Science lesson plan · 9th grade (Year 10 in England) · 45 minutes
Sex chromosomes and the chance of a boy or a girl
Cells, genetics and evolution
CCSS NGSS.HS-LS3-2England: GCSE Combined Science: Inheritance, variation and evolution
Objective and success criteria
Students will: Sex chromosomes and the chance of a boy or a girl.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Genetic crosses and Punnett squares; Count chromosomes after mitosis, meiosis and fertilisation
Leads to: the next skill in this area.
Materials: Microscope images or slides, Cell diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- 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.
- 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? → 75 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
- A body cell of a horse has 64 chromosomes. How many chromosomes are in one of its gametes (sex cells)? → 32Prompt: Mitosis makes cells identical to the parent cell.
- What is a phenotype? (a type of cell division / the characteristic that is shown, such as eye colour / the alleles an organism has, such as Bb / a chromosome) → the characteristic that is shown, such as eye colourPrompt: The genotype is the letters; the phenotype is what you see.
Hook · 5 min
- 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.
I do: model it · 10 min
- 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.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- What is the probability (in %) that a baby will be biologically male (XY)? → 50 %Prompt: Draw a Punnett square for XX × XY.
- 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 / 22 pairs, no sex chromosomes / 46 pairs, 2 pairs of sex chromosomes) → 23 pairs, 1 pair of sex chromosomesPrompt: There are 46 chromosomes in total.
You do: independent practice · 10 min
- 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.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- 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.
- Sex chromosomes: which statement is NOT true? (After two XX babies, the next baby is more likely to be XY. / The Y chromosome is smaller than the X chromosome. / The chance of a baby being XY is about 50 % for each pregnancy. / Every human egg cell carries an X chromosome.) → After two XX babies, the next baby is more likely to be XY.Prompt: Do earlier children change the odds?
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Genetic crosses and Punnett squares.
- Use concrete materials: microscope images or slides.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.
Free lesson plan from eurekami.com/lesson-plan-builder