• Quick retrieval from earlier learning (Genetic crosses and Punnett squares; Count chromosomes after mitosis, meiosis and fertilisation). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. Round seeds (R) is dominant over wrinkled seeds (r). Two parents with genotypes Rr and rr are crossed. What percentage of the offspring are expected to have round seeds? → 50 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
2. A body cell of a human has 46 chromosomes. How many chromosomes are in one of its gametes (sex cells)? → 23Prompt: Mitosis makes cells identical to the parent cell.
3. What is a phenotype? (the characteristic that is shown, such as eye colour / a type of cell division / a chromosome / the alleles an organism has, such as Bb) → the characteristic that is shown, such as eye colourPrompt: The genotype is the letters; the phenotype is what you see.

• Show a microscope image of cells and ask what students think they are looking at.
• Share the objective: Sex chromosomes and the chance of a boy or a girl. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A couple already have three daughters. What is the probability (in %) that their next baby will be XY?
• Step 1: Each pregnancy is independent: earlier children do not change the odds.
• Step 2: So the answer is: 50 %
• Answer: 50 %
• 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. What is the probability (in %) that a baby will be biologically male (XY)? → 50 %Prompt: Draw a Punnett square for XX × XY.
2. A gene on the X chromosome causes red–green colour blindness. Why is it more common in XY people? (It is caught from other people / The Y chromosome causes colour blindness / XY people have two X chromosomes / They have only one X, so one recessive allele is enough to show the condition) → They have only one X, so one recessive allele is enough to show the conditionPrompt: XX people have a second X that may carry the normal allele.

• 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 8 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. Sex chromosomes: which statement is NOT true? (Human body cells contain one pair of sex chromosomes. / The chance of a baby being XY is about 50 % for each pregnancy. / Human gametes contain two sex chromosomes. / The Y chromosome is smaller than the X chromosome.) → Human gametes contain two sex chromosomes.Prompt: Do earlier children change the odds?
2. A family has four sons and no daughters. A student says the father "only makes Y sperm". Why is this wrong? (Sperm never carry Y / About half his sperm carry X; four boys in a row can happen by chance (1 in 16) / Sons cannot have the same father / The mother decides the sex) → About half his sperm carry X; four boys in a row can happen by chance (1 in 16)Prompt: 0.5 × 0.5 × 0.5 × 0.5 = ?

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.