Form 5 · Paper 2 essay guide

Variation, Paper 2 essay guide

How Variation appears in Paper 2 essays: themes, a planning grid, a model answer structure and the keyword list.

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Essay themes this chapter supports

Explaining continuous and discontinuous variation using human examples: Define variation as the differences between members of the same species. → Define discontinuous variation and give named examples such as blood group, tongue-rolling and earlobe attachment. → Define continuous variation and give named examples such as height, body mass and skin colour. → Explain the genetic and environmental basis of each type. → Explain which type of graph correctly represents each type and why.
Explaining how meiosis, fertilisation and mutation produce genetic variation: State that genetic variation arises from meiosis, random fertilisation and mutation. → Explain how the independent separation of homologous chromosomes during meiosis shuffles alleles. → Explain how random fertilisation adds a second layer of shuffling. → Explain that mutation is the only source of an entirely new allele. → Use identical and non-identical twins to illustrate the difference between a shuffled and an unshuffled genotype.
Comparing gene mutation and chromosomal mutation with named examples: Define a gene mutation as a change within the base sequence of a single gene. → Name and briefly describe substitution, insertion and deletion. → Define a chromosomal mutation as a change in the structure or number of whole chromosomes. → Use Down syndrome, caused by an extra copy of chromosome 21, as a named example. → State one way the scale of effect differs between the two types.
Explaining how mutation supplies variation for natural selection: State that most mutations are neutral or harmful, but some can be beneficial. → Explain that a mutagen, such as radiation or certain chemicals, can raise the mutation rate. → Use antibiotic resistance in bacteria as the worked example. → Explain that the resistant mutation exists before the antibiotic is applied. → Explain how the antibiotic selects for the resistant type over successive generations.

How to structure your essay

Open with one or two sentences that define the topic and set the scope.
Devote one paragraph to each key concept, in a logical order.
Use precise biological terms and, where relevant, a labelled diagram.
Give an example or state where the process happens in the body or plant.
Close by linking the ideas back to the question.

Planning grid

Question	Answer
Introduction	State what the question asks and define the key term it uses, such as 'variation', 'mutation' or 'natural selection', in one sentence.
Point 1	Name the type of variation or mutation involved and state its defining feature, such as its category structure or its genetic basis.
Point 2	Develop the explanation or comparison the question asks for, such as the graph used, the mechanism of shuffling, or the type of chromosomal change.
Point 3	Support the answer with a specific named example, such as blood group, Down syndrome or antibiotic resistance, explained fully rather than only named.
Conclusion	Draw the ideas together, linking the point back to its significance, such as accurate classification, inheritance, or the raw material for natural selection.

Key terms to include

variation
continuous variation
discontinuous variation
gene
allele
genotype
meiosis
fertilisation
mutation
gene mutation
chromosomal mutation
substitution
insertion
deletion
frameshift
nondisjunction
Down syndrome
karyotype
mutagen
natural selection
antibiotic resistance

A model essay outline

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.

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.