Random mutation can result in new alleles of a gene.

The idea

An allele is one version of a gene, differing from another version in its base sequence. New alleles can only come from mutation: a change to the base sequence, usually arising during DNA replication, occurring at random and independently of whether the change would be useful. Meiosis and sexual reproduction do not create alleles; they only recombine alleles that already exist. Because mutation is random, two consequences follow. First, most new alleles are neutral or harmful, since a functioning protein is far easier to break than to improve. Second, whether a new allele turns out to be advantageous depends entirely on the environment: an allele conferring resistance to a herbicide is worthless in a field that has never been sprayed.

What earns the marks

• Defining an allele as a different version of a gene with a different base sequence.
• Stating that new alleles arise by random mutation.
• Stating that mutations occur independently of any need or advantage to the organism.
• Stating that only mutations in gametes, or in the cells that form them, are inherited.
• Explaining that whether the new allele is advantageous depends on the environment.

Common mistakes and how to put them right

• Writing that the environment or a selection pressure causes the useful mutation to appear.

• Saying meiosis or crossing over produces new alleles, when they produce new combinations of existing alleles.

• Using gene and allele interchangeably.

• Claiming all mutations are harmful, or that every mutation changes the protein.

• Assuming any mutation occurring in an adult will appear in its children.

Exam tips

• Open natural selection answers with 'a random mutation produced a new allele'; it is a mark almost every time.
• Keep the words random and new allele together, because examiners look for both.
• If the question describes a mutation in a body cell, state explicitly that it is not passed on.