Exam questions · Biology · Selective Breeding and Genetic Engineering
Selective Breeding
- 6 exam questions
- 22 marks
- 9 quick checks
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1 State [2 marks]
State two characteristics that have been selected by selective breeding in food crops.
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Model answer
Any two from: disease resistance, large fruit or grain, high yield, better taste.
Mark scheme
- First characteristic — 1 mark
- Second characteristic — 1 mark
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2 Describe [4 marks]
Describe how selective breeding could be used to produce a variety of wheat with a high yield.
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Model answer
Choose wheat plants that have a high yield. Breed them together. From the offspring, select the plants with the highest yield, and breed from those. Repeat the process for many generations, until the wheat reliably gives a high yield.
Mark scheme
- Choose high-yield plants — 1 mark
- Breed them together — 1 mark
- Select the best offspring — 1 mark
- Repeat over many generations — 1 mark
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3 Calculate [3 marks]
The wheat yield of a field increased from 4.0 tonnes per hectare to 7.8 tonnes per hectare after 40 years of selective breeding. Calculate the percentage increase.
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Model answer
The increase is 7.8 − 4.0 = 3.8. The percentage increase is 3.8 ÷ 4.0 × 100 = 95%.
Mark scheme
- Increase of 3.8 — 1 mark
- Divide by 4.0 and multiply by 100 — 1 mark
- 95% — 1 mark
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4 Explain [3 marks]
Explain why inbreeding in a breed of dog can lead to genetic disorders.
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Model answer
Inbreeding means breeding between close relatives, which are likely to carry the same alleles. This makes it more likely that two copies of a harmful recessive allele will be inherited by the offspring. The offspring then show the genetic disorder.
Mark scheme
- Close relatives carry the same alleles — 1 mark
- Two copies of a harmful recessive allele more likely — 1 mark
- So offspring show the disorder — 1 mark
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5 Explain [4 marks]
Explain how selective breeding reduces the genetic variation in a population, and why this could be a problem.
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Model answer
In selective breeding only a few individuals with the desired characteristic are chosen to breed, so only some of the alleles are passed on and others are lost. The population becomes genetically similar. This is a problem because a new disease or a change in the environment could affect all of the individuals, as none has a different allele that could give resistance.
Mark scheme
- Only a few individuals breed — 1 mark
- Some alleles lost — 1 mark
- Population genetically similar — 1 mark
- A disease could affect all — 1 mark
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6 Evaluate [6 marks]
Evaluate the use of selective breeding to increase the milk yield of dairy cattle.
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Model answer
Selective breeding has greatly increased the milk yield per cow, which means more milk is produced from fewer cows, and this helps to feed more people and can lower costs for farmers. However, the cows are bred from a small number of bulls, so the genetic variation is low, and a disease could affect the whole herd. Cows with very high yields can also suffer health problems such as mastitis and lameness, which is a welfare issue. It also takes many generations to achieve. Overall, selective breeding is useful, but the health and welfare of the animals must be considered, as well as the yield.
Mark scheme
- Higher yield / more food — 1 mark
- Lower costs or fewer cows — 1 mark
- Reduced variation, disease risk — 1 mark
- Welfare / health problems — 1 mark
- Slow — 1 mark
- A reasoned conclusion — 1 mark
Quick check
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1
What is selective breeding?
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B: Humans choosing which organisms to breed
In selective breeding, humans choose which organisms breed so that desired characteristics become more common.
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2
Which is the first step in selective breeding?
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A: Choose parents with the desired feature
The first step is to choose parents that have the desired characteristic.
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3
Why is selective breeding repeated for many generations?
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D: To make the desired feature gradually stronger
The characteristic gradually becomes stronger and more common with each generation.
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4
Which is a drawback of selective breeding?
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C: It reduces genetic variation
Breeding from a few individuals reduces genetic variation.
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5
Which of these came from selective breeding of wild cabbage?
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B: Broccoli
Broccoli, kale and cauliflower all came from the wild cabbage by selective breeding.
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6
What is inbreeding?
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A: Breeding between close relatives
Inbreeding is breeding between close relatives.
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7
A herd's milk yield rose from 4000 to 6000 litres per cow. What is the percentage increase?
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C: 50%
The increase is 2000, and \(2000 \div 4000 \times 100 = 50\%\).
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8
How does artificial selection differ from natural selection?
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D: People choose which individuals breed
In artificial selection, people choose which individuals breed.
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9
A farmer breeds only the sheep with the thickest wool for many years. What is likely to happen to the genetic variation of the flock?
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B: It decreases
Only some alleles are passed on, so the variation in the flock decreases.