Mutations create new alleles
Definition
Mutation
A random change in the base sequence of DNA that can produce a new allele.
Definition
Allele
A different version of a gene.
- A mutation is the source of a new allele because it changes the base sequence of DNA.
- Mutations occur randomly and continuously, rather than appearing because an organism needs a particular phenotype.
- A population contains different alleles, so its members can have different genotypes and phenotypes.
Sexual reproduction reshuffles alleles
- During meiosis, homologous chromosomes separate independently and genetic material is reshuffled into genetically different gametes.
- Random fertilisation combines one gamete from each parent to produce new allele combinations.
- Sexual reproduction increases variation by rearranging existing alleles, but it does not create new alleles.
Common Mistake
Do not write that organisms mutate because they need to adapt; mutations occur before a selection pressure favours or removes their effects.
Selection changes allele frequencies
- Most mutations have no observable effect, while a few change the phenotype.
- An allele that improves survival or reproductive success in a particular environment may be described as advantageous.
- Individuals with the advantageous allele are more likely to reproduce and pass the allele to offspring.
- The frequency of the allele can therefore increase over many generations through natural selection.
Example
- Initial variation: a random mutation gives one bacterium an allele for antibiotic resistance.
- Selection: an antibiotic kills susceptible bacteria while the resistant bacterium survives.
- Inheritance: the surviving bacterium reproduces and passes the resistance allele to its descendants.
- Outcome: the resistance allele becomes more frequent in the population.
The environment determines advantage
- An allele is advantageous only when it improves reproductive success under the current environmental conditions.
- A change in the environment can alter which phenotypes are favoured.
- The antibiotic selects resistant bacteria; it does not produce the resistance mutation.
Self review
- What process creates new alleles?
- How does sexual reproduction increase genetic variation?
- Why are mutations described as random?
- How can an advantageous allele become more common?
