Genetic crosses and Punnett squares Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Explore single gene inheritance using examples like fur colour in mice and colour blindness. Learn to construct and interpret Punnett squares, understan...

• Explain how genetic crosses work using Punnett squares to predict offspring ratios
• Calculate probabilities of inheriting specific traits from single gene crosses
• Analyse family trees to determine inheritance patterns and genotypes
• Evaluate the limitations of single gene inheritance in explaining most human traits

Key ideas to keep in view

Use this lesson to stay inside B5.1 Inheritance while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.1 Inheritance
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from Genetic crosses and Punnett squares before testing yourself from memory.

• 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.
• A section of DNA that codes for a particular characteristic, such as eye colour or blood group.
• Different versions of the same gene. For example, the gene for eye colour has brown and blue alleles.
• An allele that is expressed in the phenotype even when only one copy is present. Represented by a capital letter (e.g. B for brown eyes).
• An allele that is only expressed when two copies are present. Represented by a lowercase letter (e.g. b for blue eyes).
• 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.
• 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.
• 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.
• 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.
• Calculate the chance of inheriting a specific trait

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.