one dominant allele for a particular phenotype to appear.

The key to epistasis questions is recognising that the total number of offspring should still add up to 16 (from a dihybrid cross), but the phenotypic categories are grouped differently from the standard 9:3:3:1 ratio.

The Chi-Squared Test

The chi-squared test is used to determine whether there is a significant difference between observed and expected results. In genetics, you use it to test whether offspring ratios match a predicted Mendelian ratio.

The formula is: chi-squared = the sum of (observed - expected)^2 / expected.

To apply it, you calculate the expected numbers from your predicted ratio, compute the chi-squared value, determine the degrees of freedom (number of categories minus one), and compare your value to the critical value at the 0.05 significance level. If your chi-squared value is less than the critical value, you accept the null hypothesis -- there is no significant difference between observed and expected results. If it exceeds the critical value, the difference is significant and you reject the null hypothesis.

Practise this calculation repeatedly. Exam questions often require you to carry out the full test, interpret the result, and explain what it means in biological terms.

Variation

Continuous and Discontinuous Variation

Discontinuous variation produces distinct, separate categories with no intermediates -- for example, ABO blood groups or the ability to roll your tongue. It is usually controlled by a single gene and is largely unaffected by environmental factors.

Continuous variation produces a range of phenotypes with no clear-cut categories -- for example, height, mass, or skin colour. It is typically controlled by multiple genes (polygenic inheritance) and is influenced by both genetic and environmental factors. The distribution of a continuously variable characteristic in a population usually follows a normal distribution curve.

The Role of Genes and Environment

Most characteristics are influenced by both genes and the environment. Identical twins raised in different environments can show different phenotypes for characteristics such as body mass or academic achievement, even though their genotypes are identical. Conversely, genetically different individuals raised in the same environment will still show phenotypic differences. Understanding the interaction between genetic and environmental factors is essential for interpreting data on variation.

Population Genetics and the Hardy-Weinberg Principle

The Hardy-Weinberg principle provides a mathematical model for predicting allele and genotype frequencies in a population. It is based on two equations:

• p + q = 1 -- where p is the frequency of the dominant allele and q is the frequency of the recessive allele.
• p^2 + 2pq + q^2 = 1 -- where p^2 is the frequency of the homozygous dominant genotype, 2pq is the frequency of the heterozygous genotype, and q^2 is the frequency of the homozygous recessive genotype.

To use these equations, you typically start with the frequency of the homozygous recessive phenotype (since this is the only genotype you can identify directly from the phenotype for a dominant-recessive system). Take the square root to find q, then calculate p = 1 - q, and use these values to find the other genotype frequencies.

Assumptions of Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle only holds true when the following conditions are met:

• The population is large.
• Mating is random.
• There is no mutation.
• There is no migration (no gene flow into or out of the population).
• There is no natural selection (all genotypes are equally fit).

In reality, these conditions are rarely met perfectly, so allele frequencies change over time. This is, in fact, the definition of evolution -- a change in allele frequencies within a population over successive generations.

Selection

Natural Selection

Natural selection acts on phenotypic variation within a population. Individuals with characteristics that give them a survival or reproductive advantage are more likely to pass on their alleles to the next generation.

There are three modes of natural selection:

• Stabilising selection -- favours the intermediate phenotype and selects against both extremes. This reduces variation within the population. An example is human birth weight, where very low and very high birth weights are associated with lower survival.
• Directional selection -- favours one extreme phenotype over the other. This shifts the mean of the population in one direction. An example is the development of antibiotic resistance in bacteria, where bacteria with resistance alleles survive and reproduce.
• Disruptive selection -- favours both extremes at the expense of the intermediate phenotype. This can increase variation within the population and may eventually lead to speciation.

Sexual Selection

Sexual selection is a form of natural selection in which individuals with certain characteristics are more likely to obtain mates. Traits favoured by sexual selection -- such as the elaborate tail feathers of a peacock -- may actually reduce survival but increase reproductive success. Sexual selection can drive the evolution of sexual dimorphism, where males and females of a species look markedly different.

Speciation

Speciation is the formation of new species. It occurs when populations of the same species become reproductively isolated, so that gene flow between them ceases. Over time, different selection pressures and genetic drift cause the two populations to diverge genetically until they can no longer interbreed to produce fertile offspring.

Allopatric Speciation

Allopatric speciation occurs when a population is split by a geographical barrier -- such as a mountain range, a river, or rising sea levels. The separated populations experience different environmental conditions and selection pressures, leading to different adaptations. Over many generations, the populations diverge to the point where they are reproductively incompatible.

Sympatric Speciation

Sympatric speciation occurs within a single population without a geographical barrier. This can happen through mechanisms such as polyploidy (particularly in plants), where a mutation doubles the chromosome number and the polyploid individuals can only breed with other polyploids. It can also arise through ecological or behavioural differences that lead to reproductive isolation within the same habitat.

Reproductive Isolation

Reproductive isolation can take several forms:

• Geographical isolation -- physical barriers prevent populations from meeting.
• Ecological isolation -- populations occupy different habitats within the same area.
• Temporal isolation -- populations breed at different times of day, season, or year.
• Behavioural isolation -- differences in courtship behaviour prevent mating between populations.

Genetic Drift, the Founder Effect and the Bottleneck Effect

Genetic drift is the random change in allele frequencies that occurs in all populations but has a much larger effect in small populations. Two specific cases are particularly important:

• The founder effect -- when a small number of individuals colonise a new area, they carry only a fraction of the alleles present in the original population. The new population therefore has reduced genetic diversity, and certain alleles may be at very different frequencies compared to the original population.
• The bottleneck effect -- when a population is drastically reduced in size (by a natural disaster, for example), the surviving individuals may not be genetically representative of the original population. Even if the population recovers in size, genetic diversity remains reduced.

Both the founder effect and the bottleneck effect can accelerate divergence between populations and contribute to speciation.

Evidence for Evolution

Molecular Phylogeny and Comparative Biology

Molecular phylogeny uses DNA and protein sequence comparisons to determine evolutionary relationships between species. The more similar the sequences, the more recently two species shared a common ancestor. Molecular clocks -- based on the assumption that mutations accumulate at a roughly constant rate -- can be used to estimate when species diverged.

Comparisons of specific proteins (such as cytochrome c) or DNA sequences across species provide strong evidence for common ancestry. The universal genetic code itself is evidence that all life shares a common origin.

Fossil Evidence

The fossil record provides a chronological account of life on Earth and shows how organisms have changed over time. Transitional fossils -- such as Archaeopteryx, which shows features of both dinosaurs and birds -- demonstrate links between major groups. While the fossil record is incomplete (since fossilisation requires specific conditions), it remains one of the most important lines of evidence for evolution.

Ecosystems: Energy Flow and Nutrient Cycling

Productivity

Energy enters an ecosystem as sunlight and is converted to chemical energy by photosynthesis. The key terms you need are:

• Gross Primary Productivity (GPP) -- the total rate of energy fixed by photosynthesis in producers.
• Net Primary Productivity (NPP) -- the rate of energy that remains after respiration by the producers. NPP = GPP - R (where R is the respiratory loss). NPP represents the energy available to the next trophic level.

Ecological efficiency -- the percentage of energy transferred from one trophic level to the next -- is typically around 10%. Energy is lost at each trophic level through respiration, excretion, and the parts of organisms that are not consumed.

Nutrient Cycling

Unlike energy, which flows through an ecosystem, nutrients are recycled. You should be familiar with the carbon and nitrogen cycles and understand the role of decomposers and microorganisms in returning nutrients to the soil. Key processes in the nitrogen cycle include nitrogen fixation, nitrification, denitrification, and ammonification, each carried out by specific types of bacteria.

Gene Mutations

Types of Mutation

A gene mutation is a change in the nucleotide sequence of DNA. The three main types are:

• Substitution -- one nucleotide is replaced by another. This may be a silent mutation (if the new codon codes for the same amino acid, due to the degenerate nature of the genetic code), a missense mutation (if it codes for a different amino acid), or a nonsense mutation (if it produces a premature stop codon).
• Insertion -- one or more nucleotides are added to the sequence. This causes a frameshift, altering every codon downstream of the mutation and usually producing a non-functional protein.
• Deletion -- one or more nucleotides are removed from the sequence. Like insertion, this causes a frameshift and typically has a severe effect on protein function.

The effect of a mutation on protein function depends on where it occurs in the gene and whether it changes the amino acid sequence of the protein. Mutations in non-coding regions or those that produce synonymous codons may have no effect at all.

Meiosis in Detail

Meiosis is the type of cell division that produces haploid gametes. It is essential for generating genetic variation, and you must understand the mechanisms involved.

Independent Assortment

During meiosis I, homologous pairs of chromosomes line up at the cell equator. The orientation of each pair is random -- either the maternal or paternal chromosome can face either pole. With 23 pairs of chromosomes in humans, this produces 2^23 (over 8 million) possible combinations of chromosomes in the gametes.

Crossing Over

During prophase I of meiosis, homologous chromosomes pair up and form chiasmata -- points where non-sister chromatids exchange segments of DNA. This produces recombinant chromosomes that carry new combinations of alleles. Crossing over, combined with independent assortment, is the reason why every gamete is genetically unique.

Non-Disjunction

Non-disjunction is the failure of chromosomes to separate properly during meiosis. If homologous chromosomes fail to separate during meiosis I, or if sister chromatids fail to separate during meiosis II, the resulting gametes will have an abnormal number of chromosomes. Fertilisation involving such a gamete produces an individual with aneuploidy. Down syndrome (trisomy 21) is the most well-known example in humans.

Genetic Counselling and Screening

Genetic screening involves testing individuals or populations for the presence of specific alleles associated with genetic conditions. Genetic counselling provides individuals and families with information about the nature, inheritance, and implications of genetic conditions, helping them make informed decisions.

Ethical Considerations

Genetic screening raises significant ethical issues:

• Autonomy and consent -- individuals must be able to make informed, voluntary decisions about whether to be tested.
• Confidentiality -- genetic information is highly personal, and there are concerns about who should have access to it (employers, insurers, family members).
• Psychological impact -- receiving a positive result for a genetic condition can cause significant anxiety, particularly if no treatment is available.
• Implications for reproduction -- screening results may influence decisions about having children, raising complex moral questions.
• Social and economic concerns -- there is a risk of genetic discrimination, where individuals are treated differently based on their genetic makeup.

You should be able to discuss these ethical considerations in a balanced way, presenting multiple perspectives without asserting a single correct answer.

Statistical Tests in Biology

Statistical analysis is a required mathematical skill for AQA A-Level Biology. You need to understand when and how to use the following tests:

Chi-Squared Test

As discussed above, the chi-squared test is used to test for a significant difference between observed and expected frequencies. It is used with categorical data -- for example, testing whether offspring ratios match a predicted Mendelian ratio.

Student's t-Test

The Student's t-test is used to determine whether there is a significant difference between the means of two groups. It is appropriate when the data are continuous and normally distributed. You should understand that a t-value exceeding the critical value at the 0.05 significance level indicates a significant difference.

Correlation Coefficients and Spearman's Rank

Correlation measures the strength and direction of the relationship between two variables. A correlation coefficient ranges from -1 (perfect negative correlation) to +1 (perfect positive correlation), with 0 indicating no correlation.

Spearman's rank correlation coefficient is used when data are not normally distributed or when the relationship is not linear. You rank the data for each variable, calculate the differences between ranks, and apply the formula to obtain the correlation coefficient. You then compare this value to a critical value to determine whether the correlation is statistically significant.

For all statistical tests, you must be able to state the null hypothesis, carry out the calculation, compare your result to a critical value, and state a clear conclusion. Simply performing the arithmetic without interpreting the result will not earn full marks.

Exam Strategy for Inheritance and Genetics Questions

• Show your working clearly. In genetics calculations, draw full genetic diagrams with parental phenotypes, genotypes, gametes, a Punnett square, and offspring ratios. Even if you make an error, correct working can earn method marks.
• Read the question carefully. Identify whether the question involves codominance, sex linkage, epistasis, or simple dominance before you start writing.
• Use correct genetic notation. Distinguish between upper and lower case letters for dominant and recessive alleles. For codominance, use superscript notation. For sex linkage, show alleles on the X chromosome.
• Practise Hardy-Weinberg calculations until they are second nature. These questions are straightforward if you are confident with the algebra, but many students lose marks through careless errors.
• Know when to use each statistical test. The exam may ask you to select an appropriate test, not just carry one out.

Prepare with LearningBro

Ready to put your knowledge to the test? LearningBro offers structured question sets that cover every aspect of inheritance, evolution and genetics for AQA A-Level Biology:

• AQA A-Level Biology: Inheritance and Evolution -- covers monohybrid and dihybrid crosses, codominance, sex linkage, epistasis, Hardy-Weinberg, natural selection, speciation, and ecosystems.
• AQA A-Level Biology: Genetics and Evolution in Depth -- goes further into gene mutations, meiosis, statistical tests, genetic counselling, and the molecular evidence for evolution.

Work through both courses to build the confidence and fluency you need for Paper 2 and the synoptic Paper 3.