Allele

In the syllabus and the exam

An allele is one specific version of a gene, and monohybrid inheritance questions are built almost entirely around correctly tracking a pair of alleles for one characteristic through a cross, so precise use of this term is essential. Candidates are expected to represent alleles with letters (usually an upper-case letter for the dominant allele and the corresponding lower-case letter for the recessive allele, such as T and t), and to state that an organism normally carries two alleles for each gene, one on each of a pair of homologous chromosomes, which may be identical (homozygous) or different (heterozygous). A frequent error is using the word 'allele' when 'gene' is meant, or vice versa, especially in a sentence like 'the gene for tallness', which should more precisely say 'the allele for tallness' when referring to one specific version rather than the characteristic as a whole. Candidates should also avoid the common misconception that an allele on its own directly equals a visible characteristic; the visible characteristic (phenotype) depends on the specific combination of the two alleles present (the genotype) together with how those alleles interact, particularly whether one is dominant over the other. Exam questions frequently ask candidates to state both alleles present in a given genotype and to identify which is dominant from the information given in the question. Candidates should further note that writing an allele symbol without stating which characteristic it refers to is considered an incomplete answer, since the same letter could otherwise be reused for an entirely different gene in a later part of the same question.

Example

In pea plants, the allele for purple flowers (P) and the allele for white flowers (p) are two different versions of the same flower-colour gene, occupying the same locus on a pair of homologous chromosomes.

Related terms

GeneDominant alleleRecessive allele

Source:SRC-DSKP-EN