Recessive allele
Exam-ready definition
An allele that is only expressed in the phenotype when two copies are present (in the homozygous condition).
How this is marked
The paying phrase is 'only expressed when homozygous / when two copies are present'. Pedigree questions hinge on this: an affected child of unaffected parents proves the allele is recessive. A standard 2-mark deduction.
In an exam answer
Explaining unaffected parents with an affected child: 'The allele must be recessive, because it is only expressed when two copies are present, so both parents were carriers.'
Full topic guide: Inheritance
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