Cambridge O Level Biology · Syllabus 5090 · Inheritance
Mutation
What is Mutation?
A random change in the genetic material of a cell. A gene mutation is a change in the sequence of bases in a length of DNA, which can alter the amino-acid sequence of the protein that gene codes for. A chromosome mutation is a change in the number of chromosomes or in their structure, and therefore affects many genes at once. Mutations occur at random and are not directed by the needs of the organism; they may be harmful, neutral or, occasionally, beneficial, and only mutations in cells that give rise to gametes can normally be passed to offspring.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Mutation in context
Inheritance is the transmission of genetic information from one generation to the next. A gene is a length of DNA that codes for a protein; an allele is one of the alternative forms that gene can take. The order of the bases A, T, C and G along a strand of DNA sets the order of amino acids in a protein, which sets the protein’s shape, which sets what the protein can do — and that is how a molecule you cannot see decides a feature you can. Because gametes carry one allele of each gene, a cross between two known genotypes has a predictable outcome: Tt × Tt is expected to give 3 tall : 1 short, and Tt × tt is expected to give 1 tall : 1 short. Those are probabilities, not guarantees. Mutation makes new alleles at random, meiosis and fertilisation shuffle them into new combinations, and selection — by the environment in natural selection, by people in artificial selection — changes how common each allele becomes over generations. That change is evolution.
Common mistakes with Mutation
- “The environment caused a mutation for dark colour.” Mutation is random with respect to need; the environment selects among variants that already exist. Fix “Dark and pale forms were already present in the population through earlier mutation.”
- “The antibiotic made the bacteria mutate.” This is the whole misconception the topic exists to correct. The mutation came first, at random; the antibiotic then selected the bacteria that already had it. Fix “A resistant bacterium was already present because of an earlier random mutation.”
- “All mutations are harmful.” Most are neutral, some are harmful, and occasionally one is beneficial in a particular environment. Without that occasional useful one there would be no new variation at all. Write “Mutations may be harmful, neutral or beneficial.”
- “The organism mutated because it needed to.” Mutation is random with respect to need. Variation arises first; the environment then selects among it. Write “The mutation occurred at random, and the environment then favoured the individuals carrying it.”
- “The antibiotic made the bacteria resistant.” The resistance allele arose earlier, by random mutation. The antibiotic selected the bacteria that already had it. Write “A resistant bacterium was already present; the antibiotic killed the others, so the resistant one survived and reproduced.”
Questions students ask about Mutation
How many chromosomes does a person with Down’s syndrome have?
47, rather than the usual 46, because there is an additional copy of chromosome 21. It is classified as a chromosome mutation because what has changed is the number of whole chromosomes, not the base sequence within a gene. It arises at random during the formation of gametes.
Did the antibiotic cause the bacteria to become resistant?
No. The resistance allele arose earlier, by random mutation, in bacteria that had never encountered the drug. The antibiotic then acted as a selection pressure: it killed the bacteria that lacked the allele, leaving the resistant ones to survive and reproduce. The proportion of resistant bacteria in the population rose. The antibiotic selected variation that already existed — it did not create it.
Is a mutation always harmful?
No. Most mutations are neutral, having no detectable effect. Some are harmful. Occasionally one is beneficial in a particular environment, and those are the source of the new variation that natural selection can act on. Whether a mutation is harmful or helpful can also depend on the environment: the same allele may be a disadvantage in one place and an advantage in another.