Lecture Fourteen - Population and evolutionary genetics Flashcards
What is population and evolutionary genetics?
The study of genetic variation in populations and in evolution.
Define population, gene pool and polymorphism.
In genetics, this is defined as localised groups of interbreeding individuals of the same species.
All the alleles of a gene in a population make up the gene pool.
Many traits show variation in a population, called polymorphism - multiple morphs’ or forms of a trait.
What levels can polymorphism be examined?
Variation or polymorphism in traits can be examined at different levels:
Morphological (physical, visible things).
Physiological.
Biochemical.
Most genes are polymorphic - more than one allele present in the population.
If there’s only one allele, this is called monomorphic and the allele is fixed in the population.
How can genotype frequencies be calculated?
The number of phenotypes expressed divided by the total number of individuals in the population.
E.g.
320/500, 160/500 and 20/500 <– Genotype frequencies.
How can the allele frequency be calculated?
The number of alleles added together, then divided by total number of alleles.
E.g.
320 + 320 + 160 / 1000 <– frequency of red allele.
What is the Hardy-Weinberg principle?
Describes gene pool of a population that is not evolving.
I.e. the allele and genotype frequencies remain constant from generation to generation.
Also can be known as the HW equilibrium or law.
In many cases dominance is incomplete, so we can’t determine genotype of all individuals.
But the HW theory can be used to calculate allele frequencies and estimate genotype frequencies.
If mating is random, every male gamete unites at random with every female gamete, and frequencies of pairings depend on the allele frequencies.
What are the five assumptions of the HW theory?
1) The population is very large.
2) There is no gene flow.
3) There is no natural selection.
4) There is no mutations.
5) There is random mating. If any of these do not apply the the allele and genotype frequencies will change - microevolution.
The mechanisms that most commonly alter allele frequencies are die to violations of conditions 1-3.
How does the occurrence of genetic drift affect the HW law?
If the population size is not very large, genotype and allele frequencies can change due to random sampling effects, called genetic drift.
This can occur because not every individual will reproduce, so if this happens in a small population, only those who reproduce will pass on their genes, thus limiting the traits passed on to offspring.
Certain circumstances can increase the effect of drift:
1) Bottle neck - sudden decrease in population size, thus leaving only a small pool of genetic variation.
2) Founder effect - isolation of a few individuals to form a new population with a smaller gene pool.
How can gene flow affect the HW law?
Migration of individuals into and out of a population can alter allele frequencies if the genotypes migrate differently.
Different effects if unidirectional or bidirectional.
How can natural selection affect the HW law?
If a particular genotype is better suited to an environment these individuals will produce more offspring than other, and contribute more the next generation.
This will change allele frequencies.
What is balancing selection?
Sometimes natural selection maintains two or more forms in population, called balancing selection.
1) Heterozygous advantage - occurs when the heterozygous is more fit than under certain conditions, so the recessive allele is not lost over time.
2) Frequency dependant selection - occurs when selection favours the least common characteristic.
What is the Hardy Weinberg equation?
p2+2pq+q2 = 1
Where ‘p’ and ‘q’ represent the frequencies of alleles. It is important to note that p added to q always equals one (100%).
Frequency of MM genotype = p^2
Frequency of MN genotype = 2pq
Frequency of NN genotype = q^2
