chapter 20 Flashcards
Generations and relationship among individuals in Mendelian and Population Genetics
Mendelian: known
Population: unknown
Number of analyzed alleles
Mendelian: usually 2
Population: highly variable (one to thousands)
Forces influencing individuals/populations
Mendelian: known and controlled
Population: Unknown and inferred
Mode of reproduction (sexual vs asexual, random vs non-random mating)
Mendelian: known
Population: from known to unknown depending on organisms
Population
group of interbreeding organisms
Gene Pool
the collection of genes and alleles found among members of a population
Population genetics
the study of allele frequencies and genotype frequencies within and between populations
Evolution
changes of allele frequency and genotype frequency over time
* Influenced by mating patterns, mutation rate, genetic drift, natural selection, etc
Hardy-Weinberg Equilibrium
a model to calculate expected frequencies of alleles and
genotypes of interest in large populations
(both Godfrey Hardy & Wilhelm Weinberg independently concluded that random mating and absence of evolutionary change leads to stable allele frequencies in populations)
Assumptions of Hardy-weinberg Equilibrium (6 points)
- Population size is infinite (resist random fluctuations, small change in small population can easily skew allele frequencies)
- Random mating occurs in population (no sexual selection/no favoured phenotypes/attraction)
- No natural selection (no evolutionary forces, no favoured phenotypes - ex:white coat for snowy environment)
- No migration/gene flow (no introduction of new alleles from other populations)
- No mutations that introduce new alleles
- No genetic drift
Predictions of Hardy-Weinberg Equilibrium (4 points)
- Allele frequencies remain stable over time
- Allele distribution into genotypes is predictable
- Stable equilibrium frequencies of alleles and genotypes are maintained
- Evolutionary and non-random mating effects are predictable
H-W equilibrium calculates expected genotype and allele frequencies when…
when evolution does not occur
How to Determine Autosomal Allele Frequencies from Genotype Frequencies in Populations (Two Methods):
- Gene Counting Method
Requires genotypes of all members to be identifiable
Useful for codominant alleles - Square Root Method
Used when gene has two alleles with a dominant-recessive relationship
Simply take square root of q2, then p is calculated as 1-q
Natural Selection
H-W equilibrium is maintained when there is random mating and no
evolutionary change in a population. Allele frequencies change when evolution occurs.
Evolutionary impact can be quantified by determining change in allele frequencies
Relative fitness (w)
can quantify natural selection intensity, comparison of an organism’s reproductive success (fitness) to that of other individuals in the population
w = 1.0 represents highest reproductive success for a certain trait
Selection coefficient (s)
Individuals that reproduce less have their fitness decreased by this proportion. The coefficient of selection is usually taken to be a measure of the extent to which natural selection is acting to reduce the relative contribution of a given genotype to the next generation
E.g, if individual A has a relative fitness of 1.0 and individual B has a relative fitness of 0.8, the selection coefficient = 0.2
Therefore, individual B reproduces 80% as well as individual A
Natural Selection Favours
Balanced Polymorphism
Heterozygotes
Balanced polymorphism: this occurs when a heterozygous genotype is favoured
-Alleles reach stable equilibrium frequencies that are maintained in a steady state
-Selective pressures maintain mutant allele in heterozygotes, but act against it in homozygotes
-Keeps a “balance” of alleles since heterozygotes, being the most fit, preserve both alleles and resists allele fixation
2 types of Mutation
Mutation: the ultimate source of all new genetic variation in population
Forward mutation rate (μ): the rate of creating new alleles
Reverse mutation rate (v): rate of mutation to original allele
How do Alleles maintain stable frequencies
if forward mutation rate and reverse mutation rate remain unchanged and no other evolutionary forces are present
Gene Flow and 4 things that it can do:
Occurs by movement of organisms and genes between populations
Gene flow can:
* Introduce novel alleles
* Increase frequency of existing alleles (shift)
* Remove/reduce existing alleles
* Create admixed population
Genetic Drift
Chance fluctuations of allele frequencies due to “sampling bias”. Genetic drift occurs in all populations but is especially prominent in small populations.
Brownian Motion model of genetic drift
4 populations have allele frequencies drifting at the same time
- more individuals in pop. have greater chance of equal distribution of alleles
- less individuals in pop. have less chance of equal distribution of alleles , greater chance of drift
Inbreeding (consanguineous mating) and what it results in
mating between related individuals - reduces heterozygosity.
Increase homozygous genotypes
Biological Species concept (BSC)
a group of organisms capable of interbreeding with each other but isolated from other species
