Inheritance (A-level only)
AQA
A-Level
A genotype is the genetic constitution of an organism, representing the specific alleles it carries. It can be written for a single gene (e.g., Ff), two genes in a dihybrid cross (e.g., FfGg), or a sex-linked gene (e.g., $X^R X^r$). Alleles are different versions of a gene at a specific locus. While diploid organisms typically have two alleles for each autosomal gene, XY males possess only one allele for X-linked genes (e.g., $X^R Y$). A genotype belongs to an individual, whereas a gene pool describes a population, and a genome is the complete set of genes. The genotype is fixed at fertilisation; cellular differences arise from gene expression, not different alleles.
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Objectives
18
Exam Tips
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Pitfalls
37
Key Terms
32
Mark Points
Subtopics in this area
The genotype is the genetic constitution of an organism.
The phenotype is the expression of this genetic constitution and its interaction with the environment.
There may be many alleles of a single gene.
Alleles may be dominant, recessive or codominant.
In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous.
The use of fully labelled genetic diagrams to interpret, or predict, the results of: monohybrid and dihybrid crosses involving dominant, recessive and codominant alleles crosses involving sex-linkage, autosomal linkage, multiple alleles and epistasis.
Use of the chi-squared ( Χ ) test to compare the goodness of fit of observed phenotypic ratios with expected ratios.
Inheritance (A-level only) Revision Guide
Learning Objectives
What you need to know and understand
- Define genotype in a way that makes clear it describes one organism and the alleles it carries.
- Distinguish genotype, genome and gene pool by saying which level of organisation each one describes.
- Write the genotype of an organism using the allele symbols given in a question, for one gene and for two genes.
- Define phenotype as the expression of the genetic constitution and its interaction with the environment.
- Explain why two organisms with the same genotype can show different phenotypes, naming an environmental factor responsible.
- Explain why two organisms with the same phenotype can have different genotypes, using a dominant characteristic as an example.
- State what is meant by genetic diversity in terms of alleles, and explain how a gene with many alleles increases it.
- List all the genotypes possible for a gene with three alleles and give the phenotype produced by each.
- Explain how mutation increases the number of alleles of a gene in a population over time.
- Explain how a family tree shows that an allele is dominant rather than recessive, quoting the individuals involved and excluding codominance.
- Predict the phenotypes of all three genotypes for a pair of codominant alleles.
- Write correct allele symbols for dominant, recessive and codominant alleles within a genetic diagram.
- Explain why unaffected parents can have a child with a recessive condition, giving the parental genotypes.
- Calculate the probability that the next child of two heterozygous parents is homozygous recessive, and express it as a fraction or a percentage.
- Explain why males cannot be described as homozygous or heterozygous for most X-linked genes.
- Complete a fully labelled genetic diagram for a dihybrid cross, including gametes, offspring genotypes, phenotypes and the expected ratio.
- Write sex-linked genotypes using X and Y chromosome symbols, and explain why a male cannot be a carrier for a sex-linked gene.
- Explain why a cross involving two linked genes produces fewer recombinant offspring than a 9:3:3:1 ratio predicts.
- Calculate the expected numbers of each phenotype from a predicted ratio and a total number of offspring.
- Calculate a chi-squared value from observed and expected counts, showing the working in a table.
- Compare a calculated chi-squared value with the critical value at p = 0.05 and write a conclusion that refers back to the null hypothesis.
Marking Points
Key points examiners look for in your answers
- Defining genotype as the genetic constitution of an organism or the specific alleles it possesses.
- Distinguishing genotype (individual level) from gene pool (population level) and genome (complete set of genes).
- Identifying that diploid organisms have two alleles for autosomal genes, but XY males have only one allele for X-linked genes.
- Distinguishing genotype from phenotype, where the phenotype is the observable expression of the genetic constitution.
- Define phenotype as the expressed characteristics resulting from the genotype and its interaction with the environment.
- Explain that two organisms with the same genotype can show different phenotypes because environmental factors affect how genes are expressed, giving an example such as identical twins differing in mass or cloned plants growing taller in better light.
- Explain that two organisms with the same phenotype can have different genotypes, because a dominant characteristic is shown by both the homozygous dominant individual and the heterozygote.
- Distinguish continuous from discontinuous variation: continuous variation is influenced by both genes and environment, while discontinuous variation is largely determined by genes alone.
- Define an allele as a different version, or different base sequence, of a gene at the same locus.
- State that genetic diversity is the number of different alleles of each gene in a population.
- Recognise that an individual diploid organism carries only two of the alleles available, even when the gene has many alleles in the population.
- Name a multiple-allele example, such as the three ABO alleles I^A, I^B and I^O, and the phenotypes they produce.
- Explain that mutation creates new alleles, so the number of alleles of a gene in a population increases over time.
- Defining dominance as the allele expressed in the heterozygote, and recessiveness as the allele expressed only when homozygous, with the heterozygote showing the dominant phenotype.
- Explaining codominance as both alleles being expressed in the heterozygote, giving a third phenotype, illustrated by I^A I^B blood group AB or C^R C^W roan cattle.
- Using correct notation: a capital and lower-case letter for a dominant/recessive pair, and a shared capital with superscripts for codominant alleles, such as C^R and C^W.
- Distinguishing codominance from multiple alleles: ABO shows both, but codominance concerns expression in the heterozygote while multiple alleles means more than two alleles exist in the population.
- Explaining that dominance does not determine allele frequency, so a dominant allele may be rare and a recessive allele common in a population.
- Defining homozygous as two identical alleles and heterozygous as two different alleles at the same locus on homologous chromosomes.
- Stating that a homozygous organism produces one type of gamete for that gene, while a heterozygote produces two types in equal proportions because of segregation in meiosis.
- Explaining that two heterozygous parents can produce a homozygous recessive offspring, with a one in four probability for each pregnancy.
- Applying the terms to a named gene rather than an organism as a whole, for example stating that a person is heterozygous for the CFTR gene, not that they are heterozygous.
- Recognising that the terms apply to autosomal loci in a diploid organism, and that males have only one allele for most X-linked genes.
- one mark for the correct gametes from each parent, including the Y-carrying gamete in a sex-linked cross
- one mark for the offspring genotypes, which can still be earned by carrying forward incorrect parental gametes correctly
- one mark for all the offspring phenotypes named in words together with the expected ratio
- one mark for using the allele symbols given in the question and writing sex-linked alleles on the X chromosome
- one mark for expected values calculated by applying the predicted ratio to the total number of offspring counted
- one mark for correct substitution into the sum of (O - E) squared divided by E, with the working shown
- one mark for the degrees of freedom given as the number of phenotype classes minus one
- one mark for comparing the calculated value with the critical value at p = 0.05 and stating whether the difference is significant
- one mark for a conclusion written in terms of the null hypothesis, saying whether the difference is due to chance
Examiner Tips
Expert advice for maximising your marks
- 💡Read carefully whether the question asks for a genotype (written in allele symbols) or a phenotype (written in descriptive words).
- 💡If the question supplies specific allele symbols, use those exact symbols; inventing your own can lead to ambiguity.
- 💡If asked whether a characteristic is genetic, environmental or both, look for variation between genetically identical organisms.
- 💡Write the evidence and the explanation as two separate sentences so each mark is easy to award.
- 💡Underline the dominance information in the stem before you start; multiple-allele questions always tell you which alleles are codominant and which is recessive.
- 💡List every possible genotype before answering, because six genotypes from three alleles is easy to lose track of.
- 💡Use the superscript notation the question gives you, such as I^A and I^B, instead of inventing your own letters.
- 💡To infer dominance from a family tree, first check that codominance is excluded; then two parents with the same phenotype producing a different offspring indicates the parents are heterozygous and the offspring homozygous recessive.
- 💡For codominant alleles use the board's notation, one common capital letter carrying superscripts, such as C^R and C^W, and write them clearly.
- 💡Do not write anything you are unsure of next to your correct evidence, because a wrong statement can cancel the mark.
- 💡When a question compares two families, check whether the pattern fits nuclear inheritance from heterozygous parents. Remember that terms like homozygous and heterozygous apply to diploid loci, not to most X-linked genes in males.
- 💡Write homozygous recessive or homozygous dominant in full, because homozygous on its own rarely scores.
- 💡Probability resets with every pregnancy, so write a one in four chance for each child.
- 💡Set the diagram out in the same order every time: parental phenotypes, parental genotypes, gametes, Punnett square, offspring genotypes, offspring phenotypes, ratio.
- 💡If you get the gametes wrong, keep going, because the offspring genotype mark can still be awarded from your own gametes.
- 💡Lay the calculation out as a table with columns for O, E, O - E, (O - E) squared and (O - E) squared divided by E; the method marks come from that table.
- 💡Write down the null hypothesis before you start, even if the question does not ask for it, because your conclusion has to refer back to it.
- 💡Read the critical value from the row for your degrees of freedom and the p = 0.05 column, and quote both numbers in the conclusion.
Common Mistakes
Pitfalls to avoid in your exam answers
- Defining a genetic term using population-level wording (like all DNA in a species) when describing an individual's genotype.
- Confusing genotype with genome; the genome is the complete set of genes, while the genotype refers to the specific alleles present.
- Using unrelated letters for alleles of the same autosomal gene (e.g., G and b), obscuring their relationship as versions of the same gene.
- Treating the environment as irrelevant, so variation in a characteristic such as body mass is explained by genotype alone. Correction: the environment also affects how the genotype is expressed.
- Assuming that every individual showing a dominant characteristic is homozygous dominant. Correction: a heterozygote also shows the dominant phenotype.
- Saying the environment changes the genotype, when it changes how the genotype is expressed. Correction: the genotype stays the same; the phenotype changes.
- Describing a phenotype using allele symbols instead of words. Correction: describe the observable characteristic in words.
- Answering different genes when asked what genetic diversity means; the credit is for the number of different alleles of each gene.
- Writing three allele symbols in one genotype because the gene has three alleles.
- Assuming that with three alleles one must be dominant over the other two, when ABO has two codominant alleles and one recessive.
- Saying that a new allele appears because the organism needs it, rather than by random mutation.
- Confusing the number of alleles in the population with the number of alleles in an individual.
- Arguing from how common a characteristic is; a characteristic can be common because the allele is frequent, not because it is dominant. Correction: dominance describes expression in the heterozygote, not population frequency.
- Describing codominance as blending, so predicting an intermediate colour instead of patches of both colours. Correction: both alleles are expressed, so both characteristics appear, as in roan cattle with red and white hairs.
- Writing codominant alleles as a capital and a lower-case letter, which implies one is recessive. Correction: use a shared capital letter with superscripts, such as C^R and C^W.
- Confusing codominance with multiple alleles. Correction: codominance is about both alleles being expressed in the heterozygote; multiple alleles means more than two alleles exist for the gene in the population.
- Claiming that two parents with the same phenotype producing a different offspring proves dominance. Correction: this pattern can also arise with codominance, for example two roan parents producing red and white offspring, so codominance must be excluded first.
- Describing an organism as homozygous or heterozygous overall, when the terms apply only to a named gene. Correction: state the gene or locus, for example homozygous for the CFTR allele.
- Treating the one in four as a guarantee, so assuming that in a family of four children exactly one will be affected. Correction: each pregnancy carries a one in four chance independently.
- Using heterozygous to mean carrier in a condition caused by a dominant allele, where the heterozygote is affected. Correction: a carrier is heterozygous for a recessive allele and is unaffected.
- Writing pure bred or hybrid instead of homozygous and heterozygous in an A-level answer. Correction: use the correct genetic terminology.
- Forgetting that a male has only one allele for most X-linked genes, so he cannot be described as homozygous or heterozygous for those loci. Correction: refer to hemizygous or state that only one allele is present.
- leaving the gametes out and jumping straight to the offspring genotypes, which throws away an easy mark
- writing sex-linked genotypes as GB or Gg with no X and Y, so the linkage cannot be seen
- giving male offspring two alleles of an X-linked gene
- quoting the ratio actually counted in a small sample instead of the expected ratio
- assuming a 9:3:3:1 ratio when the two genes are linked on the same chromosome or interact via epistasis
- using percentages or proportions in the calculation instead of the actual numbers of individuals
- dividing by the observed value rather than by the expected value
- using the total number of individuals as the degrees of freedom instead of the number of classes minus one
- writing that the results are significant, when the mark is for the difference between observed and expected results being significant
- concluding that a non-significant result proves the genetic diagram is correct, rather than that there is no evidence of a real difference