Chapter 9 Flashcards
Most of Darwin’s theory involved what kind of variation?
Continuous variation (gradual change)
Compensatory mutations
These mutations do not reduce the degree of resistance, but reduce/ eliminate the fitness cost associated with resistant phenotype
Most of Mendel’s theory involved what kind of variation?
Mendelian genetics supported larger DISCRETE variation/ changes.
Define polygenic; what kind of variation do these genes exhibit?
Many traits are affected by many genes simultaneously; Polygenic traits can exhibit continuous variation
Multifactorial inheritance
Inheritance of a polygenic trait
Additive genetic effects
Phenotype of an individual can be determined by summing the effects of each allele that it carries; example: 3 genes involved in the gradation of kernel colors of wheat
Studies found that polygenic traits and continuous variation establish small graded variation is compatible with what?
Mendelian inheritance
Remember natural selection DOESN’T create variation so why in a selected population are there variants?
Variation in a relatively small number of genetic loci can generate an enormous amount of possible phenotypes
What is true when populations contain latent variation?
Not all possible genotypes could be represented
Selection could shift allele frequencies and genetic reassortment could then draw out new phenotypes from pre-existing variation
what is epistasis & give an example
When alleles at two or more loci interact in NON-ADDITIVE
ways to determine phenotype
ex. coat color in oldfield mice; effect of an allele at the Mc1R locus depends on which alleles are present at the Agouti locus
What does it mean when phenotypic effects of loci are Context Dependent? think of old field mouse alleles
Phenotypic effects of alleles at one locus depend on the context that is set by the alleles at another locus
In diploids, when two loci A and B are located on SEPARATE chromosomes…
Alleles will segregate independently according to Mendel’s law
In diploids, when two loci A and B are located on the SAME chromosome…
There is physical linkage; alleles segregate together in the absence of recombination.
What does the rate of recombination depend on?
Depends on the physical distance between the loci; closer together the lower the rate *think of two tied ropes, further apart= more chance for crossing over
Linkage disequilibrium
When there’s statistical associations present between the alleles at the A locus and B locus
How is linkage disequilibrium broken down/ dissipates?
In the absence of other evolutionary processes, linkage disequilibrium is broken down by recombination.
Where does recombination occur?
Occurs between haplotype pairs; creates new haplotypes only in double heterozygotes.
What happens when recombination occurs in a double heterozygote?
New haplotypes are produced
What is the principle of association mapping?
A technique by which loci are responsible for disease or other traits are located
One of the consequences of genetic linkage is Genetic Hitchhiking. What happens during this event?
An unselected or even disadvantageous allele is able to “ride along” with a nearby favorable allele and thus increase in frequency.
What will eventually break the association of Genetic Hitchhiking? What will the speed be dependent on?
Recombination will eventually break this association, but the speed will be dependent on distance.
If recombination breaks down genetic hitchhiking, why do medical diseases still exist?
Genetic diversity for surrounding alleles will be reduced relative to expectations of a neutral model.
What is background selection, (also a consequence of genetic hitchhiking)? analogous to hitch-hiking
Deleterious mutations carry nearby alleles to extinction analogous to hitchhiking.
Alleles can increase in frequency due to selection because…
They directly code for the beneficial mutations or are associated with other beneficial alleles on other loci.
______ tends to decrease genetic variation at loci near selected allele.
Natural selection (positive or negative)
Why does hitch-hiking dramatically effect bacteria?
Because they’re haploid; and bacterial alleles tend to be close together
Three facts about bacterial alleles/ locus (periodic selection slide)
- do not have recombination between homologous chromosome (haploid)
- no independent segregation of chromosomes
- both of these create a “single” large locus
What is periodic selection?
Result of tight linkage across bacterial genome
What is the process of periodic selection? (4 steps)
New beneficial mutation arises in a bacterial population
Mutation goes into fixation due to natural selection
Selective sweep results: alleles go to fixation due to tight linkage
after fixation, a new advantageous allele arises and the process repeats
What process is possible responsible for the persistence of antibiotic bacteria?
Periodic selection
How is antibiotic resistance transferred?
Through plasmids (small DNA molecules) or transposons
What is effective population size?
The group that has the potential to reproduce (can contribute its fitness to future generations).
What is clonal interference?
Beneficial alleles interfere with one another and slow down selection => competition
Adaptive landscapes (fitness landscapes) shows what?
Interactions among genes; generates a complicated genotype-to-phenotype map.
Why are adaptive landscapes complicated?
- pleiotropy (single gene can effect multiple aspects of phenotype)
- epistasis (phenotypic trait is often determined by complex interactions among multiple genes)
- norms of reactions (single genotype produces different phenotypes in different environments
- dominance (one allele may cover up the effects of another allele at the same locus)
- multiple pathways (a common phenotype may have different genetic basis in different individuals)
Although natural selection act on phenotype…
Next generation only gets genotype.
Quantitative genetics
studying continuously varying traits; ex. tomato size
What are three factors that contribute to phenotype?
- Genes
- Environment
- Even if genotypes and environments are identical, there still can be differences
What is developmental noise?
Random chance events during the process of development that can give rise to phenotypic differences
Variance?
Statistical measure of the variation in a sample; tells us how different trait values are from one another.
Narrow-sense heritability?
Fraction of the total variance due to additive genetic variation
How do we estimate broad sense heritability?
Compare amount of variation among genetically identical individuals with the amount among unrelated individuals ; usually use inbred lines
What is the narrow sense heritability?
A measure of what fraction of the variation is accessible to natural selection… (since natural selection can’t possibly act upon all variation, need to look at the fraction of the total variation that is due to additive genetic variation)
Selection Differential (S)
The difference between the mean trait value of the individuals who successfully contribute to the next generation and the mean trait value of all individuals in the population
Selection Response (R)
The difference between the mean trait value of the OFFSPRING population and the mean trait value of the PARENTAL population
Breeder’s equation
Expression for mean trait value (average phenotype) of a continuously valued trait that changes over time as a consequence of natural selection
*relates the narrow sense heritability
QTL mapping
way of finding at least the general region of the genome in which QTL, quantitative trait loci, resides (dead science)
Assortative mating
Individuals tend to mate with those of the same genotype/ phenotype
Disassortative mating
Individuals tend to mate with those of different genotypes or phenotypes
Inbreeding
Individuals mate with genetic relatives; a type of assortative mating
Identical by descent
Pair of gene copies at a locus that are identical because of shared descent through a recent ancestor; most extreme form is self fertilization = selfing
common in flowering plants
F-statistic
Measures the correlation between two homologous alleles in a single individual; measure of the extent of inbreeding
Inbreeding depression
Occurs when the offspring from matings between genetic relatives have reduced fitness
common when recessive alleles have deleterious consequences on fitness
ex. HTN
If inbreeding increases the frequency of homozygotes, what kind of genes are more likely expressed phenotypically?
Recessive genes
