How are crops and livestock improved to give better, more sustainable yields?
The improvement of crops and livestock by breeding, the aims of breeding programmes, inbreeding and the problem of inbreeding depression, crossbreeding and F1 hybrids, the role of genetic technology and genome sequencing, and the use of field trials to test new varieties.
An SQA Higher Biology answer on plant and animal breeding, covering the aims of breeding programmes, inbreeding and inbreeding depression, crossbreeding and F1 hybrid vigour, the role of genetic technology and genome sequencing, and the design of field trials.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this key area is asking
The SQA wants you to explain why crops and livestock are bred, describe inbreeding and inbreeding depression, explain crossbreeding and F1 hybrids, describe the role of genetic technology, and explain how field trials test new varieties.
Aims of breeding
Breeders aim to improve features such as yield, food quality, disease resistance, pest resistance and the ability to thrive in a particular environment or climate. Improving these characteristics is one way to raise food production sustainably, because a higher-yielding or disease-resistant variety produces more food from the same land and inputs.
Inbreeding and inbreeding depression
Inbreeding is useful because it fixes desirable alleles, making the line breed true, but the downside is that it also fixes harmful recessive alleles, which is why inbreeding depression appears over generations.
Crossbreeding and F1 hybrids
Crossing two different breeds, or two different true-breeding lines, can produce F1 hybrids:
- F1 hybrids are often uniform and may show improved characteristics, known as hybrid vigour. The harmful recessive alleles of one parental line are masked by dominant alleles from the other.
- Because the F2 generation shows much more variation (the masked recessive alleles reappear), new F1 hybrids must usually be produced each time by re-crossing the parental lines, rather than by breeding the hybrids together.
Genetic technology
Modern breeding is supported by genetic technology, which speeds up the slow process of conventional breeding:
- Genome sequencing identifies organisms whose genomes contain desirable alleles, so breeders can select parents more precisely without waiting to see the adult characteristics.
- Recombinant DNA technology can transfer a specific desirable gene directly into a crop or animal, even from an unrelated species.
Field trials
A good field trial is designed to give a fair, reliable comparison:
- Control of variables (such as soil, watering and spacing) so plots differ only in the treatment being tested, giving a fair test.
- Replicates, growing each variety in several plots, to take account of natural variability and improve reliability.
- Randomisation of treatments across the plots to remove bias from the results.
Examples in context
Example 1. F1 hybrid maize. Most maize grown commercially is F1 hybrid seed. Two inbred parental lines are crossed each year to produce uniform, high-yielding hybrid seed with strong hybrid vigour. Farmers buy fresh hybrid seed every season rather than saving their own, because the F2 generation would vary widely and lose the vigour. This is a worldwide example of crossbreeding overcoming inbreeding depression to boost food production.
Example 2. Marker-assisted breeding of disease-resistant rice. Plant breeders use genome sequencing to find alleles that give resistance to diseases such as bacterial blight in rice. By screening young plants for the resistance allele rather than waiting for the disease to strike, they can select parents far more quickly than by traditional breeding. This shows how genetic technology speeds up the development of improved, more sustainable crop varieties.
Try this
Q1. Explain what is meant by inbreeding depression. [2 marks]
- Cue. Repeated inbreeding makes harmful recessive alleles homozygous, so they are expressed and reduce vigour and fertility.
Q2. Give two features of a well-designed field trial. [2 marks]
- Cue. Any two of: control of variables, replicates, and randomisation of treatments.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksExplain what is meant by inbreeding depression, and explain why crossbreeding can overcome it to produce vigorous F1 hybrids.Show worked answer →
A 4-mark answer needs the cause of inbreeding depression and the reason hybrids are vigorous.
Inbreeding is the breeding of closely related individuals to make a true-breeding line. Repeated inbreeding tends to cause inbreeding depression, because harmful recessive alleles become homozygous and are expressed, reducing vigour, fertility and survival.
Crossbreeding two different true-breeding lines produces F1 hybrids. Because the two lines carry different alleles, the harmful recessive alleles from one line are masked by dominant alleles from the other, so the hybrids show improved characteristics, known as hybrid vigour.
Markers reward harmful recessive alleles becoming homozygous and expressed, and the masking of these alleles in the hybrid.
SQA Higher 20223 marksDescribe the features of a well-designed field trial used to compare a new crop variety with an existing one.Show worked answer →
A 3-mark answer needs three design features with their purposes.
The trial should control variables, so that the plots differ only in the variety being tested and the comparison is fair.
It should include replicates, growing each variety in several plots, to take account of natural variability and make the results more reliable.
It should use randomisation of the treatments across the plots, to remove bias from the results.
Markers reward the control of variables for a fair test, replicates for reliability, and randomisation to remove bias.
Related dot points
- Food supply, sustainable food production and the demands of a growing human population, the dependence of food production on photosynthesis, the capture and conversion of light energy in the light reactions and carbon fixation, and the factors limiting productivity in crops and livestock.
An SQA Higher Biology answer on food supply and photosynthesis, covering sustainable food production for a growing population, the dependence of food production on photosynthesis, the capture and conversion of light energy and carbon fixation, and the factors that limit productivity.
- The threats that weeds, pests and diseases pose to crop productivity, the characteristics of annual and perennial weeds, the use of chemical control by selective and systemic pesticides, the problems of pesticides, and the use of cultural, biological and integrated pest management.
An SQA Higher Biology answer on crop protection, covering the threats from weeds, pests and diseases, the features of annual and perennial weeds, selective and systemic pesticides and their problems, and cultural, biological and integrated pest management.
- The costs and benefits of intensive and free-range farming, the link between animal welfare and productivity, the use of indicators of poor welfare such as stereotypy and misdirected behaviour, and how observed behaviour is used to assess the welfare of farmed animals.
An SQA Higher Biology answer on animal welfare, covering the costs and benefits of intensive and free-range farming, the link between welfare and productivity, and indicators of poor welfare such as stereotypy, misdirected behaviour, failure in sexual behaviour and altered activity.
- Symbiosis as a co-evolved intimate relationship between members of two species, including parasitism and mutualism, and social behaviour including social hierarchy, co-operative hunting, social defence, altruism, kin selection and the social organisation of insects and primates.
An SQA Higher Biology answer on symbiosis and social behaviour, covering parasitism and mutualism as co-evolved relationships, and social behaviour including social hierarchy, co-operative hunting, social defence, altruism, kin selection and the social organisation of insects and primates.
- The components of biodiversity (genetic diversity, species diversity and ecosystem diversity), the measurement of species diversity from richness and relative abundance, the threats posed by human activity, and the meaning and causes of mass extinction.
An SQA Higher Biology answer on biodiversity and mass extinction, covering genetic, species and ecosystem diversity, how species diversity is measured from richness and relative abundance, the threats from human activity, and the meaning and causes of mass extinction.
Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)