Genetic Variation Within a Population (OCR Gateway GCSE Biology: Combined Science)

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Biology

Variation within populations is called intraspecific variation
Familiar examples of intraspecific variation might include
- Breeds of dogs
- Race in humans
- Varieties of plants
It occurs due to different alleles found in the population
These alleles originated from mutations of an ancestral gene
Variation in a population is responsible for the differences in characteristics but also may determine how likely an individual is to survive
Periodically, a new mutation will occur and introduce a new allele into the population
Whether this allele is successful or whether it is quickly lost again from the population depends on whether it provides any kind of advantage or disadvantage to individuals that possess it

Mutations occur continuously and are the cause of new alleles in a population
Mutations are rare, random changes that occur in the sequence of DNA bases in a gene or a chromosome
As the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for
Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
There are different ways that a mutation in the DNA base sequence can occur

Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
However, a small number of mutations code for a significantly altered protein with a different shape
This may affect the ability of the protein to perform its function. For example:
- If the shape of the active site on an enzyme changes, the substrate may no longer be able to bind to the active site
- A structural protein (like collagen) may lose its strength if its shape changes
On rare occasions mutations lead to the development of new alleles and so new phenotypes
Occasionally, the new allele (and its resulting phenotype) gives the individual a survival advantage over other members of the species
For example:
- A bird develops a mutation leading to a change in feather colours
- This makes it more attractive to birds of the opposite sex
- Which causes the bird to breed more frequently and have more chances of passing on the mutated phenotype to the next generation
Mutations can also lead to harmful changes that can have dramatic effects on the body - for example, sickle cell anaemia in humans

Mutations happen spontaneously and continuously but their frequency can be increased by exposure to the following:
- Gamma rays, x - rays and ultraviolet rays - all types of ionising radiation which can damage bonds and cause changes in base sequences
- Certain types of chemicals - for example chemicals such as tar in tobacco
Increased rates of mutation can cause cells to become cancerous, which is why the above are linked to increased incidence of different types of cancer

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