Science at key stage 3    (Year 9)

• Review pupils' ideas on variation from earlier work by asking them about ways in which humans differ from each other. Ask pupils which of these variations are likely to have been inherited from their parents, prompting them initially by showing pictures of large family groups used in unit 7D 'Variation and classification'. Extend the work to other organisms, eg farm animals, garden flowers, and ask pupils to identify characteristics that might be inherited.

• Ask pupils to suggest how offspring inherit characteristics from their parents, reminding them of the work on reproduction in unit 7B 'Reproduction' and unit 7A 'Cells'. Establish, eg using video clips, software simulation or diagrams, that during fertilisation animal and plant cells pass on 'information' in the nuclei from one generation to another. Introduce the terms 'gene' and 'genetic information' and explain them in simple terms, eg genes are instructions that control the characteristics that develop; the nucleus contains the thousands of genes needed to produce an individual. Help pupils to associate genes with particular inherited characteristics.
• Ask pupils questions to check understanding, eg
• Why are brothers and sisters similar?
• How are identical twins formed?
• Why are identical twins more similar than brothers and sisters?
• Can identical twins be different sexes?
• Will multiple births, when eggs are fertilised outside the body and then implanted, produce identical or similar offspring?
• How does fertilisation occur in plants?
• Ask pupils to record this discussion by producing a sequence of diagrams to illustrate the processes of production and fertilisation of the sex cells in both plants and animals, annotated to describe what is happening.
• Remind pupils of the structure of sperm and egg cells and how they are specialised for their functions.
• Elicit pupils' ideas about whether identical twins really are identical and how any differences between them arise.

• Show pupils specimens of the same variety of crop, eg tomatoes, garden beans, potatoes, and ask them to investigate variation within the variety. Ask pupils to suggest characteristics that vary, eg mass, length, diameter, and make measurements of a sample, tabulating data and presenting results, eg as a frequency graph.
• Ask pupils to compare the distribution and spread of results obtained from one variety with those obtained from another variety of the same species, and to decide whether the variation within a variety is greater or less than that between varieties. Point out to pupils that many varieties differ in ways that are not immediately observable, eg resistance to disease, flavour, ripening time, and ask them to think about why these factors might be important in growing crops.

• Remind pupils of the earlier discussion on twins and of the results of the previous investigation. Challenge pupils to explain why there is variation between individuals with identical or very similar genetic information and to list environmental factors that may lead to variation.

• Show pupils images, eg photographs, video clips, of different breeds, eg of dog. Ask them to identify why they are considered to be separate breeds, using questions, eg What are their unique features? What features do they share? How do breeders ensure they get the right kind of puppy? What happens if you cross a labrador with a dalmatian? Expand the discussion by asking how these breeds came about. Develop the ideas by building on pupils' notions that breeds have been produced by humans selecting dogs with particular inherited characteristics for breeding, and relate these ideas to pupils' knowledge about cells.
• Ask pupils why humans have gone to this trouble. Explain that although some varieties were bred for decorative features, most were originally functional in nature. Ask pupils to use secondary sources to find out the reasons different varieties were bred. Help pupils to summarise key points.

• Provide pupils with a series of true/false statements about individual characteristics, inherited and environmental variations, fertilisation and genes. Ask pupils in groups to agree whether they are true or false, providing an explanation for their decision. Discuss pupils' decisions with them, using those they found particularly challenging as a way of identifying difficulties. Use the outcomes to summarise work so far.

• Explain to pupils that modern farm animals have been bred by humans to have characteristics that are desirable for stock management, or in the food produced. Ask pupils to suggest what particular features may have been desirable in breeding, eg dairy cattle, pigs, beef cattle, sheep, poultry.
• Remind pupils, eg by showing some examples of pupils' work, how to make precise links between ideas when they are putting together an explanation or argument, and about skimming texts to see whether they are useful or not.
• Provide secondary sources of information for pupils to use to find out more about breeds of domestic and farm animals. They could be asked to focus on specific issues, eg
• how 'desirable' features have changed through the ages
• the origins of domesticated farm animals
• how desirable features may be very local depending on the environment, such as short-legged sheep for upland hills
• how some breeds are now no longer used on farms but are still protected to ensure that useful genes are not lost
• Ask pupils to write an account of their findings about one of the questions, supported by pictures and/or diagrams.

• Show pupils sets of varieties of food crops, eg plum, beefsteak and cherry tomatoes; curly, cos and iceberg lettuces, and explain that selective breeding is also used to produce new plant varieties. Ask pupils to think about less obvious characteristics which may be important to plant breeders, eg resistance to cold, so crops can be planted early; sweetness, colour and texture in fruit crops; long shelf life for supermarket trade and overseas shipping. Help pupils to produce a list of characteristics that selective breeding of plants might achieve.

• Review pupils' understanding of the process of pollination in a flowering plant and what happens during fertilisation, prompting if necessary by showing pupils specimens of flowering plants and images of pollen and ovules, eg photomicrographs or microscope slides.
• Ask pupils to suggest how a plant breeder could ensure that pollen from one particular flower was used to pollinate another flower, ensuring that no other pollen could be involved, and taking account of the fact that pollen and ovules may not ripen at the same time. Ask pupils to work in groups to produce a set of instructions for the process for use by a trainee breeder.
• Ask groups to share and evaluate each other's ideas. Provide pupils with an account of how a plant breeder ensures selective pollination.

• Ask pupils to investigate effects of selective breeding in plants, eg by comparing the characteristics of garden peas of different varieties in terms of size, mass, colour, taste, cooking time. Ask pupils to report on variation within each variety and to provide comparisons between the varieties. Different groups could investigate different questions and pool the results. Invite pupils to make value judgements on the 'best' type of pea.

• As an extension, ask pupils what they understand by the term 'clone' and where they have heard it, eg science fiction, Dolly the sheep. Explain the principles of cloning, emphasising that it differs from sexual reproduction in that all genetic information comes from one parent. Ask pupils to describe the consequences of this for variation between parents and offspring.
• Establish with pupils that methods of asexual reproduction, eg cuttings, grafting, have been used for many years with plants.
• Ask pupils to suggest five questions that they would ask a scientist about cloning, given the opportunity, and to explain why they think they are important. Help pupils to evaluate and refine their questions and to use some of the many internet sites providing information on the process to find the answers. Ask pupils what they have found and make a brief summary.

• Provide pupils with a set of structured questions covering the work in this unit. Ask them to use their books and other sources of information to answer the questions.
• Discuss their answers with them, identifying particular difficulties and, if appropriate, provide a set of exemplar answers.