AQA GCSE Combined Science Trilogy · 8464
Specification 4.6.1.5 · Paper 2
Revise: Inherited DisordersA dominant allele is expressed when one copy is present; a recessive allele is expressed only when two copies are present.
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GCSE Combined Science · key term
An allele expressed in the phenotype only when two copies are present.
Used by AQA and Edexcel
Specification 4.6.1.5 · Paper 2
Revise: Inherited DisordersA dominant allele is expressed when one copy is present; a recessive allele is expressed only when two copies are present.
Paper 1, Paper 2
An allele expressed in the phenotype only when two copies are present in the simple inheritance patterns studied here.
State that cystic fibrosis is a disorder of cell membranes caused by a recessive allele, whereas polydactyly means extra fingers or toes and is caused by a dominant allele.
Use the same letter for both alleles of a gene, with a capital for the dominant allele and a lower-case letter for the recessive allele. Define your symbols.
A cystic fibrosis carrier is unaffected but can pass on the recessive allele. Do not count carriers as having the disorder.
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A recessive allele is expressed only when both copies are recessive in this model.