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CAIE A-Level Biology A2 17.1 Variation Questions

Practise variation questions by classifying trait distributions, explaining genetic and environmental causes and using t-tests to compare sample means.

Syllabus
2028–2030
Course
Biology 9700
Level
A2

Exam points

  • classify continuous or discontinuous variation from categories, ranges or distribution curves
  • explain genetic and environmental causes, including polygenes, alleles and additive effects
  • calculate t-values and compare critical values to decide whether means differ significantly

Covered syllabus points

17.1.1—Genetic and environmental variation17.1.2—Discontinuous vs continuous variation17.1.3—Genetic basis of variation17.1.4—T-test to compare means of two samples

Question 1

[Maximum number: 4]

Within a population, the variation for one characteristic is usually the result of genetic and environmental causes.

Question (a)

(a)

Both genetic and environmental factors contribute to variation in body mass in a bird population.

One cause of variation in body mass in chickens is infection with the gut protoctist Eimeria. When fed with the same diet, infected chickens have a lower gain in body mass than healthy chickens in the same population.

[ 1 ]

Question (i)

(i)

Suggest one environmental factor that affects the growth and reproduction of Eimeria.

[ 1 ]

Question (b)

(b)

A study was carried out to investigate the effect of treating infected chickens with extracts from a plant, Bidens pilosa.

B pilosa is used in traditional medicine for the treatment of some infectious diseases. - Three populations, each containing 25 chickens infected with Eimeria were observed. - The body mass of each chicken was measured at the start of the study. - Each population was given a different treatment for 56 days: - standard diet with no B. pilosa extract - standard diet with low dose of B. pilosa extract - standard diet with high dose of B. pilosa extract. - The body mass of each chicken was measured at the end of the study and the gain in body mass over the 56 days was calculated. - The mean gain in body mass was calculated for each population. The results are shown in Table 3.1.

Table 3.1

Table 3.1

A statistical analysis of the results of the study of these three populations confirmed that there was a significant difference in mean gain in body mass between low dose B. pilosa and high dose B. Pilosa extract treatments.

[ 3 ]

Question (i)

(i)

Sketch on Fig. 3.1 curves to show the pattern of variation for gain in body mass in chickens treated with low dose B. pilosa extract and in chickens treated with high dose B. pilosa extract.

Label one curve 'low dose' and the other curve 'high dose'.

Fig. 3.1

Fig. 3.1

[ 3 ]

Question 2

[Maximum number: 3]

Question (a)

(a)

The sea blush, Plectritis congesta, is a flowering plant that grows on the west coast of North America.

Individual sea blush plants produce fruit that is either winged or wingless. Investigations have shown that this characteristic is controlled by a single gene with two alleles:
- a dominant winged fruit allele
- a recessive wingless fruit allele.

Fig. 3.1 shows the difference in structure between winged fruit and wingless fruit phenotypes.

Fig. 3.1

Fig. 3.1

[ 3 ]

Question (i)

(i)

A large sample of sea blush fruits was collected and their fruit-wing characteristic was recorded.

Name the type of variation that is shown for the fruit-wing characteristic of the sea blush.

[ 1 ]

Question (ii)

(ii)

Different sea blush plants can have fruit with different colours.

Suggest two ways in which a new fruit colour could naturally occur in a sea blush population.

[ 2 ]
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