Meiosis makes gametes. The chromosome pairs separate and each gamete gets one chromosome from each pair, chosen at random, so it has half the number and a different combination. With 23 pairs there are over 8 million combinations.
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.
Chromosomes come in pairs, one from each parent. This lesson: how gametes get half, and why each is different.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: copies or combinations?
ExploreTry it on the board and compare what happens with your prediction.
Explore: make gametes
Red chromosomes came from this person's mother, blue from their father. Press Make a gamete many times. How many chromosomes does each gamete get? How many different combinations can you find?
Try it yourselfYour edits are saved in this browser.
Make gametes until you have found at least six different combinations.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: meiosis
In your book: draw a cell with two pairs of chromosomes and all four gametes it could make.
With 3 pairs, each with 2 choices, how many different gametes are possible?
ElaborateUse the same idea in a new context.
Elaborate: meiosis and mitosis
Meiosis (making gametes) or mitosis (growth and repair)?
Makes sperm and egg cells
New cells have 46 chromosomes
New cells have 23 chromosomes
New cells are genetically identical to the parent cell
New cells are all genetically different
Healing a cut
Elaborate WiderUse the same idea in a new context.
Elaborate: even more variety
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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). About 20 minutes.
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.
Before class
A concept tutorial for introducing or revising the idea; it does not replace modelling meiosis with pipe cleaners or socks. Set the width with ?width=minimum, typical (default) or wider.
The hinge misconception is that gametes are identical copies. The Explore board shows variety from one cell.
Board notes: 3 pairs instead of 23; meiosis is shown as its outcome (pairs separating at random), without the stages or copying, and without crossing over. Pairs are labelled A, B and C (red, from the mother) and a, b and c (blue, from the father); the letters under each gamete name its combination.
Running the lesson
Engage: count how many chose 'all identical'.
Explore: ask how many different combinations are possible and how they know they've found them all.
Explain: the 2 × 2 × 2 question is the checkpoint.
The hard part (on purpose)
Seeing that each gamete gets one chromosome from every pair, chosen at random. When a student says the gametes are all the same, ask how many different gametes they made from the one cell, and what was different.
When students get stuck
A student thinks a gamete can get two chromosomes from one pair. Ask: Look at your gametes: was there ever a pair together? Fix: Never: the pairs separate, one of each goes to each gamete.
A student adds instead of multiplying combinations. Ask: With one pair, how many gametes? Two pairs? Fix: 2, then 4, then 8: each pair doubles it.
Stretch and support
Stretch: Why is it useful that meiosis happens only in the ovaries and testes, not everywhere in the body?
Support: Give a two-pair diagram to fill in the four possible gametes.
Where it leads
Next: Punnett squares (node 4), which track one gene through meiosis and fertilisation.
In class: sock or pipe-cleaner meiosis, including crossing over.
What each step is for
Engage: copies or combinations?: Students who think gametes are identical copies of the parent's cells won't expect variety; the board makes many different gametes from one cell.
Explore: make gametes: Making gametes from a cell with three pairs and counting the different combinations shows where the variety comes from.
Explain: meiosis: Counting combinations (2 × 2 × 2) and scaling to 23 pairs turns the board's variety into a rule.
Elaborate: meiosis and mitosis: Contrasting meiosis with ordinary cell division stops students mixing them up.
Elaborate: even more variety: Adding random fertilisation (and crossing over) shows why siblings are almost never identical.
Now you can
Explain that each gamete gets one chromosome from each pair.
Calculate the number of possible gametes for a small number of pairs.