Home · A-Level Biology 9700 · Selection and evolution · Variation and the t-test · Practice
A2 · Practice questions · Variation and the t-test

Classes, ranges and t.

Six original Cambridge-style questions on variation and the t-test, from explaining polygenes to deciding whether a difference is significant.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

How many genes, how much environment?

01
Explain
[3 marks]

Explain, with examples, how phenotypic variation can be caused by genetic factors, by environmental factors and by a combination of both.

  • Genetic: different alleles give different phenotypes, for example ABO blood group, which the environment does not change. ✓
  • Environmental: conditions alter the phenotype without changing the genotype, for example a scar, or a plant grown in shade having paler leaves. ✓
  • Combination: the genotype sets a potential range and the environment determines where the individual lies in it, for example height depends on alleles and diet. ✓
02
Recall
[3 marks]

Distinguish between discontinuous variation and continuous variation.

  • Discontinuous: distinct categories with no intermediates; continuous: a range of values between two extremes. ✓
  • Discontinuous: controlled by one gene or a few genes (with large effects); continuous: controlled by many genes (polygenes) with small additive effects. ✓
  • Discontinuous: little or no environmental effect; continuous: strongly affected by the environment. ✓
03
Explain
[4 marks]

Height in a species of plant is controlled by three unlinked genes. At each gene, one allele adds 4 cm to the height and the other allele adds 2 cm. Explain why the heights of the plants show continuous variation.

  • Each plant has six alleles for height (two of each gene), and each tall allele adds the same small amount (additive effect). ✓
  • This gives seven possible totals (0 to 6 tall alleles), so many genotypes give a range of heights from 12 cm to 24 cm in 2 cm steps. ✓
  • Intermediate numbers of tall alleles are most common, so most plants are near the middle. ✓
  • Environmental factors such as light, water and minerals add further variation, so the steps merge into a continuous range. ✓
04
Calculation
[4 marks]

Leaf lengths were measured from 10 leaves in a shady site and 10 leaves in a sunny site. Shady: mean 46.2 mm, standard deviation 3.1 mm. Sunny: mean 41.5 mm, standard deviation 2.8 mm. Use t = |x̄1 − x̄2| ÷ √(s12/n1 + s22/n2) to calculate t. The critical value of t at p = 0.05 for 18 degrees of freedom is 2.10. State your conclusion.

  • s12/n1 = 9.61 ÷ 10 = 0.961; s22/n2 = 7.84 ÷ 10 = 0.784. ✓
  • √(0.961 + 0.784) = √1.745 = 1.32. ✓
  • t = (46.2 − 41.5) ÷ 1.32 = 4.7 ÷ 1.32 = 3.56. ✓
  • 3.56 is greater than 2.10, so the probability that the difference is due to chance is less than 0.05: reject the null hypothesis; the difference in mean leaf length is significant. ✓
05
Explain
[2 marks]

State the null hypothesis for the t-test in the previous question and explain why the degrees of freedom are 18.

  • Null hypothesis: there is no significant difference between the mean leaf lengths in the sunny and shady sites (any difference is due to chance). ✓
  • Degrees of freedom = n1 + n2 − 2 = 10 + 10 − 2 = 18. ✓
06
Analysis
[3 marks]

A student finds that the mean mass of snails from two ponds differs by 3 g, but the t-test gives t = 1.2 (critical value 2.10). Suggest why the difference is not significant and how the investigation could be improved.

  • The masses within each sample are very variable (large standard deviation), so a 3 g difference could arise by chance. ✓
  • The samples may be too small: fewer readings make s2/n larger and t smaller. ✓
  • Improvement: take larger random samples from each pond (and control the time of collection) so that a real difference is more likely to be detected. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.