01. Changing allele frequencies

Outline genes vs allele

A gene is a sequence of DNA that code for a particular protein - the location of a particular gene on a chromosome is called a locus.

Humans are diploid organisms, with 2 complete sets of chromosomes in each somatic cell (called homologous chromosomes), one inherited from each parent. This means there are two copies of each gene - slight variations in the DNA sequence of a gene can result in different forms of a trait. Alternative forms of the same gene are called alleles.

Outline genotype vs phenotype

The genotype is the different combination of alleles for each particular gene - i.e. the genetic makeup of an organism.

The phenotype is the way in which the gene is chemically or physically expressed - i.e. the observable characteristics of an organism.

The phenotype is determined by the genotype as well as the environmental conditions of the individual.

What is genetic diversity/genetic variation?

The variation in genes or alleles within a population or species.

What is a population?

Members of one species living in the same geographic area at the same time, with the capability of interbreeding.

What is a species?

A group of organisms that are able to interbreed and produce viable offspring.

What is the gene pool?

The total genetic diversity in a population.

What is allele frequency and how is it calculated?

The proportion of a population that has a particular allele.

Allele frequency = number of copies of allele of interest/total number of copies of all the alleles.

When calculated, allele frequencies always add up to 1.0 or 100%

Outline the Hardy-Weinberg principle

Allele frequencies in a populations remain constant over generations when a specific set of conditions all exist.

• the population must be large

• members of the population mate at random

• all matings are equally fertile, producing equal numbers of viable offspring

• there is no migration either into or out of the population

• there is no mutation

Rarely, if ever, do these conditions exist at the same time in nature. Allele frequencies in a gene pool can therefore change over time.

What are the four main processes that drive changes in allele frequencies and the evolution of populations and species?

• mutations

• changes in environmental selection pressures leading to natural selection

• gene flow

• genetic drift

Outline mutations

Mutations are permanent changes in DNA that occur spontaneously (when DNA is copied during cell division) or due to mutagens.

Most mutations are detected and repaired by enzymes. Those that can’t be repaired can be categorised as:

• neutral mutations - have no effect on survival

• beneficial mutations - increase the likelihood of survival

• harmful mutations - decrease the likelihood of survival

What are mutagens?

A factor that increases the rate of mutation above the usual spontaneous rate (e.g. UV light and x-rays).

Outline somatic mutations and germline mutations

The type of cell a mutation occurs in determines how it affects the gene pool

Somatic mutations occur in body cells and only effect that individual, with the mutation not able to be passed onto offspring. The effect on the gene pool (if any) is temporary.

Germline mutations occur in the germline cells that produce gametes (egg, sperm, ovules, pollen grains, etc). Can be passed onto offspring. Can result in changes to the genetic makeup of a population over time through the introduction of new alleles.

What are the types of mutations?

• Point mutations

  • substitution mutations

  • frameshift mutations

• Block mutations

Outline point mutations

Gene mutations that occur within a specific DNA nucleotide triplet, where one nucleotide is substituted by another, or the order within the triplet is changed.

2 types of point mutations: Substitution mutations and frameshift mutations

(Point mutations) Outline substitution mutations

Occur when a single nucleotide in DNA is exchanged for another. Results in a different triplet or codon being transcribed and translated during protein synthesis.

Can result in a:

• silent mutation - a mutation that causes no change in the amino acids in the polypeptide.

• missense mutation - a mutation that causes a change to one amino acid in the polypeptide.

  • conservative mutation - the mutated polypeptide can carry out its normal function

  • non-conservative mutation - the mutated polypeptide is unable to carry out its normal function.

• nonsense mutation - a mutation that causes a stop codon and results in a dysfunctional protein.

(Point mutations) Outline frameshift mutations

Occur when nucleotides are inserted or deleted from a DNA sequence, resulting in a new sequence of amino acids from the point of mutation onwards.

Even a single nucleotide insertion or deletion can have a profound effect on the corresponding protein.

Outline block mutations

Large sections of a chromosome are permanently altered.

Most often occurs during crossing over in meiosis I - the homologous pairs of chromosomes swap sections incorrectly, leading to:

• deletion of a part of a chromosome

• duplication or gain of part of a chromosome

• translocation or reciprocal exchange between non-homologous chromosomes

• inversion when a segment of a chromosome rotates through 180 degrees.

Often results in faulty gamete production and developmental defects during embryo development. Duplication can provide raw material for new alleles.

Changes to an entire chromosome can result in:

Polyploidy - the entire chromosome is replicated (so instead of 2 of each chromosome there may be 3 of each chromosome). Does not occur in humans as it usually results in death during development.

Aneuploidy - loss or gain of a single chromosome. Results in an abnormal chromosome number, e.g. down syndrome.

Outline environmental selection pressures leading to natural selection

The conditions or factors that influence which phenotypes are most successful in a population - and therefore, influence allele frequencies in that population.

Selection pressures can fall under 3 main categories: physical, biological, chemical. And can be natural or artificial (brought about by humans).

Natural selection occurs when the allele frequencies in a population’s gene pool change due to environmental selection pressures.

What are the steps of natural selection?

1. Variation exists between inds in a population

2. A selection pressure occurs

3. Those with a favourable trait are more biologically fit and will be selected for, whilst unfavourable traits are selected against.

4. Those with the favourable trait are more likely to survive long enough to reproduce, so the next generation will be genetically like them.

(Alleles of the favourable trait increase in frequency in the gene pool, while alleles of the unfavourable trait decrease in frequency).

Outline gene flow

Gene flow is the transfer of alleles between populations and can result in changes in allele frequencies in a gene pool.

Occurs when new inds join a population from a different gene pool or when inds leave a population. Movement of inds in and out of populations can result in new alleles being introduced or alleles being lost, increasing or decreasing the genetic diversity in the population.

When gene flow exists between two different populations, the gene pool may remain similar. If gene flow is not possible between populations, the gene pools are isolated and each population will slowly accumulate differences. This can lead to the formation of a new species.

Smaller populations are affected greatly by gene flow, while large populations act as a buffer, preventing total loss of particular alleles and allele combination is an individual emigrates (leaves the population).

What is genetic drift?

The random change in allele frequencies of a population due to chance events, such as births and deaths.

Has the greatest impact on small populations with little to no gene flow, as the random death of one ind can significantly alter allele frequencies.

2 causes of genetic drift:

• the founder effect

• the bottleneck effect

(Genetic drift) Outline the founder effect

When a few individuals leave a population to establish a new one, with the two populations now isolated from each other.

The smaller/new population will only have a small portion of the alleles of the original population, resulting in lower genetic diversity. - recessive alleles that may be deleterious have a higher chance of meeting during fertilisation than they did in the original population.

e.g. The Afrikaner population of Dutch settlers in South Africa are mostly descended from a few colonists. They have a higher incidence of Huntington’s disease than what would be expected

(Genetic drift) Outline the bottleneck effect

Can occur when an intense selection pressure (or natural catastrophe) leads to a severe reduction in population size.

Intense selection pressure could include contagious disease, loss of habitat, hunting or a volcanic explosion.

Loss of most of the population results in the allele frequencies being altered at random, which severely limits genetic diversity within the remaining population. Results in higher levels of inbreeding, further reducing genetic diversity.

Makes the population more vulnerable to enviornmental change.

What are the biological consequences of changing allele frequencies?

A population with low genetic diversity has a gene pool with a low number of alleles for each gene. This may make organisms more vulnerable if there are changes in selection pressures, and increase the likelihood of extinction, as they are less likely to be able to adapt to changing environments.

Populations with low genetic diversity are often small in size and have limited or no gene flow with other populations’ gene pools.