Module 6: Variation Flashcards
Homologous chromosomes:
Pairs of chromosomes of the same size and shape carry genes for the same trait.
Heterozygous:
Individuals who carry different alleles of a specific gene on a pair of homologous chromosomes.
Homozygous:
Individuals who carry two identical alleles of a specific gene on a pair of homologous chromosomes.
Dominant Allele:
An allele that’s phenotype is expressed when both homozygous or heterozygous chromosomes.
Recessive Allele:
An allele that is phenotypically expressed only in the homozygous condition.
Continuous Variation:
Characteristics that can take any value within a range.
Discontinuous Variation:
Characteristics that can only appear in specific discrete values.
Continuous Variation: Causes and Genetic control
It is caused by genetic and environmental variation. Multiple genes (polygenes) code for an allele.
Discontinuous Variation: Causes and Genetic Control.
Mostly genetically caused. Coded for by only one or two genes.
What causes Genetic Variation?
Mutation, but mostly gene shuffling and independent assortment in meiosis.
Causes of continuous variation:
Genetic and environmental
Causes of discontinuous variation:
Mostly genetic.
Definition of Continuous Variation:
A characteristic that can take any value within a range. .
Definition of Discontinuous Variation:
A characteristic that can only appear in specific values.
What are “pure breeding” individuals?
Organisms that are homozygous.
Codominance:
Two different alleles for the same gene are equally dominant, resulting in both genotypes being expressed.
Multiple Alleles:
Some genes have more than 2 versions, however only two versions can be present in an individual, because the organisms only carries one on each homologous chromosome. e.g Blood Types: I0, IA, and IB
Sex Linkage:
Genes that are carried on the sex chromosomes.
Why do females have more genes than males?
Men have a Y chromosome which is very small and-so doesn’t contain all the information of the X chromosome. Therefore the man only has 1 copy of the gene if they’re present on the X chromosome.
Why might the actual ratio of offspring differ from the expected ratio?
the fertilisation of gametes is random and so a small sample can be skewed due to random chance.
If the genes studied are linked genes and no crossing over occurs, the two characteristics are always inherited together.
Autosomal Linkage:
Genes that are linked are found on one of the of the chromosomes in a homologous pair and-so are less likely to be shuffled apart.
Linked genes are inherited as _________ and cannot undergo _________ and-so the ________ ratios will not b e produced.
Linked genes are inherited as one unit and cannot undergo “shuffling” and so the expected ratios will not be produced.
Recombinant Offspring:
Have different combinations of alleles than either parent.
What effects autosomal linkage:
The closer proximity of gene loci reduces the chance of them being separated.
How to calculate the degrees of freedom (Chi^2 test):
n (“Number of categories or possible phenotypes”) -1
Epistasis:
The interaction of genes at different Loci. Gene regulation with regulatory genes controlling the activity of structural genes is an example of epistasis (e.g lac operon)
Hypostatic Gene:
A gene that is affected by another gene.
Epistatic Gene:
A gene that affects the expression of another gene. (This “masks” the effect of any subsequent expression of genes in the sequence)
Dominant epistasis:
A dominant allele results in a gene affecting another gene.
Recessive Epistasis:
Two recessive alleles result in a gene affecting another gene.
What is population genetics?
The study of how allele frequency in a population changes over time.
The relative frequency of a particular allele in the population is called the __________
The relative frequency of a particular allele in the population is called the allele frequency.
What is the Hardy-Weinberg Principle:
In a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to another and there will be no evolution.
Why is the Hardy-Weinberg principle useful?
It provides a simple model of a theoretical stable population that allows us to measure and study evolutionary changes when they occur.
Why is the Hardy-weinberg principle flawed?
None of these conditions would be met in the wild.
What are the conditions of the Harvey-Weinberg principle?
A large and isolated population of diploid organisms, with random mating, no mutations and no selection pressures.
Factors affecting evolution:
Mutation, Sexual Selection, Gene Flow, Genetic Drift, Natural Selection.
How does mutation affect evolution?
Mutation is necessary for creating new alleles, leading to genetic variation.
How does Sexual Selection affect evolution?
Sexual Selection increases the frequency of alleles which code for characteristics that improve mating success.
How does Gene Flow affect evolution?
Geneflow is the movement of alleles between populations. Immigration and emigration result in changes to allele frequency within a population.
How does Genetic Drift affect evolution?
Genetic Drift occurs in smaller populations and is caused by the random nature of mutation. In a small population the presence of an new allele is great, whereas in a large population it is not.
How does Natural selection affect evolution?
Natural Selection leads to the increased allele frequency of alleles that provide organisms a selective advantage. This is because the reproduction rates of this group are increased and-so more offspring possess the advantageous allele.
Why are small populations at risk of extinction?
Small populations have limited genetic diversity cannot adapt to changes in the environment.
What are the two limiting factors affecting populations:
Density-dependent and Density-independent
Density-Dependent Factors affecting population:
Competition, predation, parasitism, and communicable disease, waste accumulation
Density-Independent Factors affecting population:
Climate Change, Natural Disasters, Seasonal Change, and human activities.
Stabilising Selection:
The average is favoured as a selective advantage, resulting in the population of more extremes being reduced. This leads the population to tend towards the most common phenotype.
Disruptive Selection:
The extremes are selected for and the average is selected against.
Directional Selection:
A change in the environment which causes the normal phenotype to no longer be the most advantageous. Therefore one extreme is favoured, leading to it having a higher allele frequency.
Speciation:
Formation of new species through the process of evolution. Organisms belonging to the new species will be no longer able to interbreed to produce fertile offspring with organisms belonging to the original species.
Speciation is caused by what events:
Isolation of members of the population, Alleles undergoing random mutations, Accumulation of mutations and changes in allele frequencies over many generation.
Allopatric Speciation:
Geographic isolation of two populations of the same species prevents gene flow and they become reproductively isolated.
What are the two requirements that must be met for an organism to be acknowledged as a new species?
They must be either geographically isolated in the wild or unable to reproduce to form fertile offspring.
Sympatric Speciation:
An isolating barrier disrupts gene flow between populations of the same geographical area.
What can cause sympatric speciation?
Two organisms from different species reproduce to form a fertile hybrid that cannot breed with the original species.
How are asexual organisms classified?
By their morphological and physiological features,
What are the two types of isolating mechanism?
Geographic and Reproductive isolation.
What are the two types of Reproductive isolation?
Prezygotic barriers and Postzygotic barriers.
Prezygotic Barriers:
Incompatibility - anatomical or physiological differences.
Temporal - Seasonal mating
Ecological - region (niches)
Behavioural - Courtship
Postzygotic Barriers:
Hybrid Fitness - ability to survive
Hybrid Fertility - ability to reproduce
Artificial Selection:
Artificial Selection (selective breeding) - is similar to natural selection however the nature of the selection is different. It is the selection of characteristics deemed desirable by farmers and breeder. This often results in inbreeding.
What problems can be caused by inbreeding?
- It limits the gene pool, decreasing genetic diversity which reduces the chances of the population adapting to changes in their environment.
- Increased frequency of genetic disorders caused by recessive alleles.
How does inbreeding cause an increased frequency of genetic disorders?
reducing the gene pool means most organisms now genetically similar, possessing same recessive alleles -> results in offspring having a greater chance of being homozygous recessive -> overtime this reduces the ability of the species to survive and reproduce -> decreasing their biological fitness.
