Genetic Crosses and Punnett Squares — AQA GCSE Biology
Genetic crosses predict the chance of offspring inheriting particular characteristics. Punnett squares are the key tool.
Key terms
- Gene — a section of DNA coding for a characteristic.
- Allele — a different version of a gene.
- Dominant allele — only one copy needed to be expressed (capital letter).
- Recessive allele — needs two copies to be expressed (lower-case letter).
- Homozygous — two identical alleles (e.g. BB or bb).
- Heterozygous — two different alleles (e.g. Bb).
- Genotype — the alleles present.
- Phenotype — the characteristic shown.
Using a Punnett square
To predict the offspring of a cross:
- Write the genotypes of both parents.
- Find the possible gametes each parent can produce (one allele each).
- Combine them in a 2×2 grid (the Punnett square).
- Read off the offspring genotypes and work out the phenotype ratio.
Example
Cross two heterozygous parents: Bb × Bb.
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Offspring: 1 BB : 2 Bb : 1 bb → 3 dominant : 1 recessive phenotype ratio (a 75% / 25% chance).
Expressing results
Results can be given as a ratio (3:1), a fraction (¾) or a percentage/probability (75%). Remember these are probabilities, not guarantees.
Exam tips
- Learn all the key terms precisely (especially genotype vs phenotype).
- Always show the parents' genotypes and gametes, then the Punnett square.
- Convert outcomes between ratio, fraction and percentage.
- A Bb × Bb cross gives a 3:1 ratio; a Bb × bb cross gives a 1:1 ratio.