Continuous vs discontinuous variation
Variation is the differences between individuals of the same species. Continuous variation shows a range of values between two extremes with no distinct categories (height, body mass) — usually many genes plus the environment, plotting as a bell-shaped curve. Discontinuous variation shows a few distinct, separate categories with no intermediates (ABO blood group, sex) — usually a single gene, little or no environmental effect, plotting as separate bars.
Causes of variation — genetic, environmental, or both
Genetic causes are the alleles inherited from parents, shuffled into new combinations by sexual reproduction; this variation is inherited. Environmental causes are the conditions met during life — diet, light, temperature, exercise — giving a scar, a suntan or a plant stunted by poor soil; this variation is not inherited. Most characteristics, especially continuous ones like height, are shaped by both; discontinuous ones like blood group are usually genes alone.
(Extended) Mutation — the source of new alleles
Mutation is a genetic change in the base sequence of DNA. Mutations arise randomly, whether or not the new allele proves useful, and are the ultimate source of every new allele and so of all new genetic variation — meiosis and sexual reproduction only reshuffle alleles that already exist. The sickle cell allele is a mutation of the haemoglobin gene: gives severe anaemia, while carriers stay healthy and resist malaria — heterozygous advantage.
Drawn from real examiner reports.
Continuous ≠ discontinuous
Continuous variation is a range of values between two extremes with no distinct categories — height can be any value from short to tall. Discontinuous variation is a small number of separate categories with no intermediates — a blood group is A, B, AB or O, never half A and half B. Examiners report the two terms being swapped over sitting after sitting.
Continuous vs discontinuous variation confused (cambridge-0610.md Section 17, s23 Papers 12/13 Q35; s22 Paper 32 Q5a ii).
Defining the word with itself
"Continuous variation is variation that keeps happening" and "discontinuous variation is variation that is not continuous" are circular and score zero. Name the distinguishing feature instead — a range with no categories, or distinct categories with no intermediates — and add an example plus whether one gene or many genes control it.
Continuous and discontinuous variation described only in vague terms (cambridge-0610.md Section 17, s23 Papers 12/13 Q35).
Grouped data is not discontinuous
Sorting people into short, medium and tall does not make height discontinuous. Those boundaries are an arbitrary human labelling laid over an unbroken range, and someone always sits between them. A characteristic is discontinuous only when nature itself supplies the separate categories with nothing in between, as ABO blood group does.
(Extended) Mutation ≠ what it causes
Asked to define mutation, candidates give a possible result — "a disease appears" or "a new feature develops". The definition is a genetic change in the base sequence of DNA. Consequences (the protein a gene codes for may change, harmfully, neutrally or beneficially) belong in the explain part that follows, never inside the definition itself.
Definition of mutation given as a possible result of mutation instead of the definition itself (cambridge-0610.md Section 17, s22 Paper 33 Q3c i).
(Extended) The antibiotic did not cause it
"The antibiotic triggers the mutation" and "bacteria mutate on purpose to gain resistance" both score zero. Mutations happen randomly, and a resistance allele can already exist in the population before the antibiotic ever arrives. The antibiotic only selects for the bacteria that carry it. Keep random cause and later selection as two separate steps.
Misconception that mutations are caused/triggered by the selective agent rather than arising randomly beforehand (cambridge-0610.md Section 17/10, w22 Paper 41 Q4b i).
(Extended) Carriers do not have the disease
Heterozygous carriers are usually healthy and resist malaria; only homozygous individuals suffer sickle cell anaemia. Writing that carriers "have sickle cell" destroys the whole heterozygous-advantage argument, because the allele persists precisely because carriers stay well enough to survive and reproduce.
Heterozygous carriers of the sickle-cell allele resist malaria — not widely known (cambridge-0610.md Section 17, s22 Paper 23 Q33).
Classify, then justify
Naming "continuous" or "discontinuous" earns at most one mark. Add the defining feature (a range with no categories, or distinct categories with no intermediates), the example named in the question, and whether many genes or a single gene control it.
Name the environmental factor
"Affected by the environment" is too vague to credit. Name a factor that fits the context — diet or food availability for body mass, light or soil minerals for plant height — and pair it with "controlled by many genes" for any continuous characteristic.
(Extended) Write the mutation chain in order
Random change in the DNA base sequence → a new allele arises → the allele may be harmful, neutral or beneficial → if beneficial here, carriers survive, reproduce and pass it on. The marks sit in the order of those links, so write a chain, not a paragraph.
(Extended) Sickle cell: give both pressures
A full heterozygous-advantage answer names selection against (severe anaemia, often death before reproducing) AND selection for (malaria resistance). Describing only one side caps the mark, however well it is written.
Variation: the differences between individuals of the same species.
Continuous variation: a characteristic showing a range of values between two extremes, with no distinct categories (e.g. height, body mass). Usually controlled by many genes (polygenic) AND affected by the environment. Plotting continuous data gives a bell-shaped (normal-distribution) curve.
Discontinuous variation: a characteristic showing a small number of distinct, separate categories, with no intermediate values (e.g. ABO blood group, sex). Usually controlled by a single gene, with little or no environmental influence.
(Extended) Mutation: a genetic change in the base sequence of DNA. Mutations occur randomly and spontaneously; they are the ultimate source of all new alleles, and therefore the ultimate source of new genetic variation.
Define variation.
(a) Define variation.
(b) State whether each of the following characteristics shows continuous or discontinuous variation: (i) body mass in a population of mice (ii) ABO blood group in a population of humans
