Biology 11–12 · Year 12

Modelling meiosis, crossing over and fertilisation with Reebop chromosomes

Module 5: Heredity (Genetic Variation)

Practical, model not builtLow risk

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The idea

Independent assortment, crossing over and random fertilisation together give each offspring a genotype that no sibling need share, and a physical model makes each source of variation countable.

What you need

Reebop chromosome envelopes for two parents, each holding 16 chromosome cards (eight pairs) as the source sheet describes, and the decoder key
Large marshmallows 4, small coloured marshmallows up to 6, cocktail sticks 5 or 6, map pins 8, one pipe cleaner per group
Scissors and tape for modelling a crossover; recording sheet

How to do it

Lay out each parent's chromosomes in homologous pairs and confirm each parent has two alleles for every trait.
For each parent, separate the pairs at random (independent assortment) to make a gamete; before separating one pair, cut and swap the ends of the two homologues to model a crossover, and record which alleles changed partners.
Combine one gamete from each parent (fertilisation) and read the offspring genotype with the decoder.
Build the offspring from the marshmallow parts according to its phenotype.
Compare offspring across the class and count how many genotypes appear; list which differences came from assortment, which from the crossover and which from random fertilisation.
Introduce a single copying error in one allele before gamete formation (a mutation) and record its effect on the phenotype.

What you should see

Offspring differ from group to group, and the class list shows many genotypes from the same two parents. With the Reebop's eight homologous pairs, independent assortment alone sorts a parent's chromosomes 2 to the power 8 = 256 ways, and two parents give 256 x 256 = 65,536 gamete pairings. Both parents are heterozygous for every characteristic except sex, as the source states (the decoder key has XX for female and XY for male), so the seven other pairs give 3 genotypes each and the sex pair 2: 3 to the power 7 x 2 = 4,374 possible genotypes before crossing over, which the decoder key turns into the 384 phenotypes the source quotes (2 to the power 6 x 3 x 2, for six characteristics with a dominant allele, a nose of three colours and sex); with 23 pairs a human parent makes 8,388,608 assortment combinations (computed). The learner knows it worked when the class can trace each offspring trait to a specific gamete and the crossover recombines two alleles that were on the same parental chromosome.

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.

Offspring get half of each trait from each parent; they get one allele for each gene from each parent.
Every pair of siblings shares exactly half of its alleles; half is the average, and the share varies from pair to pair because each sibling is an independent draw from millions of combinations.
Crossing over swaps whole chromosomes; it exchanges segments between homologous chromatids.

Safety card

Low riskLearners carry it out

Hazards

Cocktail sticks and pins

Controls

Handle points with care; do not eat the marshmallows in the laboratory

Note

No hazardous chemicals.

Curriculum references

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

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-12 BIO11/12-4 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 12 focus area Heredity; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12-01
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-04 BI-11WS-06
Biology 11–12 Syllabus (2025), Year 12 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-12WS-04 BI-12WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities