BiologyGrade 9Reproduction and Inheritance

Genetics Lab: Punnett Square and Trait Probability

Choose parent genotypes, build a Punnett square, simulate offspring, and compare expected genotype and phenotype ratios.

SimulationsLab index
Textbook unitReproductionGrade 9 Biology Unit 4
Keywordsreproduction, inheritance, gene, allele, traitMapped to available textbook headings
Practice modeManipulate, measure, explainUse the controls, then read the live evidence

Biology · Grade 10

Cross parent genotypes and verify Mendelian ratios

Build a Punnett square, run a class-size offspring simulation, and compare observed results with Mendel's predicted 3:1 and 1:1 ratios.

Cross type75% dominant
1

Monohybrid cross: set both parents to Aa. Do you get the 3:1 dominant-to-recessive ratio? Simulate 96 offspring and compare.

2

Test cross: set one parent to Aa, the other to aa. What does a 1:1 ratio tell you about the unknown parent?

3

Try a cross that gives 100% dominant offspring. Can you get 0% recessive? Record it and explain which cross achieves this.

Cross typeMonohybrid (3:1 expected)
Gametes
A
a
A
AATall
AaTall
a
AaTall
aaShort
Simulated offspring48 dominant / 16 recessive
Genetics insight

Classic monohybrid cross: Punnett gives AA:Aa:aa = 1:2:1 genotype ratio → 3 dominant : 1 recessive phenotype. This is Mendel's Law of Segregation.

Expected genotype1:2:1
Expected dominant75%
Expected recessive25%
Observed dominant75%
Observed ratio16:32:16

Lab task

Complete all three guided crosses above. For each, record the simulated cohort and note whether observed ratios match expected Mendelian ratios.

Key principle

Mendel's Law of Segregation: alleles separate during gamete formation. Each offspring inherits one allele from each parent independently. Dominant allele A masks recessive a when both are present (Aa).

Mission

What to prove in this lab

  1. Use gametes and allele combinations to predict offspring genotypes.
  2. Compare dominant and recessive phenotype probabilities.
  3. Explain why observed class results may differ from expected ratios.