Genetic crosses and Punnett squares Revision Notes | GCSE AQA Higher Biology

1. The Language of Inheritance

Why do you have your mum's eyes but your dad's nose? The answer lies in how characteristics pass from parents to children through genes. But here's the thing - predicting inheritance isn't guesswork. It follows mathematical rules we can calculate.

Trap: Many students think dominant means 'more common' in the population. Actually, dominant just means it shows up even with one copy. Blue eyes (recessive) are rarer than brown eyes, but that's not what dominant/recessive means.

2. Building Your First Punnett Square

Think of a Punnett square like a multiplication grid. One parent's alleles go across the top, the other parent's down the side. Each box shows one possible combination in the offspring.

Worked example: Setting up a Punnett Square

Cross two parents: Bb × Bb (both have brown eyes but carry the blue allele)

1. Parent 1: Bb Parent 2: Bb — Both parents are heterozygous (have two different alleles)
2. Parent 1 gametes: B, b Parent 2 gametes: B, b — Each parent can contribute either the B or b allele
3. | B | b ----|----|---- B | BB | Bb ----|----|---- b | Bb | bb — Parent 1's alleles go across the top, Parent 2's down the side
4. BB: 1 box (25%) Bb: 2 boxes (50%) bb: 1 box (25%) — Each box represents 25% probability

Answer: Genotype ratio 1:2:1 (BB:Bb:bb). Phenotype ratio 3:1 (brown:blue eyes)

Notice the pattern: when you cross two heterozygotes (Bb × Bb), you always get a 3:1 phenotype ratio. Three-quarters show the dominant trait, one-quarter shows the recessive trait.

3. Probability and Ratios in Genetics

Genetics is all about probability. When we say '3:1 ratio', we mean if these parents had 100 children, about 75 would have brown eyes and 25 would have blue eyes. But with small families, you might not see this exact ratio.

Worked example: Calculating Inheritance Probability

What's the probability that two brown-eyed parents (Bb × Bb) will have a blue-eyed child?

1. Blue eyes = bb genotype — Only bb gives blue eyes (recessive trait)
2. From Punnett square: 1 box shows bb — Out of 4 total boxes, only 1 contains bb
3. $P = \frac{1}{4} = 0.25$ — 1 favourable outcome ÷ 4 total outcomes
4. $0.25 \times 100\% = 25\%$ — Always convert to percentage for exam answers

Answer: 25% chance of blue-eyed offspring

Trap: Students often confuse genotype and phenotype ratios. In Bb × Bb: genotype ratio is 1:2:1 (BB:Bb:bb), but phenotype ratio is 3:1 (brown:blue) because both BB and Bb look the same.

4. Reading Family Trees

Family trees (pedigrees) show inheritance patterns across generations. You can work backwards from phenotypes to figure out genotypes. The key is looking for when recessive traits appear.

Here's the detective work: if two parents with the dominant phenotype have a child with the recessive phenotype, both parents must be heterozygous. Why? Because the child got one recessive allele from each parent.

Worked example: Analysing a Family Tree

Two brown-eyed parents have three children: two brown-eyed and one blue-eyed. What are the parents' genotypes?

1. Blue-eyed child from brown-eyed parents — This tells us both parents carry the recessive allele
2. Blue eyes = bb genotype — Child must have received b from each parent
3. Both parents must be Bb — They have brown eyes (dominant) but can pass on b allele
4. Bb × Bb gives 3:1 ratio (brown:blue) — This matches the family: 2 brown, 1 blue children

Answer: Both parents are Bb (heterozygous)

5. Beyond Simple Inheritance

Here's the reality check: most human characteristics aren't controlled by single genes. Height, weight, intelligence, and skin colour involve multiple genes working together. Single gene inheritance explains some traits like ABO blood groups and some genetic disorders, but it's not the whole story.

Environmental factors also matter. Even if you have genes for being tall, poor nutrition during childhood can affect your final height. Genes set the potential, but environment influences the outcome.

Exam depth extension: In genetic crosses and punnett squares, connect the language of inheritance to building your first punnett square using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.

6. Exam Technique and Common Mistakes

Genetic crosses are exam favourites because they test both your understanding and your ability to work with ratios and probability. Here's how to nail these questions every time.

Command word focus: 'Calculate' means show your working with the Punnett square. 'Explain' means give reasons why the ratio occurs. 'Predict' means use probability to forecast outcomes.

Worked example: Exam-Style Genetic Cross Question

In cats, black fur (B) is dominant to white fur (b). A black cat (Bb) is crossed with a white cat (bb). Calculate the percentage of offspring expected to have black fur. [4 marks]

1. B = black fur (dominant) b = white fur (recessive) — [1 mark] Always start by defining your symbols clearly
2. Black cat: Bb White cat: bb — White cat must be bb (recessive phenotype)
3. | B | b ----|----|---- b | Bb | bb ----|----|---- b | Bb | bb — [2 marks] Correct grid with all genotypes filled in
4. Black offspring: 2 out of 4 boxes = 50% — [1 mark] Bb genotype gives black phenotype due to dominance

Answer: 50% of offspring will have black fur

The most common mistake? Forgetting that heterozygotes (like Bb) show the dominant phenotype. Students often think Bb means 'mixed' or 'medium' - it doesn't. Dominance means one allele masks the other completely.