The Gene Pool (VCE SSCE Biology): Revision Notes

The Gene Pool

Introduction to genetic variation

When you look around your classroom, you'll notice that everyone appears unique. There's variation in hair colour (blonde, brown, black, red), eye colour (brown, blue, green), and countless other features. Remarkably, despite these visible differences, you share 99.9% of your DNA with every other person in your class. This raises an important question: how can such small genetic differences produce such diverse appearances?

The answer lies in understanding gene pools and the mechanisms that create variation within populations. One of the most important mechanisms is mutation, which introduces new alleles into a population.

Gene pools and allele frequencies

What is a gene pool?

A gene pool is the complete set of alleles present within a particular population. To understand this concept, we need to define a few key terms:

• Gene: a section of DNA that carries the code to make a protein
• Allele: an alternate form of a gene
• Population: a group of individuals of the same species living in the same location

Understanding allele frequencies

Allele frequency refers to the proportion of certain alleles in a gene pool. We can calculate allele frequencies by counting how many copies of a particular allele exist in a population and dividing this by the total number of alleles for that gene.

Example: calculating allele frequencies in sheep

Let's examine a population of sheep with a gene that controls coat colour. This gene has two alleles: B (which produces black wool) and b (which produces white wool). Each sheep has two copies of this gene, so their genotype could be BB, Bb, or bb.

Genetic diversity and gene pools

A larger and more diverse gene pool contains a greater variety of genes and alleles. This leads to:

• A greater number of genotypes (the genetic composition of an organism at a particular gene locus)
• A greater number of phenotypes (the physical or biochemical characteristics of an organism that result from gene expression and the environment)
• Increased genetic diversity (the variation in genetic makeup or alleles within a population)

Several factors can influence gene pools and alter allele frequencies, including mutations, environmental selection pressures, genetic drift, gene flow, and artificial selection pressures.

Mutations: the source of new alleles

What are mutations?

A mutation is a permanent change to a DNA sequence. Mutations are critically important in evolution because they introduce new alleles into a population, thereby increasing genetic diversity.

Mutations can occur in two ways:

1. Spontaneously: happening naturally during DNA replication
2. Induced by mutagens: agents that cause mutations in DNA, such as UV radiation, certain chemicals, or ionising radiation

How mutations affect protein function

When mutations occur in the DNA sequence of genes, they can significantly affect the expression of that gene. This happens because the mutation may alter the folding and functionality of the resulting protein.

Mutations can be classified into three categories based on their effect on survival:

1. Advantageous: The mutation produces a protein that enhances the individual's survival or reproduction
2. Neutral: The mutation has no significant effect on the individual's survival
3. Deleterious: The mutation produces an abnormally functioning protein that reduces the individual's fitness

Deleterious alleles have an overall negative effect on individual fitness when expressed.

Germline vs somatic mutations

For a mutation to be passed on to offspring, it must occur in specific cells:

Point mutations

Point mutations are mutations that alter a single nucleotide in a DNA sequence. There are several types of point mutations, each with different effects on the resulting protein.

Types of point mutations

Mutation type	Description
Silent mutation	A nucleotide substitution that changes the codon but still codes for the same amino acid due to the degenerate nature of the genetic code (a property where a single amino acid can be coded for by more than one codon). Therefore, there is no effect on protein structure.
Missense mutation	A nucleotide substitution that changes the codon and codes for a different amino acid, altering the primary structure of the polypeptide. This affects the folding of the polypeptide and could alter the protein's function.
Nonsense mutation	A nucleotide substitution that changes the codon to a stop codon, prematurely ceasing translation of the gene's mRNA. The resulting polypeptide is too short to function as intended. These are generally considered the most dangerous type of point mutation.
Frameshift mutation	The insertion or deletion of one or two nucleotides, which alters the reading frame (the order in which nucleotide triplets or codons are divided into a consecutive, non-overlapping sequence). Since the reading frame shifts, all following codons and their corresponding amino acids are affected, causing major disruptions to protein structure and function.

Understanding silent mutations

Silent mutations occur when a nucleotide substitution doesn't change the amino acid produced. This is possible because the genetic code is degenerate - most amino acids are coded for by multiple different codons. For example, both UUU and UUC code for the amino acid phenylalanine, so a mutation changing one to the other would be silent.

Understanding missense mutations

Missense mutations change one amino acid to another in the protein sequence. Depending on the properties of the new amino acid and where it appears in the protein, this could have minor or major effects on protein function.

Understanding nonsense mutations

Nonsense mutations create a premature stop codon in the mRNA sequence. This means translation stops too early, producing a shortened, non-functional protein. These mutations typically have severe effects on the organism.

Understanding frameshift mutations

Block mutations

In contrast to point mutations (which affect single nucleotides), block mutations involve changes to larger sections of DNA. These mutations can affect multiple genes and potentially cause significant changes to an organism's DNA sequence.

Block mutations involve alterations to chromosome structure and typically occur during meiosis. Understanding these large-scale changes is important because they can have dramatic effects on an organism's phenotype.

Types of block mutations

There are four main types of block mutations:

1. Deletion: removal of a section of DNA from a chromosome
2. Duplication: replication of a section of DNA, lengthening the chromosome
3. Inversion: reversal of a section of DNA within a chromosome
4. Translocation: switching of two sections of DNA between different chromosomes

Chromosomal mutations

Mutations can also affect entire chromosomes or sets of chromosomes. These are among the most dramatic types of genetic changes and usually have significant effects on the organism.

Aneuploidy

Aneuploidy occurs when a cell or organism varies in the usual amount of chromosomes in its genome by the addition or loss of a single chromosome. For example, in humans, Down syndrome is caused by having three copies of chromosome 21 instead of the normal two copies.

Polyploidy

Polyploidy occurs when an organism contains additional sets of chromosomes in its genome. Instead of being diploid (2n), the organism might be triploid (3n) or tetraploid (4n). Polyploidy is relatively common in plants but rare in animals.

Case study: sickle cell anaemia

Sickle cell anaemia provides an excellent example of how a single missense mutation can have complex effects on an organism and demonstrates the importance of environmental context in determining whether a mutation is beneficial or harmful.

The mutation

Sickle cell anaemia arises from a missense mutation in the gene coding for haemoglobin (the protein in red blood cells that carries oxygen). This mutation changes the structure of red blood cells from their normal flattened disc shape to a crescent or "sickle" shape.

Effects of the mutation

The sickle shape causes several problems:

• Reduced surface area to volume ratio, meaning sickle cells carry oxygen less efficiently than normal red blood cells
• Sickle cells become trapped in capillaries, reducing blood flow
• In individuals with two copies of the sickle cell allele (homozygous recessive), this often leads to premature death from organ damage due to lack of oxygen

Heterozygote advantage

Despite appearing entirely deleterious, the sickle cell allele provides a fascinating example of how an allele can be both harmful and beneficial depending on the environment.

In regions where malaria (a disease caused by a parasite that infects red blood cells) is endemic, having one copy of the normal allele and one copy of the sickle cell allele provides protection against malaria. This is called a heterozygote advantage.

This example demonstrates how a mutation that appears deleterious can actually be advantageous in certain environmental contexts, illustrating the complex relationship between genotype, phenotype, and environment.

Remember!