Variation: recall that all variants arise from mutations and that: most have no effect on the phenotype; some influence phenotype; very few determine phenotype.

The idea

A mutation is a change in the base sequence of DNA. New alleles, called variants, appear only when mutation alters a gene, so every variant has a mutational origin. Most mutations are neutral because they occur in non-coding DNA or do not change the amino acid sequence, so the phenotype is unaffected. Some mutations alter the protein produced, perhaps changing an enzyme's shape or a structural protein, and so influence the phenotype; the effect may be slight or depend on the environment. Very few mutations determine the phenotype on their own, for example a mutation that causes a genetic disorder or a decisive change such as antibiotic resistance. This explains why most mutations are not harmful and why variation accumulates gradually in populations.

What earns the marks

• Defines mutation as a change in the DNA base sequence that can produce a new allele or variant.
• States that all variants ultimately arise from mutations, not from use or need.
• Explains that most mutations have no effect on the phenotype because they are in non-coding DNA or do not change the protein.
• Explains that some mutations influence the phenotype by altering the protein or how it works.
• States that very few mutations determine the phenotype, meaning the mutation alone decides the characteristic.
• Uses a suitable example, such as a mutation giving antibiotic resistance or causing a genetic disorder.

Common mistakes and how to put them right

• Saying organisms mutate because they need to adapt.

  Put it right: Mutations occur at random, and selection then acts on the variants produced.

• Claiming most mutations are harmful.

  Put it right: Most have no effect on the phenotype, some influence it, and very few determine it.

• Treating influence and determine as identical.

  Put it right: Influence means the mutation contributes to the phenotype, while determine means it alone decides the phenotype.

Exam tips

• Learn the three-part pattern: most have no effect, some influence, very few determine. Use those exact contrasts in your answer.
• When explaining a mutation's effect, refer to the protein made and whether its shape or function changes.
• Use one clear example, such as a mutation in a gene for a cell-surface receptor, to show how a variant can determine a phenotype.