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A2 · Practice questions · Selective breeding

Choose, cross, repeat.

Six original Cambridge-style questions on artificial selection and the three syllabus examples of selective breeding.

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

Humans are the selection pressure.

01
Recall
[3 marks]

Describe the principles of selective breeding.

  • Humans choose individuals with the desired characteristic(s) to be parents. ✓
  • These are crossed and the offspring showing the characteristic most strongly are selected to breed. ✓
  • This is repeated over many generations, so the frequency of the alleles for the characteristic increases. ✓
02
Explain
[4 marks]

Outline how a variety of rice that is resistant to a fungal disease and also high-yielding could be produced by selective breeding.

  • Cross a high-yielding, susceptible variety with a resistant variety (for example a wild or traditional variety). ✓
  • Select F1 offspring that are resistant. ✓
  • Backcross these repeatedly to the high-yielding variety, each time selecting only resistant offspring (testing by exposure to the fungus). ✓
  • After several generations the variety has the resistance allele(s) and the high-yield characteristics. ✓
03
Explain
[4 marks]

Explain how inbreeding and hybridisation are used to produce maize plants that are both vigorous and uniform.

  • Maize is self-pollinated for several generations to produce inbred lines that are homozygous at most loci. ✓
  • Inbred lines are uniform but show inbreeding depression (small, low yield) because harmful recessive alleles are expressed. ✓
  • Two different inbred lines are crossed; the F1 is heterozygous at many loci and shows hybrid vigour (taller, higher yield). ✓
  • All F1 plants have the same genotype because both parents are homozygous, so the crop is uniform. ✓
04
Explain
[3 marks]

Explain why a dairy farmer cannot select a bull directly for milk yield, and describe how bulls are chosen.

  • Bulls do not produce milk, so the effect of their alleles for milk yield cannot be measured in the bull. ✓
  • Progeny testing: the milk yields of the bull's daughters are recorded and compared with those of other cows. ✓
  • Bulls whose daughters have the highest yields are chosen, and their semen is used for artificial insemination of many cows. ✓
05
Data
[3 marks]

The mean annual milk yield of a breed of dairy cow rose from 4200 dm3 to 9800 dm3 over 50 years. Calculate the percentage increase and suggest two reasons, other than selective breeding, that could contribute to it.

  • Increase = 9800 − 4200 = 5600; 5600 ÷ 4200 × 100 = 133%. ✓
  • Improved diet / feed for the cows. ✓
  • Better veterinary care and disease control, or milking more often / improved housing. ✓
06
Explain
[3 marks]

Compare selective breeding with natural selection.

  • Similarity: both act on existing genetic variation, and both change allele frequencies over generations. ✓
  • Difference: in selective breeding humans choose which individuals reproduce; in natural selection the environment determines which survive and reproduce. ✓
  • Selectively bred features may not increase survival in the wild, and selective breeding often reduces genetic diversity faster. ✓

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.