• 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. 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 short stems? → 0 %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 onion plant has 16 chromosomes. It divides by mitosis. How many chromosomes are in each new cell? → 16Prompt: Mitosis makes cells identical to the parent cell.
3. What is a phenotype? (a type of cell division / the alleles an organism has, such as Bb / the characteristic that is shown, such as eye colour / a chromosome) → 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 couple's first three babies will all be XY? Give 1 decimal place. → 12.5 %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? (They have only one X, so one recessive allele is enough to show the condition / The Y chromosome causes colour blindness / It is caught from other people / XY people have two X chromosomes) → 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. How many pairs of chromosomes are in a human body cell, and how many of them are 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 chromosomes) → 23 pairs, 1 pair of sex chromosomesPrompt: There are 46 chromosomes in total.
2. Sex chromosomes: which statement is NOT true? (The chance of a baby being XY is about 50 % for each pregnancy. / The Y chromosome is smaller than the X chromosome. / Every human egg cell carries an X chromosome. / The egg cell decides whether a baby is XX or XY.) → The egg cell decides whether a baby is XX or XY.Prompt: Do earlier children change the odds?

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