75-minute lesson
Grade 12 Science: Patterns of inheritance
Lesson objective
- Use correct genetic terminology: gene, allele, genotype, phenotype, dominant, recessive.
- Predict offspring ratios from monohybrid crosses using Punnett squares.
- Apply probability to inheritance of an autosomal recessive condition.
Success criteria:
- I can define genotype and phenotype and give examples.
- I can complete a Punnett square and state genotype and phenotype ratios.
- I can calculate the probability that a child inherits a recessive condition.
Materials
- Two coins
- Masking tape and a pen to label each coin's sides
- Paper ruled into a results table
- A calculator
Introduction
8 minIntroduce today's words:
- gene: A section of DNA that codes for a particular protein or functional product.
- allele: One of the different versions of a gene.
- genotype: The combination of alleles an organism has for a gene, e.g. Aa.
- phenotype: The observable characteristic that results from the genotype and the environment.
- homozygous: Having two identical alleles for a gene (AA or aa).
- heterozygous: Having two different alleles for a gene (Aa).
Ask your child what they already know about patterns of inheritance.
Explanation
15 minMost body cells contain two copies of each chromosome, one inherited from each parent, so they carry two copies of each gene. Different versions of a gene are called alleles.
The pair of alleles is the genotype. If both are the same, the genotype is homozygous (AA or aa); if they differ, it is heterozygous (Aa). The resulting characteristic is the phenotype.
A dominant allele (capital letter) shows its effect in the phenotype whenever it is present. A recessive allele (lower-case letter) shows its effect only when no dominant allele is present (aa).
During gamete formation (meiosis), the two alleles separate, so each egg or sperm carries only one. A Punnett square shows all the equally likely combinations when gametes join.
Gregor Mendel used pea plants to discover these patterns in the 1860s. For example, purple flower colour (P) is dominant over white (p) in peas.
Cystic fibrosis is an autosomal recessive condition in humans. Two parents who are both carriers (heterozygous) are usually unaffected, but each child has a 1 in 4 chance of inheriting two recessive alleles.
Punnett squares give probabilities, not guarantees. Each child is an independent event, like each coin toss. Also, many human traits, such as height and skin colour, are controlled by many genes and the environment, so they do not follow simple ratios.
Worked examples
11 minCrossing two heterozygous pea plants
Cross two purple-flowered pea plants that are both Pp. What are the expected genotype and phenotype ratios?
Step 1: Each parent produces gametes P or p with equal probability.
Step 2: Draw a 2 × 2 Punnett square: PP, Pp, Pp, pp.
Step 3: Genotype ratio: 1 PP : 2 Pp : 1 pp.
Step 4: Phenotype: PP and Pp are purple; pp is white. So 3 purple : 1 white.
Answer: Genotypes 1 PP : 2 Pp : 1 pp; phenotypes 3 purple : 1 white.
Carrier parents
Two parents are both carriers of cystic fibrosis (Ff). What is the probability that a child has cystic fibrosis, and that a child is a carrier?
Step 1: Punnett square: FF, Ff, Ff, ff.
Step 2: Cystic fibrosis requires ff: 1 of 4 boxes = 1/4 = 25%.
Step 3: Carriers are Ff: 2 of 4 boxes = 1/2 = 50%.
Answer: 25% chance of cystic fibrosis; 50% chance of being a carrier.
Coin-toss simulation
Model an Aa × Aa cross with coins.
Step 1: Label one side of each coin 'A' and the other side 'a'. Each coin is one parent.
Step 2: Toss both coins together 40 times and record each pair (AA, Aa or aa).
Step 3: Compare your counts with the expected 10 : 20 : 10.
Step 4: Small differences are due to chance; larger samples get closer to the expected ratio.
Answer: Results approximate 1 AA : 2 Aa : 1 aa, with random variation.
Guided practice (do together)
15 min1. Which genotype is heterozygous?
(a) AA (b) Aa (c) aa (d) A
2. A pea plant with genotype pp has white flowers. 'White flowers' is its…
(a) Genotype (b) Allele (c) Phenotype (d) Gamete
3. What is the expected phenotype ratio from Aa × Aa when A is completely dominant?
(a) 1 : 1 (b) 1 : 2 : 1 (c) All dominant (d) 3 : 1
4. What is the expected outcome of AA × aa?
(a) All Aa (b) Half AA, half aa (c) All aa (d) 1 AA : 2 Aa : 1 aa
Independent practice
19 min5. Two carrier parents already have one child with cystic fibrosis. What is the chance their next child has it?
(a) 0% (b) 25% (c) 50% (d) 100%
6. In a cross Aa × aa, what percentage of offspring are expected to be aa? (number)
7. Different versions of the same gene are called what?
8. In a cross Pp × Pp, what fraction of offspring are expected to be homozygous recessive? (fraction)
Questions to check understanding
- Can you define genotype and phenotype and give examples?
- Can you complete a Punnett square and state genotype and phenotype ratios?
- Can you calculate the probability that a child inherits a recessive condition?
- What was the trickiest part today?
Answer guide
1. Aa — Heterozygous means two different alleles.
2. Phenotype — The observable trait is the phenotype.
3. 3 : 1 — AA, Aa and Aa show the dominant phenotype; aa shows the recessive one.
4. All Aa — Every gamete from the first parent is A and from the second is a.
5. 25% — Each child is an independent event, so the probability is still 1 in 4.
6. 50 — Punnett square: Aa, Aa, aa, aa, so 2 of 4 = 50%.
7. alleles — Versions of a gene are alleles.
8. 1/4 — One of the four boxes is pp.
Review
7 minRecap the success criteria together. Watch for these common misconceptions:
- Students often think dominant alleles are more common in a population. Dominance describes how alleles interact, not how frequent they are.
- Some think a 1 in 4 chance means exactly one in every four children will be affected. Each birth is independent.
- Many popular 'single-gene' examples, such as eye colour, are actually influenced by several genes, so simple ratios do not apply.
Extension activities
- Draw a pedigree chart for a fictional family showing an autosomal recessive condition across three generations.
- Research how incomplete dominance and codominance change the expected ratios.
- Find out how genetic diversity is managed in breeding programs for threatened Australian animals such as the Tasmanian devil.
Suggested follow-up
- Revisit patterns of inheritance tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
Tips for parents
- Have your student explain each Punnett square aloud, naming gametes before filling in boxes.
- Pool coin-toss results across family members to show how bigger samples get closer to expected ratios.
- Be sensitive if discussing inherited conditions that may affect your family; keep examples general.