Genetic Variants and Mutations

This lesson covers mutations, genetic variants (alleles), and the causes of variation. This is essential content for Edexcel GCSE Biology (1BI0) Topic 3: Genetics.

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What Is a Mutation?

A mutation is a random, spontaneous change in the base sequence of DNA. Mutations can occur during DNA replication (when DNA is copied before cell division) or can be caused by external factors.

How Mutations Occur

• During DNA replication, the wrong base may be inserted, a base may be deleted, or extra bases may be added.
• These changes alter the sequence of bases in the gene.
• Mutations are random — they are not directed or intentional.
• Mutations are rare but occur naturally at a low frequency.

Mutagens

Certain environmental factors can increase the rate of mutation. These are called mutagens:

Mutagen	Example
Ionising radiation	UV light, X-rays, gamma rays
Chemical mutagens	Chemicals in cigarette smoke, certain industrial chemicals
Biological agents	Some viruses can insert their DNA into the host genome

Exam Tip: Remember that mutations are random and can happen to anyone. Mutagens increase the rate at which mutations occur but do not determine which specific mutations will happen.

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Effects of Mutations

Most mutations have no effect on the organism. However, some can be harmful, and a few can be beneficial.

No Effect (Most Common)

Most mutations have no effect for one of two reasons:

1. The mutation occurs in non-coding DNA — the vast majority of the human genome does not code for proteins. A change in a non-coding region will not affect any protein.
2. The mutation is a silent mutation — because of the degeneracy (redundancy) of the genetic code, different codons can code for the same amino acid. For example, if a mutation changes the codon GGU to GGC, both still code for the amino acid glycine, so the protein is unchanged.

Harmful Mutations

Some mutations change the amino acid sequence of a protein, which can alter the protein's shape and function:

• Sickle cell disease is caused by a single base mutation in the gene for haemoglobin. The mutation changes one amino acid (glutamic acid → valine), which causes the haemoglobin protein to fold abnormally. This makes red blood cells become sickle-shaped (crescent-shaped) instead of their normal biconcave disc shape. Sickle cells can block blood vessels, causing pain, organ damage, and reduced oxygen transport.

• Cystic fibrosis is caused by a mutation (usually a deletion of three bases) in the gene for a chloride ion channel protein called CFTR. The faulty protein cannot transport chloride ions properly, leading to thick, sticky mucus in the lungs and digestive system.

Beneficial Mutations (Rare)

Very occasionally, a mutation produces a protein that works better than the original, or gives the organism a new advantageous characteristic:

• A mutation that gives a bacterium resistance to an antibiotic is beneficial (for the bacterium) because it can survive in the presence of the antibiotic while other bacteria die.
• In nature, beneficial mutations can spread through a population by natural selection — organisms with the advantageous trait are more likely to survive and reproduce, passing the mutation to their offspring.

Effect	Explanation	Example
No effect	Mutation in non-coding DNA or produces same amino acid	Most mutations
Harmful	Changes protein shape/function negatively	Sickle cell disease, cystic fibrosis
Beneficial	Produces a better protein or new advantageous characteristic	Antibiotic resistance in bacteria

Exam Tip: Exam questions often ask "What effect do most mutations have?" The answer is no effect, because they either occur in non-coding DNA or are silent mutations. Do not assume all mutations are harmful.

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Genetic Variants (Alleles)

What Are Alleles?

Alleles are different versions of the same gene. They arise because of mutations that have occurred at some point in the past. When a mutation changes the base sequence of a gene, a new allele is created.

For example, the gene for eye colour has several alleles — one allele may code for brown eyes, another for blue eyes, and another for green eyes. These are all versions of the same gene, but they differ in their base sequence and produce slightly different proteins.

Key Terms

Term	Definition
Gene	A section of DNA that codes for a specific protein.
Allele	A particular version of a gene. Different alleles have slightly different base sequences.
Dominant allele	An allele that is always expressed when present. Represented by a capital letter (e.g., B).
Recessive allele	An allele that is only expressed when two copies are present (homozygous recessive). Represented by a lowercase letter (e.g., b).
Homozygous	Having two copies of the same allele (e.g., BB or bb).
Heterozygous	Having two different alleles (e.g., Bb).
Genotype	The combination of alleles an organism has (e.g., Bb).
Phenotype	The observable characteristic that results from the genotype (e.g., brown eyes).

Exam Tip: These key terms are fundamental to genetics. You MUST know them precisely. "Allele" is frequently confused with "gene" — remember, a gene is the section of DNA, while an allele is one particular version of that gene.

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Variation

Variation refers to the differences between individuals of the same species. Variation is caused by:

1. Genetic Variation

• Caused by differences in genes (alleles) inherited from parents.
• Sources of genetic variation include:
  • Mutations — random changes in DNA that create new alleles.
  • Sexual reproduction — the combination of alleles from two parents produces unique offspring.
  • Meiosis — during gamete formation, chromosomes are shuffled randomly (independent assortment) and sections of chromosomes may be swapped (crossing over), creating genetically unique gametes.

2. Environmental Variation

• Caused by the conditions in which an organism lives.
• Examples:
  • A plant grown in the shade will be taller and thinner than one of the same genotype grown in full sunlight.
  • Scars, tattoos, and muscle development are all caused by environmental factors, not genes.
  • Language and accent are entirely determined by environment.

3. A Combination of Both

Most characteristics are influenced by both genes and the environment:

• Height is influenced by genes (tall parents tend to have tall children) but also by nutrition (a child with poor nutrition may not reach their genetic potential height).
• Weight is influenced by genes (some people are more predisposed to weight gain) but also by diet and exercise.
• Skin colour is determined by genes but also affected by sun exposure.

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Types of Variation

Feature	Continuous Variation	Discontinuous Variation
Definition	A characteristic that can take any value within a range.	A characteristic that falls into distinct categories with no intermediate values.
Controlled by	Usually multiple genes (polygenic) and the environment.	Usually one or a few genes with little environmental influence.
Examples	Height, mass, hand span, foot length, leaf length.	Blood group (A, B, AB, O), eye colour, ability to roll tongue, attached/free earlobes.
Distribution	Shows a normal distribution (bell-shaped curve) when plotted on a graph.	Shown as a bar chart with distinct categories.
Graph type	Histogram with a smooth curve.	Bar chart with gaps between bars.