Grade 9 · Genetics and Heredity · Inheritance of Traits
Punnett Squares and Probability
Predicting inherited traits using probability.
Learning objectives
- Complete a monohybrid Punnett square.
- Predict offspring ratios from parent genotypes.
- Explain why predictions are probabilities, not guarantees.
NGSS alignment
HS-LS3-3
Apply concepts of statistics and probability to explain the variation and distribution of expressed traits in a population.
HS-LS1-1
Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins which carry out the essential functions of life through systems of specialized cells.
HS-LS2-6
Evaluate the claims, evidence, and reasoning that the complex interactions in ecosystems maintain relatively consistent numbers and types of organisms in stable conditions, but changing conditions may result in a new ecosystem.
MS-LS3-2
Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information and sexual reproduction results in offspring with genetic variation.
HS-LS4-3
Apply concepts of statistics and probability to support explanations that organisms with an advantageous heritable trait tend to increase in proportion to organisms lacking this trait.
HS-LS2-2
Use mathematical representations to support and revise explanations based on evidence about factors affecting biodiversity and populations in ecosystems of different scales.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion C — Processing and Evaluating
ENGAGE
Two brown-eyed parents, one blue-eyed child
Two brown-eyed parents can have a blue-eyed child, but two blue-eyed parents almost never have a brown-eyed child.
Think about it
- What does this tell you about which allele is hidden?
- How could you show all the possible combinations?
EXPLAIN
Using a Punnett square
A dominant allele, written as a capital letter, shows in the phenotype whenever it is present. A recessive allele shows only when both alleles are recessive.
A Bb by Bb cross
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Genotype ratio 1 BB : 2 Bb : 1 bb
- Phenotype ratio 3 brown : 1 blue
- Probability of a blue-eyed child is 1 in 4, or 25 percent
Common misconception
A 3:1 ratio does not mean exactly one in four children will be blue-eyed. Each child is an independent 25 percent chance.
Homozygous means both alleles are the same, BB or bb. Heterozygous means they differ, Bb, which makes the individual a carrier of the recessive allele.
INVESTIGATION
Modelling inheritance with coins
research question
How closely does a coin model match the predicted 3:1 phenotype ratio?
hypothesis
The larger the number of trials, the closer the results will get to 3:1.
variables
Independent: number of trials (10, 50, 200). Dependent: ratio of dominant to recessive phenotypes. Controlled: same coins, same rules.
method
Let heads be B and tails be b. Flip two coins to represent one offspring and record the genotype. Repeat for each trial count.
safety
No significant hazards.
analysis
Compare each observed ratio with 3:1 and explain why sample size matters when predicting inheritance.
Watch
Monohybrids and the Punnett Square Guinea Pigs
Amoeba Sisters · 8 min
Works through Punnett squares step by step.
Before you watch: What ratio do you expect from Bb x Bb?
- Complete a Bb x bb cross.
- Why are the results probabilities?
Interactive simulation · PhET
Natural Selection
Add a dominant allele mutation and watch the proportions of each genotype over generations.
While you explore
- Which genotypes show the dominant phenotype?
- How do allele proportions change over generations?
Key vocabulary
- Heterozygous
- Having two different versions of a gene.
- Dominant allele
- The version of a gene that shows even if only one copy is present.
Practice questions
0/2 correct
Level 1 · Criterion A
In a Bb x Bb cross, what is the probability of a recessive phenotype?
Level 2 · Criterion A
An individual with two identical alleles for a gene is described as:
MYP criterion tasks
Level 3 · Criterion C
Two heterozygous parents have four children, all with brown eyes. Does this disprove the 3:1 prediction? Justify your answer.
Reflect & track
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