• 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. Purple flowers (P) is dominant over white flowers (p). Two parents with genotypes PP and pp are crossed. What percentage of the offspring are expected to have purple flowers? → 100 %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. A recessive characteristic only shows when… (there are two recessive alleles / there is a dominant allele / the organism is old / there is one recessive allele) → 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: 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 couple's first two babies will both be XX? → 25 %Prompt: Draw a Punnett square for XX × XY.
2. How many pairs of chromosomes are in a human body cell, and how many of them are sex chromosomes? (23 pairs, 1 pair of sex chromosomes / 23 pairs, all sex chromosomes / 46 pairs, 2 pairs of sex chromosomes / 22 pairs, no sex chromosomes) → 23 pairs, 1 pair of sex chromosomesPrompt: There are 46 chromosomes in total.

• 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. How many pairs of chromosomes are in a human body cell, and how many of them are sex chromosomes? (23 pairs, 1 pair of sex chromosomes / 23 pairs, all sex chromosomes / 46 pairs, 2 pairs of sex chromosomes / 22 pairs, no sex chromosomes) → 23 pairs, 1 pair of sex chromosomesPrompt: There are 46 chromosomes in total.
2. Sex chromosomes: which statement is NOT true? (Every human egg cell carries an X chromosome. / 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.) → After two XX babies, the next baby is more likely to be XY.Prompt: Do earlier children change the odds?

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