The shape of the lesson
By the end of the lesson, learners can
- Continuous variation: In the sim a class of 40 people stands on the floor.
- Discontinuous variation: Switch between pea seed shape, pea seed colour and ABO blood group, then press Sort them.
- Mutation makes new alleles: The mutation tab shows one gene as a row of 30 base pairs.
- Continuous: range between two extremes; genes and environment; histogram.
- Discontinuous: a few distinct phenotypes, no intermediates; usually genes only; bar chart.
- Mutation: genetic change; makes new alleles; rate raised by ionising radiation and some chemicals.
- Extended: sources of genetic variation are mutation, meiosis, random mating and random fertilisation.
One picture every idea returns to
Variation is the differences between individuals of the same species. Continuous variation gives a range of phenotypes between two extremes (body length, body mass) and is caused by both genes and the environment. Discontinuous variation gives a limited number of phenotypes with no intermediates (ABO blood groups, pea seed shape and colour) and is usually caused by genes only. Mutation is genetic change and is the way new alleles are formed.
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, No two alike.. Line up any class and no two people are the same height, yet every one of them belongs to exactly one blood group. 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: Continuous variation | In the sim a class of 40 people stands on the floor. Project tab 1 of the lab (Heights of a class of 40) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Discontinuous variation | Switch between pea seed shape, pea seed colour and ABO blood group, then press Sort them. Project tab 2 of the lab (Sorting peas and blood groups) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Mutation makes new alleles | The mutation tab shows one gene as a row of 30 base pairs. Project tab 3 of the lab (Mutagens changing a gene) and let a student drive it while the class predicts. | Jigsaw |
| 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 Mutation makes new alleles, 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.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
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 |
|---|---|
| Continuous variation is not caused by the environment alone. | Set the diet slider to zero in the sim: the heights still vary, because the people have different genes. |
| A feature measured in numbers is not automatically continuous. | Shoe size is recorded in whole sizes, but foot length is really continuous. Ask whether intermediate values exist in nature. |
| Mutagens do not aim at particular genes. | They increase the rate of mutation, but which base changes is random. |
Variation: worksheet
6 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Define variation and state one example each of continuous and discontinuous variation in humans.
A student measured the hand span of 30 classmates.
| Hand span / cm | 16-17 | 18-19 | 20-21 | 22-23 | 24-25 |
|---|---|---|---|---|---|
| Number of students | 2 | 7 | 12 | 6 | 3 |
(a) State the type of variation shown and the type of graph you would draw. (b) Describe the pattern. (c) Suggest why hand span shows this type of variation.
Pea plants produce seeds that are either round or wrinkled. Explain why this is described as discontinuous variation and state what usually causes it.
Describe what is meant by mutation and name two factors that increase the rate of mutation.
A gardener grows plants from cuttings of one parent plant, so they are genetically identical. Some grow in shade and some in full sun. The plants in the sun grow taller. Explain what this shows about the cause of variation in height, and why the plants all have the same flower colour.
Extended. Describe a gene mutation and name the other sources of genetic variation in a population. State which source is the only one that produces new alleles.
Answer key and marking notes
- Variation: differences between individuals of the same species.
- Continuous: body length (height) or body mass.
- Discontinuous: ABO blood group.
- (a) Continuous variation; a histogram (bars touching).
- (b) Most students are in the middle class (20-21 cm) and fewer towards each extreme.
- (c) Hand span is affected by many genes and by the environment (for example diet during growth),
- so every value between the extremes is possible.
- There is a limited number of phenotypes (two).
- There are no intermediates: no seeds are partly wrinkled.
- It is usually caused by genes only, not by the environment.
- Mutation is a genetic change; it is how new alleles are formed.
- Ionising radiation, for example X-rays or gamma rays.
- Some chemicals, for example tar in tobacco smoke.
- The plants have the same genes, so the difference in height must be caused by the environment (light).
- Height shows continuous variation, which is affected by the environment as well as genes.
- Flower colour is (usually) discontinuous and controlled by genes only, and the genes are identical, so the colour is the same.
- A gene mutation is a random change in the base sequence of DNA.
- Other sources: meiosis (it produces gametes that are genetically different from each other),
- random mating and random fertilisation (any gamete can fuse with any other).
- Only mutation produces new alleles; the others create new combinations of existing alleles.
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 variation?
- ADifferences between individuals of the same species
- BDifferences between individuals of different species
- CChanges in an organism as it grows older
- DChanges in the DNA of an organism caused by radiation
The histogram shows the lengths of 50 leaves from one tree. What does it show?
- AContinuous variation, with most leaves between 60 and 80 mm
- BDiscontinuous variation, with six types of leaf
- CContinuous variation, with most leaves between 40 and 50 mm
- DDiscontinuous variation, with most leaves 70 mm long
Which pair are both examples of discontinuous variation?
- AABO blood group and pea seed colour
- BBody mass and pea seed shape
- CBody length and ABO blood group
- DBody mass and body length
Which row correctly states the usual causes of each type of variation?
- AContinuous: genes and environment. Discontinuous: genes only.
- BContinuous: genes only. Discontinuous: genes and environment.
- CContinuous: environment only. Discontinuous: genes only.
- DContinuous: genes and environment. Discontinuous: environment only.
Workers in a hospital X-ray department wear a badge that records how much radiation they receive. Why is it important to limit their exposure?
- AIonising radiation increases the rate of mutation
- BX-rays change a person's blood group
- CRadiation makes all mutations beneficial
- DX-rays cause continuous variation in height
Extended: which statement about a gene mutation is correct?
- AIt is a random change in the base sequence of DNA
- BIt is a change caused only by the environment during growth
- CIt is a planned change made by a cell to survive
- DIt is the swapping of whole chromosomes at fertilisation
Extended: two parents have three children who all look different from each other. Which process does NOT contribute to this genetic variation?
- AMitosis in the cells of the growing children
- BMeiosis when the parents make gametes
- CRandom fertilisation of an egg by any one sperm
- DRandom mating between individuals in a population
Genetically identical cloned calves were raised on two farms. Calves given more food grew to a greater body mass, but all of them had the same coat colour. What is the best conclusion?
- ABody mass is affected by the environment; coat colour here is controlled by genes
- BBody mass is controlled by genes only; coat colour is affected by food
- CBoth body mass and coat colour are caused only by the environment
- DThe calves given more food must have mutated to grow heavier
Quiz key
Variation is the differences between individuals of the same species, for example the different heights of people in a class. Differences between species are what we use to classify organisms, not variation. Changes with age are growth and development. A change in DNA is a mutation, which is one cause of variation rather than variation itself.
Leaf length can take any value between the shortest and the longest, so it is continuous variation, plotted as a histogram with touching bars. The two tallest bars are 60-70 mm (14 leaves) and 70-80 mm (13 leaves). The six bars are measurement classes chosen by the scientist, not six types of leaf. The 40-50 mm class has only 3 leaves, one of the extremes.
ABO blood group (A, B, AB or O) and pea seed colour (yellow or green) each have a limited number of phenotypes with no intermediates. Body mass and body length are continuous, with a range of values between two extremes, so any pair that includes either of them is wrong.
Continuous variation such as body mass is caused by both genes and the environment (diet, exercise). Discontinuous variation such as blood group is usually caused by genes only. It is a common error to swap them, or to say continuous variation is caused only by the environment: genetically different people on the same diet still differ in mass. The environment does not set a person's blood group.
X-rays are ionising radiation, which damages DNA and increases the rate of mutation; some mutations can lead to cancer. Blood group is fixed by genes and is not changed by exposure. Most mutations are harmful or have no effect; radiation does not make them beneficial. Height variation is caused by genes and environment, and radiation is not a normal cause of it.
A gene mutation is a random change in the base sequence of DNA, which may produce a new allele. Environmental effects during growth change the phenotype but not the DNA. Cells do not plan mutations to help them survive; mutations are random. Fertilisation brings together chromosomes from two gametes but does not change the base sequence.
Mitosis produces genetically identical cells for growth and repair, so it adds no genetic variation. Meiosis produces gametes that are genetically different from each other. Random fertilisation means any sperm can fuse with the egg, giving a different combination each time. Random mating in a population also mixes alleles. With mutation, these are the sources of genetic variation.
The calves have identical genes, so the difference in mass must come from the environment (food): body mass is continuous variation, affected by genes and environment. Coat colour did not change with food, consistent with a feature controlled by genes. Saying mass is genes only ignores the result. Coat colour stayed the same, so it was not caused by the environment. Mutations are random and rare; they would not make a whole group heavier.
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.