• Quick retrieval from earlier learning (Genetic crosses and Punnett squares; Genes, alleles, genotypes and phenotypes). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Tall stems (T) is dominant over short stems (t). Two parents with genotypes TT and tt are crossed. What percentage of the offspring are expected to have tall stems? → 100 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
2. Which word means: having two identical alleles, such as BB or bb? (allele / homozygous / heterozygous / dominant) → homozygousPrompt: “Geno-” is about the genes you carry; “pheno-” is about what you see.
3. A recessive characteristic only shows when… (there is a dominant allele / there is one recessive allele / there are two recessive alleles / the organism is old) → there are two recessive allelesPrompt: A dominant allele hides it.

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: Two-gene (dihybrid) crosses and 9 : 3 : 3 : 1 ratios. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Two pea plants, both RrYy (round, yellow seeds), are crossed. They produce 16 seeds. How many would you expect to be round and yellow (both dominant traits)?
• Step 1: 16 ÷ 16 = 1
• Step 2: 1 × 9 = 9
• Step 3: So the answer is: 9
• Answer: 9
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Two pea plants, both RrYy (round, yellow seeds), are crossed. They produce 48 seeds. How many would you expect to be round and yellow (both dominant traits)? → 27Prompt: A cross between two double heterozygotes gives a 9 : 3 : 3 : 1 ratio.
2. Assuming the genes are on different chromosomes, how many different kinds of gamete can an organism with genotype Aa make? → 2Prompt: Each heterozygous gene gives 2 options.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Two pea plants, both RrYy (round, yellow seeds), are crossed. They produce 160 seeds. How many would you expect to be round and green (one dominant, one recessive)? → 30Prompt: A cross between two double heterozygotes gives a 9 : 3 : 3 : 1 ratio.
2. Two guinea pigs, both BbRr, are crossed. What ratio of phenotypes is expected in the offspring? (1 : 1 : 1 : 1 / 1 : 1 / 3 : 1 / 9 : 3 : 3 : 1) → 9 : 3 : 3 : 1Prompt: List the gametes each parent can make.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.