KK1 - gene pool and types of mutations

Gene pool

the total aggregation of all the genes and alleles present within a particular population or species

Allele frequencies

• the proportion of a particular allele appearing at a certain gene locus in a gene pool

  • Frequencies could be calculated by totalling the number of a particular allele by the total number of alleles present in the population

• Larger and more diverse gene pool will have a larger variety of genes and alleles

  • Leads to a greater number of genotypes and phenotypes therefore increasing genetic diversity

mutations

• Mutations - permanent changes to the DNA sequence of an individual

  • Can occur either spontaneously or be induced by agents known as mutagen (e.g. UV radiation)

• When mutations occur in DNA sequence of genes, they can have a significant downstream effect of the expression of the gene

  • Alters the folding and functionality of the resultant protein 

• Can be classified as advantageous, neutral or deleterious

  • Depending on the effect on the survivability of the individual affected

evolutionry signficance - mutations

• Evolutionary significance - increases genetic diversity through mutations

  • Ability to create and introduce new alleles into a population 

• In order for mutations to be heritable, must occur in an individual's germline cells

  • If mutation occurs in somatic cell, then it is not heritable

POINT MUTATIONS

• A change to a single nucleotide in a gene

• Modifications can include substitution of a base

  • Broken down into silent, missense and nonsense mutations depending on their effect on the protein produced

  • Can also include in the addition and deflection of a single nucleotide

• Triggers what is known as frameshift mutation

Silent mutations

• Substitution mutations have no effect on the resulting amino acid sequence

• Due to degenerate nature of genetic codes, multiple different codons code for the same amino acid

  • Despite changes to original DNA sequence, the same amino acid is incorporated into the protein

Missense mutation

• Substitution mutations which code for a different amino acid, altering the primary structure of the polypeptide

• Affects of the folding of polypeptide and could alter the functioning of the protein

Nonsense mutation

• Substitution mutations which prematurely end the translation of a gene’s mRNA

• Substitution of nucleotide can cause codon to become a stop codon

• Gene will not be completely translated

  • Leads to a polypeptide being too short to function to intended

• These mutation considered most dangerous

Frameshift mutation

• Addition or deletion of one or two nucleotides, which alters the reading frame of all the following nucleotides.

  • Reading frame - how DNA or mRNA is divided into triplets or codons

• The amino acids they code for are affected, which can cause major disruptions to the structure and function of the protein

Block mutations

• Involves changes to larger sections of DNA, potentially causing significant changes to DNA sequence of the organism

• Involves the alteration of the structure of a chromosome by deleting, duplicating, inverting or translocating a cluster of nucleotides

  • May involve multiple genes

• Occur during the process of meiosis

Aneuploids and polyploids

• Can cause genomes to gain or lose a chromosome or in extreme cases, entire sets of chromosomes

  • Referred to aneuploidy and polyploid

• Aneuploidy - a chromosomal abnormality in which an organism possesses an incorrect number of total chromosomes caused by the addition or loss of an individual chromosome

• Polyploidy - to a chromosomal abnormality in which an organism has an incorrect number of sets of each chromosome - there is an entire set of chromosomes added or missing

Environmental selection pressures

• factors within the environment that influence the survivability of a species within a given environment

  • predation, disease, competition, and climate change. 

• The process of natural selection can occur

  • involves the selection of the phenotype most suited to overcome the environmental selection pressure

Genetic fitness

• higher genetic fitness, due to the presence of their advantageous phenotype, which arises due to the presence of certain alleles

• Fitter organisms with the advantageous phenotype have a selective advantage and are more likely to survive

  • More likely to pass on their alleles to the next generation

  • increases allele frequency of the allele that codes for the advantageous phenotypes

Natural selection

• Relies on the heritability of the advantageous allele and the presence of variation with the existing population

  •  ensure that the alleles that confer an advantage are present within the environment

• Variation allows for disadvantages alleles to arise

  • can be selected against and subsequently removed from the population

FOUR CONDITIONS OF NATURAL SELECTION

variation

individuals in a population vary generically, which leads to phenotypic differences

Selection pressure

an environmental selection pressure impacts the survivability of organisms within a population and their ability to reproduce

Selective advantages

• Individuals with phenotypes that are fitter or more advantageous under the environmental selection pressure are conferred a selective advantage

• allows them to survive and reproduce more successfully.

Heritability

• The advantageous trait must be heritable, allowing it to be passed on from the parents to their offspring

• Therefore, over time, the frequency of the advantageous allele will increase.

GENETIC DRIFT

• Involves changes to a population's allele frequencies due to sudden and random occurrences

• Genetic drift can occur through the bottleneck effect or the founder effect

Bottleneck effect

• Occurs when a large portion of a population is wiped out by a random event such as a natural disaster

  • Dramatically decrease the population size, impacting allele frequencies

• Reduction in population size can cause individuals carrying unique alleles to be lost

  • New population has lower genetic diversity than the pre-disaster population

Founder effect

Occurs when a small unrepresentative sample of individuals separates from a larger population to colonise a new region and start new population

For example

• Population has beetles of many different colours

• If 10 beetles left their original population to form a new population, it would not mirror the initial gene pool

  • Would be considered an unrepresentative sample

• Genetic diversity of the new population is lower than the original

If the original population were all green, the green between would not be considered an example of the founder effect as the founding colony resembles the initial gene pool

gene flow

• Involves the introduction or removal of alleles between populations through migration or interbreeding

• Migration occurs when populations are physically close together or due to external forces such as the clearing of geographical barrier between populations

• Entering a population via migration means that their alleles are added to the gene pool of that particular population

• Existing a population via emigration means that their alleles are removed from the gene pool

• When individuals temporarily enter a population and interbreed with local individuals before leaving again, they contribute to the gene pool of that particular population

• populations in different geographic locations can exchange alleles through either migration or interbreeding