Population genetics 3 Flashcards
Examples of traits that vary quantitavely
Body size
Resource utilisation
Reproductive capacity
Fruit/ seed yield
Fur, feather, bark properties
Basic quantitative model
Phenotype= population mean + Genotypic effects + Environmental effects
G is total Genotypic variation
- Additive variance (VA)
- Dominance variance (VD)
- Epistatic variance (VI)
E is total Environmental variation
- Pure environmental impact (VE)
- Interaction between genotype and environment (VGE)
So phenotype = population mean + total phenotypic variance (VG+VE)
Heritability
Heritability is the proportion of phenotypic variation that is due to genetic causes/ differences and is therefore, heritable
Broad sense is possible with full-sib analyses:
H2= VA + VD + VI / VP
Narrow sense is possible with half sib analyses as only the additive genetic variance can be calculated:
h2= VA/ VP
VA can be much larger than VD+VI so the narrow sense heritability calculation can be useful
How to estimate heritability
Parent-offspring correlation:
- Compare the genotypic variation between parents and offspring to the total phenotypic variation
Twin analyses:
- Compare phenotypic variance among twins
- The assumption is that the variance is due to the environment
Sib analyses:
- Compares phenotypic variance and genotypic variance between full sibs and half sibs.
- This requires the pedigree of the population
Sib analyses example: Forest trees
Forest trees use full sib analyses.
Seeds are grown in several locations. Within each location, there is limited variation.
The genetic variation and the total variation are compared between the sites.
- traits (e.g. growth rate) are measured over many years.
Limitations and solutions to heritability estimates with selective breeding
Pedigree may inaccurately describe actual genetic resemblance (e.g. sib =0.5)
Heritability is population-specific
The environment impacts heritability
Hertaibilty varies through life
Applications of heritability studies to breeding
Once you have calculated heritability, you can calculate the genetic gain/ response of different populations to selection.
Delta P (genetic gain) = heritability x selection intensity x phenotypic variation
This allows you to predict how populations will respond to selection and whether it will be effective.
Example of selection: Dairy cows
7,500 kg/cow increase in milk yield over 70 years (375% increase)
Breeding value
Breeding value is another helpful measure to work out the value of breeding different organisms.
BV is the genetic merit of individuals.
EBV is estimated from Heritability and the phenotypic difference between individual and the population means (how extreme)
During selective breeding, the individuals with the highest EBV are selected.
The change in the mean of the population (genetic gain) only shifts a small amount due to environmental factors and multigenic effects.
Challenged with selective breeding
The heritability estimate may be wrong as the pedigree describes the genetic resemblance between individuals inaccurately.
Solution: Use DNA markers to calculate the actual resemblance
Different populations have different H2 and H2 changes thorugh time.
Solution: Combine populations and re-calculate H2 or keep populations separate.
Breeding reduces diversity through directional selection, reducing adaptability.
Solution: maintain base population with wide diversity and breed with wild relatives to inject diversity.
Natural selection
Natural selection acts more effectively in populations that have a higher phenotypic variation. The fitness is moved further.
Natural selection leads to different types of selection
- Directional
- Stabilising
- Disruptive
Selection acting on Multiple traits
Where selection acts on multiple traits and trait correlation impacts genotypic change. Trait correlation occurs when selection for one trait effects the selection on another trait as these trait are coded for by some of the same genes.
Multivariant selection gradients measure selection acting on the trait of interest after accounting for selection on other measured traits which effect this trait of interest.
This leads to the multivariant breeder’s equations, which take into consideration genetic correlation and selection index, which quantifies the indirect effects of selection for a given trait on other traits.
Quantitative genetics combined with genetic analyses
Quantitative genetics when combined with genomic analyses, can help to identify genetic loci and mechanisms that underpin phenotypic traits
Example: Localising quantitative trait loci (QLT analyses)
- This is a technique used to identify the loci that are responsible for a trait.
1) The phenotype and genotype of the population are worked out (recombination mapping)
2) Look for a correlation between genotype and phenotype to work out which loci variation leads to the phenotype.
