• 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. Brown fur (B) is dominant over white fur (b). Two parents with genotypes Bb and Bb are crossed. What percentage of the offspring are expected to have brown fur? → 75 %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 dog has 78 chromosomes. How many chromosomes are in one of its gametes (sex cells)? → 39Prompt: Mitosis makes cells identical to the parent cell.
3. What does "heterozygous" mean? (having two identical alleles, such as BB / having four alleles / having two different alleles of a gene, such as Bb / having no alleles) → having two different alleles of a gene, such as BbPrompt: "Hetero" means different.

• 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. A couple already have three daughters. What is the probability (in %) that their next baby will be XY? → 50 %Prompt: Draw a Punnett square for XX × XY.
2. Which gamete decides whether a baby is XX or XY? (both equally / the egg cell / the sperm cell / neither: it depends on diet) → the sperm cellPrompt: What sex chromosomes can each gamete carry?

• 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? (The Y chromosome is smaller than the X chromosome. / After two XX babies, the next baby is more likely to be XY. / The chance of a baby being XY is about 50 % for each pregnancy. / Human body cells contain one pair of sex chromosomes.) → After two XX babies, the next baby is more likely to be XY.Prompt: Do earlier children change the odds?
2. A gene on the X chromosome causes red–green colour blindness. Why is it more common in XY people? (The Y chromosome causes colour blindness / They have only one X, so one recessive allele is enough to show the condition / XY people have two X chromosomes / It is caught from other people) → 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.

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