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GCSE Biology · key term
An allele that affects the phenotype when one or two copies are present.
Used by AQA, Edexcel and OCR
Specification 4.6.1.7 · Paper 2
An allele that is expressed in the phenotype when one or two copies are present.
Revise: Inherited DisordersFor a simple single-gene disorder, let an upper-case letter represent a dominant allele and the matching lower-case letter represent a recessive allele. A dominant disorder allele causes the disorder when just one copy is present. A recessive disorder allele causes the disorder only when two copies are present.
Specification 3.16 · Paper 1
Do not say that a dominant allele is more common, stronger or better. Dominant means that one copy is enough for the associated phenotype to be expressed.
Use the same letter for both alleles: a capital letter for the dominant allele and the corresponding lower-case letter for the recessive allele.
Do not assume that a dominant allele is more common, more beneficial or ‘stronger’ than a recessive allele.
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A dominant allele is usually written as a capital letter and the recessive allele as the corresponding lower-case letter.
Specification B5.1i, B5.1m · Paper 2 (Foundation), Paper 4 (Higher)
An allele whose associated phenotype is seen when one or two copies are present in a simple dominant–recessive pattern.
Revise: Single gene inheritanceFor a simple dominant–recessive trait, write the dominant allele with a capital letter and the recessive allele with the corresponding lower-case letter.
An allele whose associated characteristic can appear when just one copy is present.
Paper 1 (Foundation), Paper 2 (Foundation), Paper 3 (Higher), Paper 4 (Higher)
An allele that affects the phenotype when one or two copies are present in a simple dominant–recessive pair.
Revise: Inheritance TermsIn a simple dominant–recessive relationship, a dominant allele affects the observable characteristic when at least one copy is present. A recessive allele affects the characteristic only when there is no dominant allele present—usually when the genotype contains two recessive copies.