AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
(HT only) Students should be able to construct a genetic cross by Punnett…
Full specification wording
Genetic inheritance: (HT only) Students should be able to construct a genetic cross by Punnett square diagram and use it to make predictions using the theory of probability.
Higher tier students construct a Punnett square from parental genotypes and use probability to predict outcomes.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 20 of 20
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Higher tier students construct a Punnett square from parental genotypes and use probability to predict outcomes. For a cross such as Bb × Bb, the square gives BB, Bb, Bb and bb. The probability of a dominant phenotype is 3/4, 0.75 or 75%, and of a recessive phenotype is 1/4, 0.25 or 25%. Probability means the chance for each offspring, not a guarantee about a particular family; a 75% chance does not mean that every litter of four will contain exactly three dominant offspring. Express predictions as fractions, decimals or percentages, and apply the same reasoning to sex determination, where XX × XY gives a 1/2 chance of each sex. Ratios describe relative frequencies but are distinct from probabilities.
What earns the marks
- Construct the Punnett square correctly from the parental genotypes, placing one parent's alleles across the top and the other's down the side.
- Count the total number of equally likely outcomes in the square and identify how many give the phenotype or genotype asked about.
- Express the prediction as a probability, for example 1/4, 0.25 or 25%, ensuring it is not confused with a ratio.
- Explain that probability predicts the chance for each individual offspring rather than the exact outcome of a small number of offspring.
- Apply the same method to a named context, such as predicting the chance of a child being male or female from an XX × XY cross.
Common mistakes and how to put them right
Treating a probability as a fixed result for a small number of offspring.
Put it right: State that each offspring independently has a specific chance of a phenotype, so small samples may differ.
Counting cells twice or missing a cell when working out the probability.
Put it right: Total the four cells of the square and count only those matching the required outcome.
Confusing a probability with a ratio.
Put it right: A probability is a fraction of the total (e.g., 1/4 or 25%), whereas a ratio compares parts to parts (e.g., 3:1).
Exam tips
- Show the completed Punnett square even when the question only asks for a probability, because the grid supports your reasoning.
- Convert between fractions, decimals and percentages confidently so you can give the prediction in the form the question uses.
Try the lesson
The start of the Genetic inheritance: (HT only) Students should be able to construct a genetic cross by Punnett square diagram and use it to make predictions using the theory of probability. lesson, made from checked teaching notes for this specification point.
Have a go first
Two heterozygous parents, Tt and Tt, are crossed. What is the probability that an offspring will be heterozygous?
Listen
Higher tier students construct a Punnett square from parental genotypes and use probability to predict outcomes. For a cross such as Bb × Bb, the square gives BB, Bb, Bb and bb. The probability of a dominant phenotype is 3/4, 0.75 or 75%, and of a recessive phenotype is 1/4, 0.25 or 25%. Probability means the chance for each offspring, not a guarantee about a particular family; a 75% chance does not mean that every litter of four will contain exactly three dominant offspring. Express predictions as fractions, decimals or percentages, and apply the same reasoning to sex determination, where XX × XY gives a 1/2 chance of each sex. Ratios describe relative frequencies but are distinct from probabilities. A strong answer earns marks for these points. Construct the Punnett square correctly from the parental genotypes, placing one parent's alleles across the top and the other's down the side. Count the total number of equally likely outcomes in the square and identify how many give the phenotype or genotype asked about. Express the prediction as a probability, for example 1/4, 0.25 or 25%, ensuring it is not confused with a ratio. Explain that probability predicts the chance for each individual offspring rather than the exact outcome of a small number of offspring. A common slip: Treating a probability as a fixed result for a small number of offspring. Instead, state that each offspring independently has a specific chance of a phenotype, so small samples may differ. Exam tip: Show the completed Punnett square even when the question only asks for a probability, because the grid supports your reasoning.
Key words
Say what each word means before you turn the card.
Punnett square diagram
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Probability
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Ratio
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See every definition
- Punnett square diagram
- A diagram used to construct a genetic cross, with one parent's alleles across the top and the other's down the side.
- Probability
- The chance for each offspring, expressed as a fraction, decimal or percentage, and not a guarantee about a particular family.
- Ratio
- A way of describing relative frequencies that is distinct from probabilities.
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