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Science lesson plan · 9th grade (Year 10 in England) · 60 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 · 6 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.
  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? → 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. It divides by mitosis. How many chromosomes are in each new cell? → 78Prompt: Mitosis makes cells identical to the parent cell.
  3. In humans, which pair of sex chromosomes does a typical male have? (XY / XX / YY / XXY only) → XYPrompt: Females typically have two of the same.

Hook · 6 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 · 12 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 · 12 min

  • 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 / 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.

You do: independent practice · 17 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 · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  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. Sex chromosomes: which statement is NOT true? (Human gametes contain two sex chromosomes. / Human body cells contain one pair of sex chromosomes. / Every human egg cell carries an X chromosome. / The chance of a baby being XY is about 50 % for each pregnancy.) → Human gametes contain two sex chromosomes.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

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Sex chromosomes and the chance of a boy or a girl

Name

Date

Score/ 8

  1. Question 1

    Sex chromosomes: which statement is NOT true?

    • A. The chance of a baby being XY is about 50 % for each pregnancy.
    • B. After two XX babies, the next baby is more likely to be XY.
    • C. Every human egg cell carries an X chromosome.
    • D. The Y chromosome is smaller than the X chromosome.

  2. Question 2

    A couple already have three daughters. What is the probability (in %) that their next baby will be XY?

    Answer%

  3. Question 3

    A family has four sons and no daughters. A student says the father "only makes Y sperm". Why is this wrong?

    • A. Sperm never carry Y
    • B. The mother decides the sex
    • C. About half his sperm carry X; four boys in a row can happen by chance (1 in 16)
    • D. Sons cannot have the same father

  4. Question 4

    What is the probability (in %) that a couple's first two babies will both be XX?

    Answer%

  5. Question 5

    Which gamete decides whether a baby is XX or XY?

    • A. both equally
    • B. neither: it depends on diet
    • C. the sperm cell
    • D. the egg cell

  6. Question 6

    What is the probability (in %) that a couple's first three babies will all be XY? Give 1 decimal place.

    Answer%

  7. Question 7

    How many pairs of chromosomes are in a human body cell, and how many of them are sex chromosomes?

    • A. 23 pairs, all sex chromosomes
    • B. 23 pairs, 1 pair of sex chromosomes
    • C. 46 pairs, 2 pairs of sex chromosomes
    • D. 22 pairs, no sex chromosomes

  8. Question 8

    A gene on the X chromosome causes red–green colour blindness. Why is it more common in XY people?

    • A. The Y chromosome causes colour blindness
    • B. It is caught from other people
    • C. They have only one X, so one recessive allele is enough to show the condition
    • D. XY people have two X chromosomes

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Answer key: Sex chromosomes and the chance of a boy or a girl

  1. 1. B. After two XX babies, the next baby is more likely to be XY.

  2. 2. 50 %

  3. 3. C. About half his sperm carry X; four boys in a row can happen by chance (1 in 16)

  4. 4. 25 %

  5. 5. C. the sperm cell

  6. 6. 12.5 %

  7. 7. B. 23 pairs, 1 pair of sex chromosomes

  8. 8. C. They have only one X, so one recessive allele is enough to show the condition