Dominant allele
Exam-ready definition
An allele that is expressed in the phenotype when only one copy is present (in the heterozygous condition).
How this is marked
Mark schemes reward 'expressed when one copy present' or 'expressed in the heterozygote'. Avoid 'stronger allele'. Strength language never scores and signals weak understanding to the examiner.
In an exam answer
Explaining a pedigree: 'The allele is dominant because it is expressed in the heterozygous condition, when only one copy is present.'
Full topic guide: Inheritance
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