Selective breeding questions

Selective breeding syllabus

Questions on selective breeding can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 5.10 understand how selective breeding can develop plants with desired characteristics
• 5.11 understand how selective breeding can develop animals with desired characteristics

How to answer selective breeding questions

1. Describe each stage of the biological technique in order and name the cells, enzymes or vectors involved.
2. Distinguish selective breeding, genetic engineering and cloning before comparing their outcomes.
3. Balance a stated benefit against a specific biological, environmental or ethical limitation.
4. Use evidence from the question when evaluating yield, disease resistance, biodiversity or food security.

Selective breeding example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Comparing egg production as chickens age

Adapted from January 2017, Paper 1B · Question 2(b)

Question 1

The photograph shows a chicken with feathers.

Selective breeding is also used to breed chickens for egg production and chickens for meat production.

The graph shows how age affects the average (mean) number of eggs produced per chicken per year.

The graph shows this information for chickens bred for egg production and for chickens bred for meat production.

Describe how age affects the number of eggs produced by each group of chickens.

Final answer

As chickens get older, the mean number of eggs produced per chicken per year decreases in both groups. It decreases faster in chickens bred for egg production.

Mark scheme points

1. M1 Egg production decreases as chickens get older.
2. M2 The decrease is faster or greater in chickens bred for egg production.

Explanation

Describe the overall trend in both groups first: egg production falls with age. Then compare the lines: the fall is steeper for chickens bred for egg production, so their egg production decreases faster.

Common mistakes

• Saying only that egg production decreases misses the comparison needed for the second mark.

Example 2: Selecting against a harmful recessive allele

Adapted from January 2023 (R), Paper 1B · Question 8(b)(iv)

Question 2

The photograph shows a breed of dog called a border collie.

Some border collies have a genetic condition called sensory neuropathy.

Border collies with sensory neuropathy may be injured as their reflexes do not work well.

Sensory neuropathy is caused by a recessive allele, n. The dominant allele for not having sensory neuropathy is N.

Dog breeders use selective breeding to try to remove harmful alleles from dog breeds.

Explain how selective breeding could be used to remove the allele for sensory neuropathy from a population of border collies.

Final answer

Use a test cross to identify dogs with genotype NN, then breed only these dogs. Select NN offspring to breed and repeat this process over several generations, so dogs carrying the n allele are not used for breeding.

Mark scheme points

Any three of the following points are needed for full marks; they are alternative or compatible parts of a breeding strategy.

1. M1 Use a test cross to identify dogs that are NN (homozygous dominant).
2. M2 Breed using dogs with no family history of sensory neuropathy.
3. M3 Mate dogs that do not have sensory neuropathy, or do not breed from affected dogs. It is also acceptable to mate dogs that did not produce any affected offspring.
4. M4 Allow only homozygous dominant dogs to breed.
5. M5 Select offspring without sensory neuropathy, or offspring that are homozygous dominant, to breed.
6. M6 Repeat the selection and breeding process over several generations.

Explanation

Because n is recessive, a dog with genotype Nn does not have sensory neuropathy but can still pass n to its offspring. So selecting dogs only because they are unaffected may leave the allele in the population. A test cross can help identify dogs that are NN. Breeding only NN dogs, then selecting suitable offspring and repeating this over several generations, reduces the chance that n is passed on.

Common mistakes

• Choosing only dogs that appear unaffected is not enough: some may be Nn carriers.
• The examiner report says many candidates found this question challenging; strong answers specified breeding homozygous dogs without the condition and repeating the process through generations. Include both the genotype and the repeated selection process.

Practise Selective breeding questions

Build a focused practice set from this part of the Edexcel IGCSE Biology syllabus.