Inheritance (A-level only): There may be many alleles of a single gene.

The idea

A gene occupies a fixed locus, but the base sequence at that locus can differ between individuals, and each different base sequence is a different allele. Mutation creates new alleles, so over time a population accumulates many alleles of one gene, even though each diploid individual can carry only two of them. The human ABO blood group gene is the standard example, with three alleles, I^A, I^B and I^O, giving six possible genotypes and four phenotypes. Genetic diversity within a population is measured as the number of different alleles of each gene, so a gene with many alleles contributes more diversity and gives natural selection more variation to act on. Some genes, such as those coding for the proteins used in cell recognition, have hundreds of alleles.

What earns the marks

• Define an allele as a different version, or different base sequence, of a gene at the same locus.
• State that genetic diversity is the number of different alleles of each gene in a population.
• Recognise that an individual diploid organism carries only two of the alleles available, even when the gene has many alleles in the population.
• Name a multiple-allele example, such as the three ABO alleles I^A, I^B and I^O, and the phenotypes they produce.
• Explain that mutation creates new alleles, so the number of alleles of a gene in a population increases over time.

Common mistakes and how to put them right

• Answering different genes when asked what genetic diversity means.

  Put it right: The credit is for the number of different alleles of each gene.

• Writing three allele symbols in one genotype because the gene has three alleles.

• Assuming that with three alleles one must be dominant over the other two, when ABO has two codominant alleles and one recessive.

• Saying that a new allele appears because the organism needs it, rather than by random mutation.

• Confusing the number of alleles in the population with the number of alleles in an individual.

Exam tips

• Underline the dominance information in the stem before you start; multiple-allele questions always tell you which alleles are codominant and which is recessive.
• List every possible genotype before answering, because six genotypes from three alleles is easy to lose track of.
• Use the superscript notation the question gives you, such as I^A and I^B, instead of inventing your own letters.