Patterns of inheritance — OCR A-Level Biology
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Patterns of inheritance explained
Patterns of inheritance covers 11 specification points, including (a) (i) the contribution of both environmental… and (ii) how sexual reproduction can lead to….
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Open a point below for its explanation and revision guidance.
(a) (i) the contribution of both environmental and genetic factors to phenotypic variation
Phenotype is the observable characteristic of an organism, and variation in phenotype can arise from genetic factors, environmental factors, or both acting together. Genetic factors include different alleles and gene mutations, which are inherited and set the range of possible phenotypes. Environmental factors include diet, temperature, light and other conditions that influence how genes are expressed or how an organism develops. Some characteristics, such as blood group, are determined largely by genes, while others, such as body mass in many organisms, are influenced strongly by environment. Many traits show combined effects: genotype sets a potential and the environment determines where within that potential the phenotype falls. Recognising both contributions explains continuous variation and why identical genotypes can produce different phenotypes.
(ii) how sexual reproduction can lead to genetic variation within a species
Sexual reproduction generates variation because offspring receive a new combination of alleles from two parents. Meiosis produces gametes that are haploid and genetically different: crossing over between non-sister chromatids of homologous chromosomes in prophase I, and independent assortment of homologous chromosomes in metaphase I, shuffle alleles. Independent assortment of chromatids in metaphase II adds further combinations. Random fertilisation then pairs one gamete from each parent, so many allele combinations are possible. Mutation creates new alleles in the first place. The result is that members of a species differ genetically, which is the raw material for natural selection. For example, a parent heterozygous at two unlinked loci can produce four gamete types in equal proportions, and fertilisation multiplies the possible offspring genotypes.
(b) (i) genetic diagrams to show patterns of inheritance
Genetic diagrams predict the genotypes and phenotypes of offspring from parental genotypes. For a monohybrid cross, assign alleles (for example A dominant, a recessive), state parental genotypes, set out gametes, then combine them in a Punnett square or cross diagram to give offspring ratios. A cross between two heterozygotes Aa × Aa gives a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. Diagrams must show gametes correctly (each gamete carries one allele) and combine them systematically. Sex linkage, codominance and multiple alleles can also be shown, adjusting the expected ratios. Multiple-choice questions test whether you can read a diagram, deduce parental genotypes or predict offspring proportions accurately.
(ii) the use of phenotypic ratios to identify linkage (autosomal and sex linkage) and epistasis
Phenotypic ratios from genetic crosses act as a diagnostic tool. For two genes on different chromosomes, independent assortment gives the familiar 9:3:3:1 ratio in a dihybrid F₂. When the genes are linked on the same autosome, parental allele combinations tend to be inherited together, so the offspring ratio departs from 9:3:3:1 and shows an excess of the two parental phenotypes with fewer recombinant types. Sex linkage is revealed when reciprocal crosses give different results and when phenotype frequencies differ between the sexes, because males carry one X and express X-linked alleles directly. Epistasis occurs when one gene masks or modifies another; a 9:3:4, 12:3:1 or 9:7 ratio signals interaction rather than independent assortment.
(c) using the chi-squared (χ²) test to determine the significance of the difference between observed and expected results
The chi-squared (χ²) test compares observed counts with expected counts to judge whether a difference is due to chance. First state a null hypothesis that there is no significant difference between observed and expected results. Calculate expected values from the genetic ratio and total, then compute χ² = Σ (O − E)² / E for each class and sum them. Determine degrees of freedom as the number of categories minus one. Compare the calculated χ² with the critical value at the chosen probability level (commonly p = 0.05) for those degrees of freedom. If the calculated value is greater than the critical value, reject the null hypothesis: the difference is significant and not due to chance. If it is less, accept the null hypothesis. The test uses raw counts, not percentages or ratios, and each expected value should be at least five.
(d) the genetic basis of continuous and discontinuous variation
Variation between individuals is either continuous (a smooth range of phenotypes, such as human height or mass) or discontinuous (distinct categories, such as ABO blood group or flower colour). The genetic basis differs. Continuous variation is usually polygenic: many genes, each with a small additive effect, plus environmental influence, give a normal distribution. Discontinuous variation is usually controlled by one gene or a few genes with distinct alleles, so phenotypes fall into clear classes; the environment has little effect on the category. Meiosis also contributes: independent assortment of chromosomes and crossing over create new allele combinations, and random fertilisation adds further genetic variation. Mutation supplies new alleles.
(e) the factors that can affect the evolution of a species
Evolution is a change in allele frequencies in a population over generations. Several factors drive or constrain it. Mutation creates new alleles and so provides the genetic variation on which selection acts. Meiosis, including crossing over and independent assortment, and random fertilisation reshuffle alleles into new combinations. Natural selection then changes allele frequencies when heritable variation affects survival and reproduction: individuals with advantageous alleles reproduce more, passing those alleles on. Directional selection shifts a phenotype in one direction; stabilising selection favours the mean and reduces extremes. Genetic drift, especially in small populations, changes frequencies by chance rather than by fitness, and the founder effect and bottlenecks reduce variation. Gene flow through migration between populations can introduce or remove alleles.
(f) the use of the Hardy–Weinberg principle to calculate allele frequencies in populations
The Hardy–Weinberg principle models allele and genotype frequencies in an idealised population that is large, randomly mating, with no mutation, no migration and no selection. For a gene with two alleles, let the dominant allele frequency be p and the recessive allele frequency be q, so p + q = 1. Genotype frequencies are then p² (homozygous dominant), 2pq (heterozygous) and q² (homozygous recessive), summing to 1. To calculate allele frequencies, start from the observable recessive phenotype frequency, which equals q²; take its square root to obtain q, then p = 1 − q. For example, if 9% of a population shows the recessive phenotype, q² = 0.09, so q = 0.3 and p = 0.7. Allele frequencies are proportions of the gene pool, not counts of individuals, and results are estimates valid only while the assumptions hold.
(g) the role of isolating mechanisms in the evolution of new species
Speciation occurs when populations become reproductively isolated so that gene flow between them stops. Isolating mechanisms prevent interbreeding and so allow the separated gene pools to diverge. Geographical (allopatric) isolation, such as a river or mountain range separating populations, is a common starting point. Once separated, different mutations arise in each population, and natural selection acts differently where environments or selection pressures differ, changing allele frequencies. Over many generations the populations accumulate enough genetic and phenotypic difference that they can no longer interbreed to produce fertile offspring, so they are separate species.
(h) (i) the principles of artificial selection and its uses
Artificial selection is selective breeding in which humans choose which individuals reproduce, so allele frequencies in the population shift towards desired phenotypes. The principles: heritable variation must exist in the trait; humans identify individuals with the desired phenotype and use them as parents; offspring vary, so selection is repeated over successive generations; the population's gene pool changes as favoured alleles increase. Selection is directional towards a human-defined goal, not an environmental one. Uses include increasing crop yield, improving disease resistance in livestock, producing high-yielding dairy cattle, and developing breeds or varieties with particular traits.
(ii) the ethical considerations surrounding the use of artificial selection.
Ethical considerations around artificial selection weigh benefits against harms. Benefits include improved food security, higher yields, disease resistance and economic gains for producers. Concerns include animal welfare where selection for productivity causes health problems, for example extreme conformation or reduced mobility in some breeds; loss of genetic diversity and increased vulnerability to disease when populations become genetically uniform; narrowing of breed and variety range; and questions about whether humans should direct the evolution of other organisms. Ethical reasoning should identify the stakeholder affected (animal, farmer, consumer, environment), state the principle at stake (welfare, autonomy, biodiversity, fairness) and weigh it against the benefit. A balanced answer considers both sides and reaches a justified judgement rather than listing opinions.
Your focus
- Define phenotype and distinguish it from genotype.
- Describe genetic and environmental factors that contribute to phenotypic variation.
- Explain how both factors together can produce variation, including in organisms with identical genotypes.
Show all 33 objectives
- Explain how crossing over and independent assortment during meiosis produce genetically different gametes.
- Describe how random fertilisation combines alleles from two parents to produce variation in offspring.
- Explain why genetic variation within a species is important for natural selection.
- Construct a genetic diagram from given parental genotypes.
- Deduce offspring genotypes, phenotypes and ratios from a cross.
- Adapt genetic diagrams to represent codominance or sex-linked inheritance.
- Distinguish independent assortment, autosomal linkage, sex linkage and epistasis from the phenotypic ratios they produce.
- Interpret a given set of offspring phenotypes to identify the most likely genetic cause.
- Explain why sex-linked ratios differ between male and female offspring.
- Apply the chi-squared test to observed and expected genetic data.
- Determine degrees of freedom and select the appropriate critical value.
- Interpret a chi-squared result to accept or reject a null hypothesis.
- Distinguish continuous from discontinuous variation using phenotypic examples.
- Explain the polygenic and environmental basis of continuous variation.
- Explain the single-gene basis of discontinuous variation and the role of meiosis and fertilisation in generating variation.
- Identify factors that affect the evolution of a species, including mutation, selection, drift, gene flow and isolation.
- Explain how each factor changes allele frequencies in a population.
- Distinguish natural selection from genetic drift in a given scenario.
- Apply p + q = 1 and p² + 2pq + q² = 1 to a two-allele gene.
- Derive q from a given recessive phenotype frequency by taking the square root.
- Calculate p from q and interpret the result as an allele frequency in a population.
- Explain how geographical isolation can lead to separate gene pools.
- Describe how mutation and natural selection cause divergence between isolated populations.
- Explain how reproductive isolating mechanisms result in the formation of new species.
- Explain the principles by which artificial selection changes allele frequencies in a population.
- Identify the selecting agent in artificial selection and distinguish it from natural selection.
- Describe at least two uses of artificial selection and the traits selected for.
- Identify ethical concerns arising from artificial selection and the stakeholders affected.
- Explain how loss of genetic diversity creates risks for selected populations.
- Weigh benefits against concerns and reach a justified conclusion.
Patterns of inheritance exam tips
Marking Points
- Phenotype is the observable characteristic of an organism.
- Genetic factors such as different alleles and mutations contribute to phenotypic variation.
- Environmental factors such as diet, temperature or light contribute to phenotypic variation.
- Many characteristics are influenced by both genetic and environmental factors acting together.
- Identical genotypes can produce different phenotypes when environments differ.
- Some characteristics are determined mainly by genes while others are influenced strongly by environment.
- Meiosis produces haploid gametes, halving the chromosome number so gametes carry one allele of each gene.
- Crossing over between non-sister chromatids of homologous chromosomes in prophase I exchanges alleles and creates recombinant chromatids.
- Independent assortment of homologous chromosomes in metaphase I, and of chromatids in metaphase II, gives many random combinations of maternal and paternal alleles.
- Random fertilisation of two gametes combines alleles from two parents, producing genetically distinct offspring.
- Mutation is the original source of new alleles, which sexual reproduction then reshuffles.
- Genetic variation within a species provides the variation on which natural selection can act.
- Parental genotypes are stated using correct allele symbols, with dominant and recessive alleles distinguished.
- Gametes are shown each carrying a single allele, reflecting segregation during meiosis.
- Gametes are combined systematically in a Punnett square or cross diagram to give all possible offspring genotypes.
- Offspring genotypes and phenotypes are deduced, giving the expected ratio such as 3:1 for a heterozygous monohybrid cross.
- Codominance or sex linkage can be represented by adjusting allele symbols and expected ratios appropriately.
- Independent assortment of two unlinked genes in a dihybrid cross gives an expected 9:3:3:1 phenotypic ratio in the F₂ generation.
- Linkage on the same autosome reduces recombination, so parental phenotypes appear in excess and the ratio deviates from 9:3:3:1, with recombinant classes fewer.
- Sex linkage is identified when reciprocal crosses differ or when phenotype frequencies differ between males and females, because males express a single X-linked allele.
- Epistasis is identified by a modified ratio such as 9:3:4, 12:3:1 or 9:7, showing one gene masking or altering the expression of another.
- Comparing observed ratios with the expected independent-assortment ratio allows the cause (linkage, sex linkage or epistasis) to be distinguished.
- State a null hypothesis that there is no significant difference between observed and expected results.
- Calculate expected values from the expected ratio and the total number of offspring, using counts rather than percentages.
- Compute χ² = Σ (O − E)² / E, squaring each difference, dividing by the expected value and summing across categories.
- Determine degrees of freedom as the number of phenotypic categories minus one.
- Compare the calculated χ² with the critical value at the chosen probability level; if calculated exceeds critical, reject the null hypothesis and conclude the difference is significant.
- Continuous variation is controlled by many genes (polygenic) whose alleles have small additive effects, producing a range of phenotypes.
- Continuous variation is also influenced by the environment, so the phenotype forms a normal distribution rather than discrete classes.
- Discontinuous variation is controlled by one gene or a few genes with distinct alleles, giving clearly separate phenotypic categories.
- Discontinuous variation is little affected by the environment, so individuals fall into distinct groups such as blood group.
- Meiosis increases genetic variation through independent assortment of chromosomes and crossing over between homologous chromosomes.
- Random fertilisation of gametes and mutation as a source of new alleles also contribute to genetic variation.
- Mutation introduces new alleles, increasing genetic variation that selection can act on.
- Meiosis (crossing over and independent assortment) and random fertilisation produce new allele combinations in offspring.
- Natural selection changes allele frequencies when heritable variation affects survival and reproductive success.
- Directional selection shifts the phenotype in one direction; stabilising selection favours the mean phenotype.
- Genetic drift, founder effect and bottlenecks change allele frequencies by chance, especially in small populations.
- Gene flow via migration between populations can introduce or remove alleles.
- Reproductive isolation prevents interbreeding and allows populations to diverge into separate species.
- States that for two alleles p + q = 1, where p and q are the frequencies of the dominant and recessive alleles.
- States that genotype frequencies are p² for homozygous dominant, 2pq for heterozygous and q² for homozygous recessive, summing to 1.
- Identifies that the frequency of the recessive phenotype equals q², so q is the square root of that frequency.
- Calculates p as 1 − q after finding q, and can convert a proportion to a percentage or vice versa.
- Recognises that the calculation gives estimated allele frequencies and depends on assumptions such as a large population, random mating and no selection, mutation or migration.
- States that speciation requires reproductive isolation so that gene flow between populations stops.
- Describes geographical (allopatric) isolation, where a physical barrier separates populations and prevents interbreeding.
- Explains that separated populations experience different selection pressures and accumulate different mutations, so allele frequencies diverge.
- Identifies reproductive isolating mechanisms such as temporal, behavioural or mechanical isolation, or hybrid infertility, that maintain separation.
- Concludes that accumulated differences eventually prevent the populations producing fertile offspring, so they become separate species.
- Heritable variation in the trait must already exist in the population before selection can act.
- Humans select which individuals become parents, based on a desired phenotype, rather than the environment doing so.
- Selected parents pass on alleles, so the frequency of favourable alleles increases over successive generations.
- Selection is repeated across many generations, producing cumulative change in the population.
- Artificial selection is directional towards a human-defined goal, unlike natural selection which follows environmental pressures.
- Uses include improving crop yield, disease resistance, livestock productivity and particular breed or variety traits.
- Identifies benefits of artificial selection such as improved yield, food security and disease resistance, and explains who gains.
- Identifies welfare concerns where selection for productivity or conformation harms the selected animals, giving a specific example.
- Explains loss of genetic diversity and the resulting vulnerability to disease or environmental change.
- Considers the narrowing of breed and variety range and its effect on biodiversity or future breeding options.
- Raises the question of whether humans should direct the evolution of other organisms, linking to a stated ethical principle.
- Weighs competing considerations and reaches a justified overall judgement rather than only listing views.
Examiner Tips
- 💡Name a specific environmental factor rather than saying 'the environment' in general.
- 💡Use an example of a characteristic influenced by both factors to show combined contribution.
- 💡State clearly that genotype sets a potential range and environment influences the actual phenotype expressed.
- 💡Name the stage of meiosis for each source of variation (prophase I for crossing over, metaphase I and II for independent assortment).
- 💡Use precise terms: haploid gametes, homologous chromosomes, non-sister chromatids, random fertilisation.
- 💡Link variation explicitly to its importance for natural selection to gain credit for the 'within a species' clause.
- 💡Check the parental genotypes given in the stem before predicting offspring ratios.
- 💡Confirm each gamete contains one allele, then combine systematically to avoid missing combinations.
- 💡Match the ratio to the cross type, remembering codominance and sex linkage change expected proportions.
- 💡State the expected ratio for independent assortment first, then describe how the observed ratio departs from it.
- 💡When a ratio is given, name the specific modified ratio (for example 9:3:4) and link it to the mechanism it indicates.
- 💡For sex linkage, explicitly compare phenotype numbers in males and females rather than only totals.
- 💡Always write the null hypothesis before any calculation so the final conclusion refers back to it.
- 💡Show the working for each (O − E)² / E term, then the summed total, to secure method marks.
- 💡State the probability level and degrees of freedom used when quoting the critical value, and phrase the conclusion in terms of rejecting or accepting the null hypothesis.
- 💡Identify the number of genes involved first, then decide whether environment smooths the phenotype.
- 💡Link each type of variation to a concrete example, such as height for continuous and blood group for discontinuous.
- 💡When asked about genetic variation, name meiosis processes (crossing over, independent assortment) and random fertilisation explicitly.
- 💡Name the specific factor (mutation, selection, drift, gene flow, isolation) rather than writing 'evolution' vaguely.
- 💡State the direction of effect on allele frequency, for example 'increases the frequency of the advantageous allele'.
- 💡Link each factor to variation and to reproductive success to show the mechanism.
- 💡Write down p + q = 1 and p² + 2pq + q² = 1 before substituting numbers, so the algebra is visible.
- 💡Identify which phenotype frequency is given and decide whether it corresponds to q² or to p² + 2pq before calculating.
- 💡Keep frequencies as decimals through the working and only convert to a percentage at the end, checking that p + q = 1.
- 💡Use the sequence: isolation, no gene flow, different selection pressures, allele frequency change, reproductive isolation, new species.
- 💡Name a specific isolating mechanism and explain how it prevents interbreeding rather than just naming it.
- 💡Link divergence explicitly to mutation and natural selection acting on each population's gene pool.
- 💡State clearly that humans choose the breeding parents and explain why that changes allele frequencies.
- 💡Link each use to a specific desired trait rather than saying only 'better crops' or 'better animals'.
- 💡Contrast artificial with natural selection in one sentence to show understanding of the selecting agent.
- 💡Structure the answer around stakeholders (animal, producer, consumer, environment) to ensure balance.
- 💡Use a concrete example, such as welfare problems from selecting for extreme productivity, to support each concern.
- 💡End with a short justified judgement that follows from the points you have weighed.
Common Mistakes
- Claiming phenotype is determined only by genotype; correction: environmental factors also contribute to phenotypic variation.
- Assuming identical genotypes always give identical phenotypes; correction: differing environments can produce different phenotypes.
- Confusing genotype with phenotype; correction: genotype is the genetic makeup while phenotype is the observable characteristic.
- Thinking sexual reproduction itself creates new alleles: correction — mutation creates new alleles; meiosis and fertilisation reshuffle existing alleles.
- Confusing crossing over with independent assortment: correction — crossing over exchanges sections between non-sister chromatids, whereas independent assortment is the random alignment and separation of whole chromosomes or chromatids.
- Believing offspring are identical to parents or to each other: correction — random fertilisation and meiosis produce genetically distinct offspring.
- Assuming crossing over occurs between sister chromatids: correction — it occurs between non-sister chromatids of homologous chromosomes.
- Putting two alleles in a gamete: correct this by showing each gamete carries only one allele of a gene.
- Assuming a 3:1 ratio always applies: correct this by checking parental genotypes, since other crosses give different ratios.
- Mixing up genotype and phenotype in the diagram: correct this by labelling genotypes with allele symbols and phenotypes separately.
- Assuming any departure from 9:3:3:1 means epistasis: correction is to check first whether parental classes dominate (linkage) or whether the sexes differ (sex linkage) before concluding epistasis.
- Treating a 3:1 ratio as proof of linkage: correction is that 3:1 arises from a monohybrid cross of one gene and says nothing about linkage between two genes.
- Ignoring sex when analysing ratios: correction is to tabulate male and female offspring separately, since unequal sex frequencies indicate sex linkage.
- Believing linked genes always give a fixed ratio: correction is that recombination frequency varies, so the ratio shifts with crossing over rather than being a single constant value.
- Using percentages or ratios instead of raw counts in the calculation: correction is to convert all data to actual numbers of individuals before computing χ².
- Setting degrees of freedom equal to the number of categories: correction is that degrees of freedom equals categories minus one.
- Rejecting the null hypothesis when the calculated value is below the critical value: correction is that a calculated value smaller than critical means the difference is not significant, so the null hypothesis is accepted.
- Omitting the null hypothesis: correction is to state it explicitly before calculating so the conclusion has a clear basis.
- Assuming all continuous variation is purely genetic: correction — environment also contributes, which is why the distribution is smooth.
- Thinking discontinuous variation is caused by many genes: correction — it is typically single-gene or few-gene with distinct alleles.
- Confusing the source of new alleles with reshuffling: correction — mutation creates new alleles, whereas meiosis and random fertilisation recombine existing alleles.
- Thinking evolution happens to an individual during its lifetime; correction: evolution is a change in allele frequencies in a population across generations.
- Confusing genetic drift with natural selection; correction: drift is a chance change in allele frequency, not driven by fitness differences.
- Assuming mutation is always harmful; correction: mutations may be neutral, harmful or advantageous, and are the ultimate source of new alleles.
- Believing selection acts directly on genotype; correction: selection acts on the phenotype, which reflects the genotype and environment.
- Treating the recessive phenotype frequency directly as q rather than q²; the correction is to take the square root first.
- Assuming the dominant phenotype frequency equals p² when it actually equals p² + 2pq; the correction is to use the recessive phenotype to find q².
- Reporting allele frequencies as whole numbers of individuals; the correction is that frequencies are proportions of the gene pool between 0 and 1 (or percentages).
- Forgetting that p + q must equal 1 and producing values that do not sum correctly; the correction is to check the sum after calculating.
- Confusing geographical isolation with reproductive isolation; the correction is that a physical barrier starts separation but reproductive isolation is what maintains separate gene pools.
- Claiming that individual organisms evolve or choose to change; the correction is that allele frequencies in populations change over generations through selection.
- Assuming isolation alone creates a new species immediately; the correction is that divergence through mutation and selection over many generations is required.
- Treating all isolating mechanisms as geographical; the correction is to recognise temporal, behavioural and mechanical reproductive barriers too.
- Thinking artificial selection creates new variation: correction — variation must already exist; selection only changes allele frequencies.
- Confusing artificial with natural selection: correction — in artificial selection humans choose the parents; in natural selection the environment determines survival and reproduction.
- Assuming one round of breeding is enough: correction — cumulative change requires repeated selection over many generations.
- Listing opinions without linking them to a stakeholder or ethical principle: correction — name who is affected and which principle (welfare, biodiversity, fairness) is at stake.
- Treating the issue as one-sided: correction — present benefits and concerns and then weigh them.
- Confusing ethical concerns with scientific limitations: correction — keep welfare, diversity and human responsibility as the ethical focus, not technical feasibility.