Genetics Chapter 27: Population Genetics
Disruptive selection
- 2 or more peaks in the population -fitness values for different genotypes in different environments - works on traits that are determines by multiple genes
Polymorphism
- Many traits have different variations within a population - all the individuals are the same species they just have different alleles so they appear phenotypically different - 2 or more alleles that influence phenotype - monomorphic: genes that exist as a single allele ( has to be in 99% of the population in order to be monomorphic)
Gene Pool
- all of the alleles in a population - only individuals that reproduce can be included as part of the gene pool
Modern description of natural selection
- allele variation arise from mutations in the DNA - Some alleles encode proteins with differing functions, increase reproductive capability - individuals with beneficial alleles are more likely to survive and reproduce - allele frequencies may change due to natural selection: alters the characteristics of the species or makes the species better adapted to its environment
Practice calculating the allele frequency
- always less than or equal to 1 -monomorphic: close to 1 -polymorphic: all alleles should add up to about 1
Non-homologus crossover - double crossover
- axons and the flanking introns are inserted into another gene
DNA fingerprinting
- comparing the DNA from different individuals
Hardy Weinberg equation
- equilibrium under certain set of conditions
effects of inbreeding in a population
- f is the consequence on the HW equilibrium due to nonrandom mating, varies from 1- -1 - increase the frequency for the homozygotes and decreases the frequency for the heterozygotes
population
- group of the same species that occupy the same region -local population: far more likely to breed with other members ( the populations is separated by geographic barriers ) -dynamic change from one generations to the next
consequence effects of inbreeding
- higher homozygosity, which can be used to yield a desirable trait - increasing the likely for homozygosity of genetic disease - in natural populations, inbreeding lowers the overall fitness of the population
Microevolution
- in natural population, gene variations changes between the generations -microevolution: changes in the gene pool from generation to generation Driven by: -mutation - random genetic drift -migration -natural selection -nonrandom mating
horizontal gene transfere
- incorporating the genetic material of one organism without the new organism being the offspring of the organism the incorporated genetic material was taken from - usually in prokaryotes about 20/30 % of mutation happens this way - can occur from prokaryotes and eukaryotes and vice versa
genetic variation due to receptive sequences
- introduction of repetitive sequences - microsatilites: 1-6 repeat sequences - tandem repeat: hundreds of BP long - minisatilites: larger repeats 6-80 covering 1,000-20,000 bp
what is true about large populations and the elimination and the fixation of certain alleles
- larger populations are more likely to have a mutation than the smaller populations - larger populations have a greater chance of the new mutation being eliminated compared to the smaller populations
Darwinian fitness
- likelihood that individual will survive and contribute to the gene pool - different genotypical classes assigned different fitness values, according to their reproductive success
Mutations the source of genetic variations
- mutations can alters the chromosomal structure / number - mutations occur at a low rate - mutagens increase the mutation rate - mutations provide the materials for evolution but they don't do the evolution themselves
Populations and population genetics
- mutations need to be considered because they affect the reproductive success of an individual: beneficial, deleterious, neutral -beneficial mutations are far less likely to occur than neutral or deleterious mutations
conditions to reach equilibrium
- no new mutations - no genetic drift - no migration - random mating - no natural selection + no population really satisfies these requirements +null hypothesis not rejected: population in HW equilibrium + null hypothesis rejected, population not in HW equilibrium
Inbreeding hw
- since the population is smaller when inbreeding, there is a smaller gene pool -inbreeding coefficient analyzes the degree of relatedness within the pedigree - increases the proportion of homozygotes - the likelihood of the path is calculated by calculating all individuals in the path besides the individual of interest + in this case n=5
Heterozygous advantages deleterious alleles
- the Hb^s is the allele for the human globin gene - individuals who have heterozygous allele have the highest fitness in the populations where malaria is a threat
fixation coefficient
- the allele because fixed because it is in the homozygous state - f increases as the population size decreases
mutation effect allele frequencies over time
- the occurrence of gene mutations very slowly changes the allele frequency
How do allele frequencies change as a result of non-random mating
- the species chose who they mate with regardless of their phenotype of genotype - in humans, this principle is violated frequently
production of new genes via exon shuffling
- two discrete domains with different functions - the domains tend to be encoded by one or a series of Exons - exon shuffling is when there is an exon and introns inserted into a gene - duplication or rearrangement of the exons - promotion by transposable elements - also can be the result of non homologs crossover
changes in population (3)
-Size -Geographic location - Genetic composition
Mean fitness for values with A.5 a.5
-after 1 generation the AA phenotype frequency should be increasing In this case, the allele frequency of big A will increase and little a is going to decrease because the AA genotype has the highest fitness
Time for fixations
-allele fixation takes much longer in the larger populations
Directional selection
-favors one extreme phenotype equation: p2wAA 2pqWAa q2waa + the equations don't add up to 1
Migration gene flow
-gene flow is the transfer of genes from the donor population to the recipient population, which changes the gene pool - new population is called the conglomerate To calculate: -Must know the allele frequency in the recipient and donor population - the proportion of the conglomerate that is due to migrants
calculating the fitness values
-highest reproductive gene gets value of 1 - values used to calculate how the allele frequency will change from one generation to the next
Migration
-local population separated -moving from one population to another
Natural selection key points
-phenotypes different reproductive success -natural selection acts on the phenotypes which are influenced by the genotypes -reproductive success varies by: + ability to reach reproductive age + factors that directly affect fertility
Genetic drift (3)
-populations are relatively small -allele frequencies change due to chance - more of an factor the smaller the population
mutation rate
-probability that a gene will be altered by a new mutation -new mutation / generation - 10^-5 - 10^-6
Genetic drift concept
-random changes in the allele frequencies as a random chance -favors the loss or the gain of an allele - monomorphic alleles cannot fluctuate -large populations the drift Is more subtle than compared to the drift in the small populations
Negative frequency dependent selection
-rare individuals are more likely to reproduce -common individuals are less likely to reproduce
SNP
-single change in a base pair of the DNA -humans 2-3 thousand bp long (10 polymorphic sites)
nonrandom mating
-some phenotypes more preferable than other. In this case, individuals chose who they want to mate with
Natural selection
-some populations have greater reproductive success than other populations
stabilizing selection
-the extreme phenotypes are selected against - tends to reduce the diversity
Balancing selection (4)
-when polymorphism reached, forces balance each other -heterozygote advantage is one mechanism that can cause this -homezygotes have a lower fitness than the heterozygote -Negative frequency dependent selection
Four patterns of natural selection
1) directional selection: one extreme phenotype 2) Balancing selection: favors 2 or more alleles maintained 3) disruptive selection: survival of 2 or more phenotypes 4)stabilizing selection: survival of individuals with intermediate phenotypes
Bidirectional migration
1) reduces the allele frequency differences between the two populations 2) it can enhance the genetic diversity in the population
new mutations caused by the genetic drift in populations (where is the mutation more likely to occur)
2nu n= individuals u= mutation rate - new mutation is more likely to occur in a large population compared to a smaller population because large population have more individuals
Inbreeding/outbreeding
In: genetically related individuals out: individuals not genetically related - in absence of other forces, allele frequencies not altered by in or out breeding - affect the HW phenotypes
fundamental calc to population genetics
allele frequency: total copies of allele in population/total number of alleles in the gene population Genotype frequency: particular number of specific genotype in specific population/ total number of the individuals in the population
sources of genetic variations in eukaryotes and prokaryotes
eukaryotes: sexual reproductive gives new combination of alleles prokaryotes: gene transfere - rare DNA mutations give rise to new variants - changes to the structure or number of chromosomes
2 explanations for genetic drift
founder effect: small group of individuals that separate from the larger population (founding population less genetic variation, population also have allele frequencies much different than the general population) bottleneck effect: population reduced by disaster (go from large genetically diverse population to a large non genetically diverse population
Changes in the allele arise through:
genetic changes
mutation
genetic changes arise -provide the material for the changes in the population
differences in reproductive success
likelihood to: survive, be fertile, mate
calculating the probability of fixation or elimination of an allele
look at the screenshot for equations
Heterozygous population will reach equilibrium when
sAAP=saaP
Assortative mating
when mating does not occur randomly Positive: mating more likely between individuals who are phenotypically similar negative: mating is more likely to occour between individuals with dissimilar phenotypes
