Investigating Science 11–12 · Year 11

Mendel’s law of dominance: his counts, a bead model and a chi-square test

Module 4: Theories and Laws

Practical, model not builtLow risk

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The idea

Mendel’s law of dominance predicts a 3 : 1 ratio in the second generation; his own counts and a bead model test whether departures from 3 : 1 are within chance.

What you need

two opaque bags per pair, each with 50 red beads (allele R) and 50 white beads (allele r)
tally sheet, calculator or spreadsheet
Mendel’s 1865 paper in English translation (Electronic Scholarly Publishing edition) for his counts

How to do it

Draw one bead from each bag without looking to make one offspring’s genotype; record it and return the beads.
Make 100 offspring and count dominant (RR or Rr) and recessive (rr) phenotypes.
Calculate χ² = Σ (O − E)²/E against 75 : 25 and compare with 3.84 (1 degree of freedom, p = 0.05).
Pool the class results and repeat the test.
Apply the same test to Mendel’s seed-shape and seed-colour counts.
Explain what the tests show about the evidence for the law and what they cannot show.

What you should see

Mendel reports 5474 round and 1850 wrinkled seeds (7324 in all, ratio 2.96 : 1), χ² = 0.263, and 6022 yellow and 2001 green (ratio 3.01 : 1), χ² = 0.015, both far below 3.84. For 100 bead offspring the expected dominant count is 75 with a standard deviation of 4.3; counts from 67 to 83 give χ² below 3.84, so most pairs’ results are consistent with 3 : 1, and the pooled class ratio lies closer to 3 : 1 than most single pairs.

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.

Dominant alleles are more common in a population.
A 3 : 1 ratio means exactly 3 of every 4 offspring show the dominant trait.
A good fit proves the law is true.

Safety card

Low riskLearners carry it out

Hazards

beads are a choking hazard for young children at home

Controls

keep beads in bags and count them back in

Note

No hazardous chemicals or heat sources: record the activity in the school's RiskAssess risk assessment, following the NSW Department of Education Science safety and compliance page; the Chemical Safety in Schools package is not triggered.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-11 INS11/12-5 INS11/12-6 INS11/12-7
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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