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Quantitative Genetics - Coggle Diagram
Quantitative Genetics
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Phenotypic Variation
Gausian variation in traits, give a normal distribution of trait values
Trait values may correlate with fitness
- Therefore under selection
Phenotypic selection: Consistent differences in fitness among phenotypes, acting within a single generation
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Covariance between Traits and Fitness, S
S = cov(w,z)
S
Selection differential
= covariance between trait (z) and fitness (w)
(when fitness is continuous)
= mean of selected - mean of population
(when fitness is dichotomous)
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In Artificial selection, Fitness is determined by the selector.
- Decisions on how strongly to select should be influenced by the amount of genetic variance present
- If you select strongly on one trait, you reduce your effective population size.
- This increases effect of drift
- Over many generations, likely to create inbred individuals
- There's such a thing as selecting too fast
Heritable Variation, h^2
- Narrow-sense heritability
- Additive genetic variance / total phenotypic variation
- Relationship between trait values in offspring vs mid-parents (average of two parents)
Slope of this line is h^2
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Sexual Dimorphism
Differences in morphology, colouration etc. between the two sexes in dioecious species
Types of Sexual Dimorphism
Qualitative: Difference in Presence (absent or present)
Quantitative: Difference in mean
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Sexual dimorphism is one of the major axes of phenotypic diversity
- Sex can be considered a genotype x environment effect (sex is the environment)
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Sex Differences in Human Disease
- Evident sex differences in prevalence of:
- heart disease, autoimmune disease, drug metabolism and recovery, mental illnesses, respiratory illnesses, viral illness
- More men died of COVID-19
- Exact reason unknown; could be variation in risk factors
- Sex differences were largely neglected in medical research and drug approvals
How did Sex Evolve?
Anisogamy
There is variation in the size of gametes produced
- Increased size leads to:
- Increased zygote fitness
- Increased chance of fertilisation (by colliding with others)
- Increased cost
Selection for the evolution of eggs and sperm
- Imagine a species with variation in gamete size within a free-spawning environment
- Fertilisation occurs by two gametes meeting and fusing
- Larger gametes are more likely to collide with other gametes, but more costly to produce and so less are produced
- Smaller gametes are less costly to produce, but less likely to collide with others, but more are produced
- Larger zygotes have greater fitness, but this plateaus at a certain point
- Disruptive selection favours both:
- smaller gametes (proto-sperm) and
- larger gametes (proto-eggs)
A: Normal distribution of gamete size
B: Model of zygote fitness by zygote size
C: Outcome of these two factors
Sperm is cheap, but eggs can be expensive
Therefore individuals that reproduce sperm rahter than eggs maximise their fitness in very different ways
Therefore forms of selection acting on those individuals can be very different, leading to diversification
Results in Sexual Selection
Results from traits that confer differential reproductive success to certain individuals relative to other individuals of the same sex
If eveyone creates less large gametes, the probability that they will encounter others in a free-spawning environment is low
If everyone creates many small gametes, they're likely to collide but the fitness of their zygotes will be low
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Quantitative Genomics
Two broad approaches to dissecting genetic architecture:
- Decomposition of Phenotypic variance into:
- Additive genetic (co)variances using phenotypic resemblance between relatives
- Mendelian factors (genes, SNPs, genomic regions) via QTL mapping, association studies
Variance component approach is NOT informative of:
- Number of genes affecting quantitative traits
- Small number --> greater chance of genetic intervention
- Larger number --> greater likelihood of new mutation
- Genes' positions in the genome
- Effect size
- Individual (vs collective) modes of action
- i.e., additive, dominant, epistatic
- Individual (vs collective) pleiotropic effects
- Natural allele frequencies
Evolutionary applications of genotype-phenotype mapping studies
- Trade-offs, G X E
- "Speciation genes" - loci affecting reproductive isolation
- Genetic basis of adaptation
- Parallel evolution (gene reuse during adaptation)
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Quantitative Trait Locus (QTL) Mapping
- Cross breed individuals with extreme values for a quantitative trait of interest
- F2 population will have recombined genomes
- Measure associations between molecular markers and trait values
- Greater associations indicate closer proximity between the marker and causal loci
E.g. Bristle number in Drosophila
- Cross breed individuals with low number and high number of bristles
- Assumptions:
- Alleles for low number are in high frequency in individuals down low end of bristle number distribution
- Inbred lines
- Have anonymous molecular markers in the genome - not the causal locus/loci
- Trying to measure associations between markers and trait values
- Further away each marker is from causal locus, the less likely / weaker the assocation will be
No association
Association
Overdominance
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Association Studies
Association studies
- Candidate genes or whole genome
- Find SNPs
- Phenotype and Genotype large sample
- Associate genotype with phenotype to find Quantitative Trait Nucleotides (QTNs)
- Exploits variation and linkage disequilibrium that already exists in a population
- Useful when breeding designs aren't feasible (e.g., in humans)
Fruit fly example
Wanting to investigate associations between male mating success (fitness), pheromones (phenotypes) and genotypes
- Collect male flies from nature
- Assay males for mating success AND pheromone phenotype
- Sequence gene, find SNPS
- Associate genotype with phenotype
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Genome scan in an association study
- Common elements:
- Genome position on x-axis
- y-axis is a measure of statistical significance
- Significance level is not p = .05, because doing so many scans
This example is for the Desaturase 2 gene, which affects the synthesis of pheromones
- Blue dots above line are showing significant associations between loci and pheromone
- Red dots show assocations between loci and mating success
- More positions affecting mating success
- not necessarily associated with success through the phenotype of interest
- Others affect the pheromone, but not in ways that are associated with mating success
Expanded Fruit Fly Example
- Flies are collected from nature
- Measures taken of CHCs and mating success
- Positive covariance between CHC traits and male mating success
- Under positive sexual selection
- CHCs also under natural selection for Desiccation resistance
- Create a wax coat - hence the shine
CHCs divergence: latitudinal clines
- Incredible geographic diversity in CHC compounds
- Increased expression of short-chain compounds in Southern sections of Australian coast
- Expression of long-chain compounds greater in hot tropical environments
- believed to act as waterproofing agent
- similar effects in other species and compounds
- Individuals that don't express short-chained compounds have been observed
- Shows there's a polymorphism
What is this gene? and where is it?- 1 more item...