Recessive allele

In the syllabus and the exam

A recessive allele is expressed in the phenotype only when an organism is homozygous recessive, carrying two copies of that allele, one on each chromosome of the homologous pair; when paired with a dominant allele in a heterozygote, its effect is masked and does not appear in the phenotype. Candidates are expected to recognise that a recessive characteristic can 'skip' a generation, appearing in a grandchild whose parents both show the dominant phenotype but are heterozygous carriers, which is a very common structured-question scenario used to test understanding of genotype versus phenotype rather than simple recall of a definition. A key exam skill is reasoning backward from an observed recessive phenotype in offspring to conclude that both parents must carry at least one copy of the recessive allele, even though both parents display the dominant phenotype themselves. A common error is assuming a recessive characteristic is automatically rarer or less healthy than a dominant one; recessive simply describes the pattern of expression in a heterozygote, not the frequency of the allele in a population or any measure of its biological significance. Candidates should also be careful to write recessive alleles consistently in lower case in genetic diagrams, matching the corresponding dominant allele's letter, since inconsistent labelling is penalised even when the underlying reasoning is correct. Candidates preparing for higher-order questions should also be able to calculate the probability that a specific child from a given cross will show the recessive phenotype, expressing this as a fraction, percentage or ratio depending on how the question is phrased.

Example

A child with the genotype tt has attached earlobes because two copies of the recessive allele (t) are present, since a single copy paired with the dominant allele (T) would instead produce free earlobes.

Related terms

Dominant alleleAllelePhenotype

Source:SRC-DSKP-EN