Form 5 · Worked answers

Variation, worked answers

Fully worked answers for Variation, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Classifying a given variation as continuous or discontinuous.
Explaining how meiosis and fertilisation produce variation.
Describing a cause of mutation and its possible effect.
Sketching and labelling a distribution curve for a continuous variation and a bar chart for a discontinuous variation.
Distinguishing gene mutation from chromosomal mutation, with one named example of each.
Explaining, using antibiotic resistance, how a mutation can give a survival advantage.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

State whether each of the following is an example of continuous or discontinuous variation, giving a reason for each: (a) blood group, (b) body mass.

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(a) Blood group is **discontinuous variation** because every person falls into one of four distinct categories, A, B, AB or O, with no intermediate blood group possible. (b) Body mass is **continuous variation** because it can take almost any value across an unbroken range within a population, with no natural break between one person's mass and the next.

discontinuous variationdistinct categoriescontinuous variationunbroken range

Explain why height is usually shown as a histogram rather than a bar chart.

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Height is a **continuous variation** controlled by many genes together with environmental factors such as diet and exercise, so measured values form an **unbroken range** rather than a small number of separate categories. A **histogram**, whose bars touch, correctly represents this continuous spread and typically produces a **bell-shaped** normal distribution curve, whereas a bar chart with separated bars is appropriate only for a small number of distinct categories, which height does not have.

continuous variationunbroken rangehistogrambell-shaped

Distinguish between a gene mutation and a chromosomal mutation, giving one named example of each.

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A **gene mutation** is a change confined to the base sequence within a single gene, caused by substitution, insertion or deletion of a base, and it may or may not change the protein produced. A **chromosomal mutation** instead changes the structure of a whole chromosome, through deletion, duplication, inversion or translocation of a segment, or changes the total number of chromosomes through **nondisjunction**. An example of a gene mutation is a substitution that alters a single base within a gene; an example of a chromosomal mutation is **Down syndrome**, caused by an extra copy of chromosome 21, giving 47 chromosomes instead of the usual 46.

gene mutationchromosomal mutationsubstitutionnondisjunctionDown syndrome4647

Explain why an insertion or deletion mutation usually has a more serious effect on a protein than a substitution mutation.

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Because bases are read in fixed groups of three during protein synthesis, adding a base (**insertion**) or removing one (**deletion**) shifts every codon that follows the mutated site, an effect called a **frameshift**, which typically scrambles the entire amino acid sequence downstream and usually produces a non-functional protein. A **substitution** changes only the single codon in which it occurs and, because more than one codon can code for the same amino acid, it can sometimes be **silent** and leave the protein completely unaffected, so its effect is generally more limited than a frameshift.

insertiondeletionframeshiftsubstitutionsilentcodon

Explain, using the example of antibiotic resistance, how a mutation can provide a survival advantage.

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A random **mutation** that happens to protect a bacterium from a particular antibiotic can already exist in a population **before** that antibiotic is ever used. When the antibiotic is applied, non-resistant bacteria are killed while the resistant bacterium **survives**, reproduces and passes the resistance allele to its offspring, so resistant bacteria come to **dominate** the population over successive generations. The antibiotic does not cause the mutation to appear; it selects for a mutation that already existed, which is why this is an example of **natural selection**.

mutationantibiotic resistancesurvivesdominatenatural selection

Explain two ways in which genetic variation is produced in a population without any mutation occurring.

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During **meiosis**, homologous chromosome pairs separate independently of one another, so each gamete receives a different, essentially random combination of maternal and paternal chromosomes, shuffling existing alleles. At **fertilisation**, which of the many gametes produced by each parent happens to combine is also a matter of chance, adding a second layer of shuffling. Both processes redistribute alleles that already exist into new combinations rather than creating a new allele.

meiosisindependentlyfertilisationshufflingexisting alleles

Explain why identical twins can still differ slightly from each other despite having an identical genotype.

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**Identical twins** develop from a single fertilised egg that splits into two embryos, so both share exactly the same **genotype** from the start. Because their genes are identical, any difference that later appears between them, such as a slight difference in height or weight, must be caused by **environmental** factors such as diet, prenatal conditions or lifestyle rather than by genetic differences.

identical twinssingle fertilised egggenotypeenvironmental

State two examples of a mutagen and explain what a mutagen does.

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Examples of a mutagen include **ionising radiation**, such as X-rays, gamma rays or ultraviolet light, and certain **chemicals**, such as those found in tobacco smoke. A mutagen is an external agent that damages DNA directly, which **raises the rate** at which mutations occur above the spontaneous rate that occurs during normal DNA replication.

mutagenionising radiationchemicalsraises the rate

Phrasing that earns marks

Continuous variation: Variation with a range of values and no clear categories, such as height or mass, usually controlled by many genes and the environment.
Discontinuous variation: Variation with distinct categories and no in-between, such as blood group, usually controlled by one or a few genes.
Genetic and environmental factors: Variation comes from genes, from the environment, or from both acting together.
Sources of genetic variation: Meiosis and random fertilisation shuffle alleles, and mutation creates new alleles.
Mutation: A change in the DNA (gene mutation) or in the chromosomes (chromosomal mutation) that can be passed on if it occurs in a gamete.
Causes of mutation: Mutations can be spontaneous or caused by mutagens such as radiation and certain chemicals.
Examples of human variation: Tongue-rolling, earlobe attachment and blood group are commonly used discontinuous examples because each person fits into one clear category. Height, body mass and skin colour are common continuous examples because they form an unbroken range with no natural break between one value and the next.
Distribution curves: Continuous variation, when measured in a large sample and plotted as a histogram, typically produces a bell-shaped normal distribution curve with most individuals near the average and fewer at the extremes. Discontinuous variation is instead shown as a bar chart with separate bars for each distinct category, since there are no intermediate values.

Frequently asked questions

What is the difference between continuous and discontinuous variation?

Continuous variation shows a smooth range of values with no distinct groups, such as human height or body mass, and is usually controlled by many genes together with the environment. Discontinuous variation shows distinct categories with nothing in between, such as blood group or tongue-rolling, and is usually controlled by one or a few genes with little environmental effect.

Where does variation come from?

Variation comes from genetic and environmental sources. Genetic variation arises during meiosis, when alleles are shuffled, and at random fertilisation, when different gametes combine; mutation adds entirely new alleles. Environmental variation comes from factors such as diet, climate and lifestyle. Many characteristics result from both genes and the environment acting together.

What is a mutation?

A mutation is a change in the genetic material. A gene mutation is a change in the DNA sequence of a single gene, while a chromosomal mutation changes the structure or number of whole chromosomes. Mutations happen spontaneously or are caused by mutagens such as radiation and certain chemicals. If a mutation occurs in a gamete it can be passed to offspring and become a source of new variation.