The shape of the lesson
By the end of the lesson, learners can
- Three breeding programmes: Choose disease-resistant wheat and press Next generation four times: the orange rust spots become rarer each generation, and the ears shrink in the F1 and then return to full size.
- Maize: inbreeding, then hybrid vigour: Maize is naturally cross-pollinated by wind.
- Dairy cattle: breeding for milk: Milk yield is a continuous, polygenic trait strongly affected by the environment, and only cows produce milk.
- Select parents with the trait, cross, select the best offspring, repeat.
- Wheat and rice: cross with a resistant variety, then backcross to the high-yielder.
- Maize: inbreed to homozygous lines, then cross for uniform, vigorous F1 hybrids.
- Cattle: select cows by yield, bulls by progeny testing; use AI.
One picture every idea returns to
In selective breeding (artificial selection), humans choose individuals with desirable characteristics and allow only these to breed. Over many generations the frequency of the alleles for the desired features rises. It differs from natural selection because the selection pressure is set by humans, and the selected features need not help the organism survive. Disease resistance is introduced into wheat and rice by crossing a high-yielding but susceptible variety with a resistant variety, then backcrossing repeatedly to the high-yielder and selecting resistant, high-yielding offspring. Maize is inbred for several generations to give homozygous, uniform inbred lines, then two lines are crossed (hybridisation) to give F1 hybrids with hybrid vigour that are also uniform. The milk yield of dairy cattle is improved by selecting high-yielding cows and bulls chosen by progeny testing, using artificial insemination. Selective breeding reduces genetic diversity.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, When humans do the choosing.. Farmers have been choosing which plants and animals breed for thousands of years. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Three breeding programmes | Choose disease-resistant wheat and press Next generation four times: the orange rust spots become rarer each generation, and the ears shrink in the F1 and then return to full size. Project tab 6 of the lab (Selection and evolution lab, selective breeding) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Maize: inbreeding, then hybrid vigour | Maize is naturally cross-pollinated by wind. | Pairs |
| 21 to 30 min | Teach and check: Dairy cattle: breeding for milk | Milk yield is a continuous, polygenic trait strongly affected by the environment, and only cows produce milk. | Think-Pair-Share |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Dairy cattle: breeding for milk, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Selective breeding does not create new alleles. | It can only increase the frequency of alleles that already exist in the breeding population or its wild relatives. |
| The vigorous hybrids are not the inbred lines. | Inbreeding makes plants uniform but weak; it is crossing two inbred lines that gives vigour, and the F1 is uniform because both parents are homozygous. |
| A bull does carry alleles affecting milk yield even though he produces no milk. | That is exactly why progeny testing works: his daughters show the effect of the alleles he passes on. |
Selective breeding: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the principles of selective breeding.
Outline how a variety of rice that is resistant to a fungal disease and also high-yielding could be produced by selective breeding.
Explain how inbreeding and hybridisation are used to produce maize plants that are both vigorous and uniform.
Explain why a dairy farmer cannot select a bull directly for milk yield, and describe how bulls are chosen.
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.
Compare selective breeding with natural selection.
Answer key and marking notes
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
Which statement describes the principle of selective breeding?
- Ahumans choose individuals with desired features to breed, and repeat this over many generations
- Bhumans cause mutations in the desired genes so that offspring have the features
- Cthe environment chooses which individuals survive, and humans collect the offspring
- Dindividuals are crossed at random, and the best offspring appear by chance after one generation
A high-yielding wheat variety is susceptible to rust. A wild wheat is resistant but low-yielding. After the two are crossed, why are the offspring backcrossed to the high-yielding variety for several generations?
- Ato recover the high-yield alleles while selecting plants that keep the resistance allele
- Bto remove the resistance allele, which lowers the yield
- Cto make the plants heterozygous at every locus for hybrid vigour
- Dto allow the wild variety to adapt to the farm conditions
Why are inbred lines of maize small and low-yielding?
- Athey are homozygous at many loci, so harmful recessive alleles are expressed
- Bthey are heterozygous at many loci, so the alleles interfere with each other
- Cself-pollination produces mutations that reduce growth
- Dinbred lines contain fewer chromosomes than hybrid maize
Why are farmers keen to grow F1 hybrid maize that is uniform?
- Athe plants reach the same height and ripen together, which suits machine harvesting
- Buniform plants are immune to all diseases
- Cuniform plants can be self-pollinated to give the same crop next year
- Duniform plants need no fertiliser because they show hybrid vigour
How is artificial insemination used in improving the milk yield of dairy cattle?
- Asemen from a bull with high-yielding daughters is used to inseminate many cows
- Bsemen from the cow with the highest yield is used to fertilise other cows
- Cembryos from low-yielding cows are destroyed before implantation
- Dbulls are injected with milk-producing hormones before mating
The bar chart shows the yields of two inbred maize lines, their F1 hybrid and the F2. Which statement explains the results?
- Athe F1 is heterozygous at many loci (hybrid vigour); in the F2 alleles segregate, so some plants are homozygous again
- Bthe F1 has more chromosomes than the inbred lines, and the F2 loses some of them
- Cthe inbred lines are heterozygous, and the F1 is homozygous at all loci
- Dthe F2 yield falls because F1 plants were given less fertiliser
Daughters of bull X have a mean milk yield of 8400 dm3 per year; the herd mean is 7000 dm3 per year. By what percentage do bull X's daughters exceed the herd mean?
- A20%
- B17%
- C1.2%
- D14%
A rice breeder wants a variety resistant to a bacterial disease. Which source of the resistance allele and which disadvantage of the programme are both correct?
- Aa wild or traditional variety; repeated selection reduces genetic diversity in the crop
- Bthe susceptible high-yielding variety itself; the crop becomes more genetically diverse
- Cthe bacterium that causes the disease; the rice becomes unable to reproduce
- Dany plant exposed to the bacterium for a long time; the resistance is not inherited
Quiz key
Selective breeding means humans select parents showing the desired characteristic, breed them, select the best offspring, and repeat for many generations so the frequency of the wanted alleles rises. It works on existing variation; it does not need humans to cause mutations. Survival in the environment is natural selection. Random crossing for one generation would not consistently increase the desired alleles.
Each backcross to the high-yielder increases the proportion of its alleles in the offspring, while selection for resistance each generation keeps the resistance allele, giving a high-yielding, resistant variety. Removing resistance would defeat the purpose. Backcrossing increases similarity to one parent; it does not create heterozygosity at every locus (that is the idea behind maize hybrids). Individual plants do not adapt; the breeder changes allele frequencies by selection.
Self-pollination over several generations makes more and more loci homozygous, so harmful recessive alleles that were masked in heterozygotes are now expressed: inbreeding depression. Inbred lines are homozygous, not heterozygous. Self-pollination does not cause mutations. Inbred and hybrid maize have the same chromosome number; the difference is in which alleles they carry.
Uniform plants have the same height and mature at the same time, so a whole field can be harvested by machine in one pass. Uniformity does not give immunity to disease; a genetically uniform crop may all be susceptible to the same pathogen. Seed from F1 plants gives a variable F2, so farmers must buy new F1 seed. Hybrid vigour increases yield but does not remove the need for minerals from fertiliser.
Bulls proven by progeny testing to have high-yielding daughters provide semen that is stored and used to inseminate many cows, spreading their alleles widely. Cows produce eggs, not semen. Destroying embryos is not part of artificial insemination. Hormone injections into bulls would not change the alleles passed to offspring.
Each inbred line is homozygous at most loci and shows inbreeding depression (2.6 and 3.0 t ha−1). The F1 (P × Q) is heterozygous at many loci, masking harmful recessives, so yield is much higher (9.0). In the F2, alleles segregate, so many plants are homozygous at some loci and yield falls (5.8) and becomes variable. Chromosome number does not change. The inbred lines are homozygous, not heterozygous, and the F1 is heterozygous. The difference is genetic; no treatment difference is described.
Difference = 8400 − 7000 = 1400 dm3; 1400 ÷ 7000 × 100 = 20%. 17% divides by the daughters' yield (1400 ÷ 8400) instead of the herd mean. 1.2% is the ratio 8400 ÷ 7000 = 1.2 misread as a percentage. 14% uses 1400 ÷ 10 000, a wrong denominator.
Resistance alleles are found in wild relatives or traditional local varieties, and intense selection of a few lines narrows the gene pool. The susceptible variety lacks the allele, and selection reduces rather than increases diversity. Selective breeding crosses rice with rice; it does not take genes from the bacterium (that would be genetic engineering), and the bred rice is still fertile. Exposure does not create resistance alleles in a plant; resistance from breeding is genetic and inherited.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.