Genetic inheritance: allele

The idea

An allele is one of the different versions of a gene that can occupy the same locus on a chromosome. Humans are diploid, so body cells carry two copies of most genes; the two copies may be the same allele (homozygous) or different alleles (heterozygous). Alleles differ in their DNA base sequence, so they may code for slightly different versions of the same protein, producing different phenotypes. For example, the gene for eye colour has several alleles, and the combination inherited from each parent helps determine eye colour. In genetic diagrams, alleles are represented by letters: a dominant allele by a capital letter and a recessive allele by the lower-case version of the same letter. A gamete contains only one allele from each pair because meiosis separates homologous chromosomes.

What earns the marks

• An allele is a version or variant of a gene, not a whole chromosome or a whole gene.
• Alleles of the same gene occupy the same locus on homologous chromosomes.
• A diploid organism has two alleles for each gene, one inherited from each parent.
• Alleles differ in base sequence, which can lead to different amino acid sequences and different proteins.
• Homozygous means both alleles are the same; heterozygous means the two alleles are different.
• Gametes carry one allele per gene because homologous chromosomes separate during meiosis.

Common mistakes and how to put them right

• Writing that an allele is a type of gene rather than a version of a gene.

  Put it right by saying an allele is one of the alternative forms of a gene.

• Confusing alleles with chromosomes.

  Put it right by stating that alleles are found at the same locus on homologous chromosomes.

• Assuming a dominant allele is always more common in a population.

  Put it right by explaining that dominance describes the effect on the phenotype, not the frequency of the allele.

Exam tips

• Define allele using the words version, gene and locus to show precise understanding.
• When using a genetic diagram, state the parental genotypes, gametes and offspring genotypes clearly.
• Use capital and lower-case versions of the same letter for dominant and recessive alleles, and keep the letter consistent throughout a question.