Year 13 · Science · Biological sciences
Patterns of inheritance
Students use Punnett squares and probability to predict the outcomes of monohybrid crosses and analyse inheritance of autosomal recessive conditions.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- 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 looks like
- 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.
The big idea
Most 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
Crossing two heterozygous pea plants
Cross two purple-flowered pea plants that are both Pp. What are the expected genotype and phenotype ratios?
- Each parent produces gametes P or p with equal probability.
- Draw a 2 × 2 Punnett square: PP, Pp, Pp, pp.
- Genotype ratio: 1 PP : 2 Pp : 1 pp.
- 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?
- Punnett square: FF, Ff, Ff, ff.
- Cystic fibrosis requires ff: 1 of 4 boxes = 1/4 = 25%.
- 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.
- Label one side of each coin 'A' and the other side 'a'. Each coin is one parent.
- Toss both coins together 40 times and record each pair (AA, Aa or aa).
- Compare your counts with the expected 10 : 20 : 10.
- 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.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
Which genotype is heterozygous?
Aa — Heterozygous means two different alleles.
A pea plant with genotype pp has white flowers. 'White flowers' is its…
Phenotype — The observable trait is the phenotype.
What is the expected phenotype ratio from Aa × Aa when A is completely dominant?
3 : 1 — AA, Aa and Aa show the dominant phenotype; aa shows the recessive one.
What is the expected outcome of AA × aa?
All Aa — Every gamete from the first parent is A and from the second is a.
Two carrier parents already have one child with cystic fibrosis. What is the chance their next child has it?
25% — Each child is an independent event, so the probability is still 1 in 4.
In a cross Aa × aa, what percentage of offspring are expected to be aa? (number)
50 — Punnett square: Aa, Aa, aa, aa, so 2 of 4 = 50%.
Different versions of the same gene are called what?
alleles — Versions of a gene are alleles.
In a cross Pp × Pp, what fraction of offspring are expected to be homozygous recessive? (fraction)
1/4 — One of the four boxes is pp.
Watch out for
- 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.
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.
Go further
- 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.