AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Polydactyly (having extra fingers or toes) is caused by a dominant allele.
Full specification wording
Inherited disorders: Polydactyly (having extra fingers or toes) is caused by a dominant allele.
Polydactyly is a condition in which a person has extra fingers or toes.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 2 of 4
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Polydactyly is a condition in which a person has extra fingers or toes. It is caused by a dominant allele, so only one copy of that allele is needed for the condition to appear. A person with polydactyly may be heterozygous, with one dominant allele and one recessive allele, or homozygous for the dominant allele. If a parent with polydactyly is heterozygous and the other parent is homozygous recessive, each child has a 50% chance of inheriting the dominant allele and therefore the condition. Genetic diagrams using letters such as P for the dominant polydactyly allele and p for the recessive allele show these possible outcomes and help to explain why the condition can appear in every generation.
What earns the marks
- Polydactyly means having extra fingers or toes.
- A person with polydactyly can have the genotype PP or Pp.
- A person without polydactyly has the genotype pp.
- In a cross between a heterozygous affected parent and an unaffected parent, the expected probability of an affected child is 50%.
- Genetic diagrams using P and p alleles can predict the possible genotypes and phenotypes of offspring.
Common mistakes and how to put them right
Writing that two dominant alleles are needed for polydactyly.
Put it right by stating that a dominant allele causes the condition when at least one copy is inherited.
Using the same letter case for dominant and recessive alleles.
Put it right by using an uppercase letter for the dominant allele and the matching lowercase letter for the recessive allele.
Confusing genotype with phenotype.
Put it right by stating that genotype is the allele combination, such as Pp, while phenotype is the observable feature, such as having extra fingers or toes.
Exam tips
- State clearly that polydactyly is dominant and that one dominant allele is enough.
- Show parent genotypes and gametes in a genetic diagram before working out offspring probabilities.
- Use the terms genotype and phenotype correctly when describing the results of a cross.
Try the lesson
The start of the Inherited disorders: Polydactyly (having extra fingers or toes) is caused by a dominant allele. lesson, made from checked teaching notes for this specification point.
Have a go first
What type of allele causes polydactyly?
Listen
Polydactyly is a condition in which a person has extra fingers or toes. It is caused by a dominant allele, so only one copy of that allele is needed for the condition to appear. A person with polydactyly may be heterozygous, with one dominant allele and one recessive allele, or homozygous for the dominant allele. If a parent with polydactyly is heterozygous and the other parent is homozygous recessive, each child has a 50% chance of inheriting the dominant allele and therefore the condition. Genetic diagrams using letters such as P for the dominant polydactyly allele and p for the recessive allele show these possible outcomes and help to explain why the condition can appear in every generation. A strong answer earns marks for these points. Polydactyly means having extra fingers or toes. A person with polydactyly can have the genotype PP or Pp. A person without polydactyly has the genotype pp. In a cross between a heterozygous affected parent and an unaffected parent, the expected probability of an affected child is 50%. A common slip: Writing that two dominant alleles are needed for polydactyly. Instead, stating that a dominant allele causes the condition when at least one copy is inherited. Exam tip: State clearly that polydactyly is dominant and that one dominant allele is enough.
See it
Key words
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Polydactyly
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Genotype
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Phenotype
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Genetic diagram
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See every definition
- Polydactyly
- Having extra fingers or toes, caused by a dominant allele so that one copy of the allele is sufficient for the condition to be expressed.
- Genotype
- The allele combination, such as PP or Pp, with an uppercase letter for the dominant allele and the matching lowercase letter for the recessive allele.
- Phenotype
- The observable feature, such as having extra fingers or toes.
- Genetic diagram
- A diagram using letters such as P for the dominant polydactyly allele and p for the recessive allele to show possible outcomes and predict genotypes and phenotypes of offspring.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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