Science 7–10 · Year 10

Modelling meiosis with model chromosomes: independent assortment and gamete variety

Science understanding: Biological sciences (ACARA Year 10 heredity, AC9S10U01, which names meiosis; SC5-GEV-02 is cited for the NSW Stage 5 content point on how genetic information is passed on to offspring by sexual and asexual reproduction, and SC5-GEV-01 for the natural selection content point whose example reads that variation occurs through mutation, meiosis and fertilisation, the one place NSW Stage 5 content names meiosis; NSW content does not name homologous pairs or independent assortment)

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The idea

Meiosis halves the chromosome number and, because each homologous pair lines up independently of the others, one parent can make 2 to the power n different gametes from n chromosome pairs before crossing over is counted.

What you need

Model chromosomes for each group: two homologous pairs (one long pair and one short pair) made from socks, pipe cleaners or pop beads, each homologue in a different colour to show which parent it came from
A second identical set to add as sister chromatids, and pegs, tape or velcro to join each chromatid pair at the centromere
Sticky labels marking alleles: A and a on the long pair, B and b on the short pair
A coin, chalk or string to outline cells on the bench, and a tally table for gamete types AB, Ab, aB and ab

How to do it

Outline a large cell on the bench and place the four chromosomes in it (2n = 4). Model DNA replication by joining a matching sister chromatid to each chromosome.
Meiosis I: line up each homologous pair side by side across the middle of the cell. Toss the coin for the short pair: heads, its first-parent colour faces the same side as the long pair's first-parent colour; tails, the opposite side.
Separate the homologues of each pair into two new cell outlines. Record the chromosomes in each cell: each cell now holds one chromosome from each pair, still made of two chromatids.
Meiosis II: in each of the two cells, separate the sister chromatids into two gamete outlines. Record the allele combination of all four gametes.
Return the chromosomes and repeat steps 1 to 4 eight times, tossing the coin each time. Tally the gamete types across all runs.
Model fertilisation: join one of your gametes with one from another group, chosen at random, and record the zygote genotype; repeat five times.
Run the simulation with 2, 3 and 23 chromosome pairs and compare the count of possible gametes with your tally.

What you should see

Every modelled meiosis gives four gametes of exactly two complementary types (for example AB, AB, ab, ab or Ab, Ab, aB, aB), each gamete holding two chromosomes, half the four in the parent cell. Across eight runs all four types AB, Ab, aB and ab appear unless every run lines up the same way, which has a probability of 2 times (1/2)^8, or 1 in 128; pooled class tallies are expected to show the four types in roughly equal numbers. If only one gamete were sampled from each of eight meioses, the expected number of different types seen would be 3.60. Two pairs give 2^2 = 4 possible gametes, three pairs 2^3 = 8, and the 23 human pairs 2^23 = 8,388,608, the about 8.4 million the University of Pennsylvania handout gives before crossing over. The learner knows it worked when every gamete holds exactly one chromosome of each pair and the pooled class tally contains all four types.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Meiosis is mitosis done twice; the first division separates homologous pairs, not sister chromatids.
Each gamete gets a random half of all the chromosomes; it gets exactly one from every pair.
Brothers and sisters differ only because of mutations; independent assortment and crossing over alone make almost every gamete different.

Safety card

Low riskLearners carry it out

Hazards

None beyond ordinary classroom movement

Controls

Keep model pieces off the floor

Note

No hazardous chemicals; record a RiskAssess risk assessment for the activity as school procedure requires.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC5-GEV-02 SC5-GEV-01 SC5-WS-05 SC5-WS-06
Australian Curriculum v9AC9S10U01AC9S10I04AC9S10I05

Sources

The pages the author read to write this activity.

All Concept Studio activities