Offline Classroom Activities — Science 8
Term 1 · Week 4 · CG Quarter 1, competency 5 · Predicting offspring ratios
Three individual and two group activities. No electricity, nothing to buy, no laboratory equipment. All five are written for a room of 45 or more with fixed desks.
One caution before any of these. Do not ask learners about traits in their own families. A class survey of the room is fine and useful; a homework task asking about parents is not, because some learners are adopted, some do not know a parent, and none of them should have to say so in front of the class to explain a blank worksheet.
Activity 1 — Six Crosses, Twelve Answers (individual)
| Time | 20 minutes |
| Materials | Paper and pencil |
| Preparation | Five minutes: write the six crosses on the board. |
The six crosses
T = tall, dominant. t = semi-dwarf, recessive.
1. Tt × Tt · 2. TT × tt · 3. Tt × tt · 4. TT × Tt · 5. tt × tt · 6. TT × TT
Instructions
- Draw and complete a Punnett square for each.
- For each one, write two ratios: the genotype ratio and the phenotype ratio.
- Circle every cross where the two ratios are the same, and say why.
- Circle in a different way every cross where every offspring has the same genotype.
Why steps 3 and 4 are the activity
Steps 1 and 2 are drill. Steps 3 and 4 ask the learner to look across all six and notice something — that the two ratios coincide only when there is nothing hidden to hide, and that a cross produces uniform offspring only when at least one parent has nothing to vary.
A learner who does steps 1 and 2 correctly and cannot do 3 has the procedure without the idea, which is the exact failure this week is written against.
Differentiation
| One step down | Three crosses — 1, 3 and 5 — and one ratio each, the phenotype one. |
| One step up | Add a seventh cross where the genotypes are unknown: a tall plant crossed with a semi-dwarf gives half tall and half semi-dwarf offspring. What was the tall parent? (Tt.) |
Assessment
| Seen | What it means |
|---|---|
| All twelve ratios right, and steps 3 and 4 answered | The competency is met with understanding |
| Twelve ratios right, steps 3 and 4 blank | Procedure only. Ask one of them aloud on Monday |
| One ratio given per cross | Genotype and phenotype have not separated. This is the most common error and it is not carelessness — the words are new |
Activity 2 — A Hundred Flips (group)
| Time | 35 minutes |
| Group size | 5 — works at 4 to 6 |
| Materials | Two coins per group; paper for tallies |
| Preparation | Ten minutes: collect coins, or cut card squares marked T and t. |
Roles — everybody has one
| Role | What they do | How many |
|---|---|---|
| Flipper | Flips both coins together, twenty times. Then hands over | 1 |
| Caller | Reads each result aloud as TT, Tt or tt. Only the Caller decides what a result was | 1 |
| Tallier | Marks the tally. Nothing goes on the sheet that the Caller did not say | 1 |
| Checker | Counts the tally at 50 and at 100 and announces the running ratio | 1 |
| Reporter | Takes the group's hundred to the board and adds it to the class total | 1 |
Every role rotates after twenty flips, so all five have flipped and all five have tallied.
Instructions
- Flip both coins together. Heads is T, tails is t. Record the pair.
- Do this one hundred times, rotating roles every twenty.
- At fifty flips, the Checker announces the ratio so far. Write it down.
- At one hundred, the Checker announces it again. Write it down.
- The Reporter adds the group's hundred to a class total on the board.
- Each learner writes: their group's ratio at 50, at 100, and the class total.
What the three numbers show
| Typical result | |
|---|---|
| 50 flips | Anywhere from 2 : 1 to 5 : 1. Visibly scattered |
| 100 flips | Usually between 2.5 : 1 and 3.5 : 1 |
| Class total, 800 to 2000 | Close to 3 : 1, often within two or three per cent |
Draw all three on the board as bar charts side by side. The convergence is the lesson, and it cannot be told — the class has to watch its own numbers settle.
The question to close with
"So who should trust a Punnett square — a farmer with four hundred seeds, or a family with four children?"
The farmer. And the square is not wrong for the family; it is simply answering a different question from the one the family is asking.
Rubric — 12 marks
| Criterion | 3 | 2 | 1 | 0 |
|---|---|---|---|---|
| A hundred flips completed | 100, tallied accurately | 80 or more | 50 or more | Fewer than 50 |
| Roles rotated | All five roles held by all five members | Rotated at least twice | Rotated once | One person did it all |
| The three ratios recorded | 50, 100 and class total, all three | Two of the three | One | None |
| The conclusion | States that larger numbers land closer, in their own words | States that the numbers differ | Records the numbers only | Absent |
Activity 3 — Traits in This Room (group)
| Time | 30 minutes |
| Group size | 6 — works at 5 or 7 |
| Materials | Paper; a mirror is useful and not needed |
| Preparation | Five minutes: agree with the class which three traits will be counted, and check that none of them is something a learner might be self-conscious about. |
The three traits
| Trait | Usually described as | How to check |
|---|---|---|
| Tongue rolling | Dominant | Can you roll your tongue into a U? |
| Earlobes | Free is dominant, attached recessive | Does the lobe hang free of the jaw? |
| Widow's peak | Dominant | Does the hairline come to a point in the middle? |
⚠️ These are textbook simplifications and the class should be told so. Real tongue rolling and real earlobe shape are influenced by more than one gene, and honest sources disagree about them. They are used here because they are visible and harmless, not because the genetics is simple — and saying that out loud teaches something truer than the survey does.
Roles — everybody has one
| Role | What they do | How many |
|---|---|---|
| Surveyor | Asks each person in the group and records their answer | 1 |
| Counter | Totals the group and takes the total to the board | 1 |
| Class Tallier | Adds every group's total for one trait | 1 |
| Predictor | Writes, before the count, which trait the group thinks will be commonest — and why | 1 |
| Analyst | Compares the prediction with the result and writes one sentence | 1 |
| Reporter | Says the group's finding to the class in one sentence | 1 |
With five, the Analyst and the Reporter combine.
Instructions
- The Predictor writes the group's prediction before anybody is asked anything.
- Survey the group for all three traits.
- Pool with the class. Write the three class totals on the board.
- Answer: is the dominant trait the more common one in this room?
- Answer the harder one: should it be?
The answer to step 5
No. Dominance describes what happens inside one individual when both alleles are present. How many people in a room carry an allele is a separate question, decided by history, migration and chance, not by dominance.
The two clean counter-examples: extra fingers is dominant and very rare; blood type O is recessive and is the most common type in the Philippines.
Rubric — 12 marks
| Criterion | 3 | 2 | 1 | 0 |
|---|---|---|---|---|
| Prediction made first | Written before the survey, with a reason | Written before, no reason | Written after | None |
| Survey completed | All three traits, everybody counted | Two traits | One trait | Incomplete |
| Step 5 answered | No, with a counter-example | No, with a reason | No, unexplained | Yes |
| Everybody worked | All six roles carried out | Five | Four | Three or fewer |
Activity 4 — Working It Backwards (individual)
| Time | 20 minutes |
| Materials | Paper and pencil |
| Preparation | Five minutes: write the four situations on the board. |
The four situations
- Two tall plants produce a semi-dwarf offspring. What were the parents?
- A tall plant crossed with a semi-dwarf gives all tall offspring. What was the tall parent?
- A tall plant crossed with a semi-dwarf gives half tall and half semi-dwarf. What was the tall parent?
- A cross gives all offspring the same genotype, and that genotype is Tt. What were the parents?
Instructions
- For each, write the parents' genotypes.
- For each, write how you know — the reasoning, not the answer.
- For situation 3, explain why this cross is the one a plant breeder uses to find out what a tall plant is carrying.
Why step 3 matters
Crossing an unknown tall plant with a tt plant is called a test cross, and it is the practical use of everything in this week. If any semi-dwarf offspring appear, the unknown parent was Tt. It is a real technique, it is used at IRRI, and it exists because you cannot tell TT from Tt by looking — which was §A of Day 1.
Assessment
| Seen | What it means |
|---|---|
| All four with reasoning, and step 3 explained | Full understanding, running in both directions |
| All four, reasoning thin | Ask one aloud. The reasoning is usually there and unwritten |
| Situation 3 answered as TT | The learner has not noticed that half the offspring got no T at all |
Activity 5 — The Four Denials (individual)
| Time | 15 minutes |
| Materials | Paper and pencil |
| Preparation | None. |
The four statements
Each one is wrong. All four are said by adults, regularly.
- "They have three children with the trait, so the fourth one won't have it."
- "It's the dominant version, so most people have it."
- "He's Tt, so he's a bit tall — halfway between."
- "The T cancels out the t, so it's gone."
Instructions
- For each, write one sentence saying what is wrong with it.
- For each, write one sentence saying what the speaker should have said instead.
- Choose the one you think you were closest to believing at the start of the week. Not marked.
- Write one sentence explaining statement 1 to somebody in Grade 6.
Why step 3 is unmarked and asked anyway
Naming the misconception you were closest to is the beginning of not making it, and a learner being graded on the answer will pick a safe one. Say it is unmarked before they start.
Assessment
Step 4 is the one to read. Explaining independence to a younger learner without using the word independent is genuinely difficult, and a learner who manages it has the idea rather than the vocabulary.
| Seen | What it means |
|---|---|
| All four corrected, and step 4 is clear to a younger reader | Full understanding |
| Statements 1 and 4 corrected, 2 and 3 not | The probability half has landed and the biology half has not |
| Statement 2 defended | Dominance is still being read as commonness. It is the most persistent of the four |
If there are no coins, no mirror and no board space
- No coins — card squares with T on one face and t on the other, cut from a cereal box. Two per group. They land less randomly than a coin, so have learners drop them from a height rather than flip them.
- No mirror for Activity 3 — pairs check each other. Earlobes and widow's peaks are easier to see on somebody else anyway.
- No board space for the class tally — a single sheet of paper passed from group to group, with each Reporter adding a row. It is slower and the running total is just as visible.
- A class where the trait survey feels uncomfortable — drop it and run Activity 4 twice instead. Nothing in the competency requires counting the room, and the point about dominance and commonness can be made with extra fingers and blood type O alone.
Part of the E-turo MATATAG artifact set. Companion files:
syllabus.md · lesson-plan.md ·
lesson-plan-ilaw.md · worksheet.html