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.
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.
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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