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An allele that affects the phenotype only when two copies are present in a simple dominant–recessive pair.

An allele that is expressed in the phenotype only when two copies are present and no dominant allele is present.

For 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.

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.

Use the same letter for the two alleles: a capital letter for the dominant allele and the corresponding lower-case letter for the recessive allele, such as $A$ and .

A dominant allele is usually written as a capital letter and the recessive allele as the corresponding lower-case letter.

An allele whose associated phenotype is seen only when two copies are present in a simple dominant–recessive pattern.

For 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 appears in a simple Mendelian cross when two copies are present.

A recessive allele affects the characteristic only when there is no dominant allele present—usually when the genotype contains two recessive copies.

An allele that affects the phenotype only when two copies are present in this simple inheritance model.

Recessive allele

Definition

How each exam board covers it

AQA GCSE Biology · 8461

Edexcel GCSE Biology · 1BI0

Exam tips

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OCR GCSE Biology A · J247

OCR GCSE Biology B · J257