AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to understand the concept of probability in…
Full specification wording
Genetic inheritance: Students should be able to understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are the result of multiple genes rather than single gene inheritance.
A single gene cross predicts the chance of each offspring genotype, not the exact family outcome.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 17 of 20
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A single gene cross predicts the chance of each offspring genotype, not the exact family outcome. If two heterozygous parents (Aa × Aa) are crossed, each child has a 1 in 4 chance of aa, so probability describes long-run proportions across many offspring. A Punnett square shows the four equally likely fertilisation outcomes, and ratios such as 3:1 express those probabilities. However, most phenotype features, including human height, skin colour and eye colour, are polygenic: several genes plus environmental factors contribute, giving continuous variation rather than two clear categories. So a single gene cross is a model that works for clear-cut traits such as cystic fibrosis or pea seed shape, but it does not explain most features.
What earns the marks
- Probability in a single gene cross is the chance that a particular genotype or phenotype appears in one offspring, calculated from the equally likely gamete combinations.
- A Punnett square for Aa × Aa gives four equally likely outcomes: AA, Aa, Aa and aa, so the probability of aa is 1 in 4, or 25%, and the dominant phenotype probability is 3 in 4, or 75%.
- Probability predicts proportions over many offspring, not the exact result in a small family; random fertilisation means observed numbers can differ from expected ratios.
- Most phenotype features are polygenic, controlled by multiple genes, often with environmental influence, so they show continuous variation rather than simple dominant or recessive categories.
- Single gene inheritance explains clear-cut traits such as cystic fibrosis, where genotypes produce distinct phenotypes, but it is a limited model for most features.
Common mistakes and how to put them right
Treating a 3:1 ratio as a guarantee for four offspring, for example expecting exactly three dominant and one recessive child.
Put it right: Probability describes chance, and small samples often deviate from expected ratios.
Assuming every feature with a dominant and recessive form is controlled by one gene.
Put it right: Most features are polygenic and also influenced by the environment.
Confusing genotype probability with phenotype probability when dominance is incomplete or when both alleles are expressed.
Put it right: Read the question carefully and map genotypes to phenotypes using the stated inheritance pattern.
Exam tips
- State clearly that probability predicts chance, not certainty, and refer to large numbers of offspring when explaining ratios.
- When asked why a family does not match a 3:1 ratio, link the answer to random fertilisation and small sample size.
- For questions about continuous variation, name multiple genes and environmental factors rather than saying only that it is genetic.
Try the lesson
The start of the Genetic inheritance: Students should be able to understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are the result of multiple genes rather than single gene inheritance. lesson, made from checked teaching notes for this specification point.
Have a go first
In a single gene cross, what does probability predict about the offspring?
Listen
A single gene cross predicts the chance of each offspring genotype, not the exact family outcome. If two heterozygous parents (Aa × Aa) are crossed, each child has a 1 in 4 chance of aa, so probability describes long-run proportions across many offspring. A Punnett square shows the four equally likely fertilisation outcomes, and ratios such as 3:1 express those probabilities. However, most phenotype features, including human height, skin colour and eye colour, are polygenic: several genes plus environmental factors contribute, giving continuous variation rather than two clear categories. So a single gene cross is a model that works for clear-cut traits such as cystic fibrosis or pea seed shape, but it does not explain most features. A strong answer earns marks for these points. Probability in a single gene cross is the chance that a particular genotype or phenotype appears in one offspring, calculated from the equally likely gamete combinations. A Punnett square for Aa × Aa gives four equally likely outcomes: AA, Aa, Aa and aa, so the probability of aa is 1 in 4, or 25%, and the dominant phenotype probability is 3 in 4, or 75%. Probability predicts proportions over many offspring, not the exact result in a small family; random fertilisation means observed numbers can differ from expected ratios. Most phenotype features are polygenic, controlled by multiple genes, often with environmental influence, so they show continuous variation rather than simple dominant or recessive categories. A common slip: Treating a 3:1 ratio as a guarantee for four offspring, for example expecting exactly three dominant and one recessive child. Instead, probability describes chance, and small samples often deviate from expected ratios. Exam tip: State clearly that probability predicts chance, not certainty, and refer to large numbers of offspring when explaining ratios.
See it
Key words
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Probability
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Punnett square
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Polygenic
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Continuous variation
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See every definition
- Probability
- The chance that a particular genotype or phenotype appears in one offspring, calculated from the equally likely gamete combinations.
- Punnett square
- A diagram showing the four equally likely fertilisation outcomes for a single gene cross, such as AA, Aa, Aa and aa.
- Polygenic
- Controlled by multiple genes, often with environmental influence, so features show continuous variation rather than simple dominant or recessive categories.
- Continuous variation
- The range of phenotype features produced when several genes plus environmental factors contribute, rather than two clear categories.
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Test your understanding of Genetic inheritance: Students should be able to understand the concept of probability in predicting the results of a single gene cross, but recall that most phenotype features are the result of multiple genes rather than single gene inheritance.
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