--- title: "Why is every sperm and egg different?" subtitle: "Year 10 Biology · Genetics · Lesson 2" --- # Why is every sperm and egg different? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** You and your brother or sister came from the same two parents. The chance that two of their sperm or eggs carry exactly the same chromosomes is tiny. :::teacher-note **In one sentence:** Students model meiosis, explain how it halves the chromosome number and produces genetically different gametes, and contrast it with mitosis (AC9S10U01, node 3, a hinge node). **Students should be able to say:** I can explain that each gamete gets one chromosome from each pair. I can calculate the number of possible gametes for a small number of pairs. I can distinguish meiosis from mitosis. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **One person makes millions of sperm (or thousands of eggs). What do you predict about their chromosomes?** ☐ Each has a random number of chromosomes ☐ Each has half the chromosomes, but the combination of which ones differs from gamete to gamete ☐ Each has the full 46 ☐ They are all identical My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Each has 23, one from each pair, but which one of each pair is random, so almost every gamete is a different combination. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: meiosis :::keep-together ### One of each pair **On the board, every gamete had three chromosomes. Which is true?** ☐ Each gamete gets the whole pair from one chromosome ☐ Each gamete gets exactly one chromosome from each pair ☐ Each gamete gets all the red ones or all the blue ones ☐ Each gamete gets any three chromosomes ::: :::teacher-note **Answer:** Each gamete gets exactly one chromosome from each pair ::: :::keep-together ### Three pairs **With 3 pairs, each with 2 choices, how many different gametes are possible?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8 2 × 2 × 2 = 8. ::: :::keep-together ### 23 pairs **Humans have 23 pairs. About how many different combinations are possible?** ☐ 2 × 23 = 46: two choices for each of 23 pairs ☐ 23: one new combination for each pair ☐ 23 × 23 = 529 ☐ 2 to the power 23: over 8 million ::: :::teacher-note **Answer:** 2 to the power 23: over 8 million ::: :::typical ## Elaborate: meiosis and mitosis :::keep-together **Meiosis (making gametes) or mitosis (growth and repair)?** Groups: Meiosis, Mitosis - Makes sperm and egg cells: {{field-15}} - New cells have 46 chromosomes: {{field-15}} - New cells have 23 chromosomes: {{field-15}} - New cells are genetically identical to the parent cell: {{field-15}} - New cells are all genetically different: {{field-15}} - Healing a cut: {{field-15}} ::: :::teacher-note **Answer:** Makes sperm and egg cells → Meiosis; New cells have 46 chromosomes → Mitosis; New cells have 23 chromosomes → Meiosis; New cells are genetically identical to the parent cell → Mitosis; New cells are all genetically different → Meiosis; Healing a cut → Mitosis ::: ::: :::wider ## Elaborate: even more variety :::keep-together **Each parent can make over 8 million different gametes. Ignoring crossing over, roughly how many different children could two parents have?** ☐ About 8 million × 8 million: around 70 trillion ☐ About 16 million ☐ 46 ☐ About 8 million ::: :::teacher-note **Answer:** About 8 million × 8 million: around 70 trillion Crossing over (pairs swapping pieces during meiosis) adds far more variety still. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Meiosis** makes gametes: chromosome pairs separate; each gamete gets **one from each pair**, so it has **half** the number. - Which of each pair goes into a gamete is **random**: 2 × 2 × 2 … for 23 pairs, over **8 million** combinations. - Random fertilisation (and crossing over) multiply the variety. ## Exit ticket :::keep-together **Why does meiosis make every gamete different?** {{lines-3}} ::: :::teacher-note **Answer:** Each gamete gets one chromosome from each pair, and which one is random, so the combination differs. :::