Lecture 29 Flashcards Preview

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Flashcards in Lecture 29 Deck (18):

Traits to genes:

- We want to associate particular traits with particular genes


Traditional trait analyses in ecological genetics:

- Focussed on morphological polymorphisms eg. the peppered moth and response to pollution levels
- Pretty snail with shells with different banding patterns, this is a polymorphism, some habitats have higher numbers of dense bands than other habitats


Quantitative genetics:

- Focus on phenotype with continuously distributed variation (vs discrete)
- Study statistically
- Trait variation is assumed to be controlled by many genes, major and minor


Why are quantitative traits important?

- most of the key characteristics considered by plant and animal breeders are quantitative
- Many of the traits that allow species to adapt to its environment are quantitative


From discrete phenotypes to quantitative traits:

- Add intermediate categories and genes
- Add environmental effects
- The data will become less and less discrete, and will form a normal distribution


Analysing quantitative traits:

- NS operates on differences between individuals in populations and is measured using variance


Variation P =

variation G + variation E


Phenotype P =

genotype G + environment E


Measuring adaptive genetic diversity: Vp = (Va+Vd+Vl) +Ve

- Va = additive genetic variance, determines resemblance between relatives across generations
- Vd = dominance variance - interactions within a locus, affects similarity between offspring (within generations)
Vi = epistasis variance - interactions between loci


Additive gene action is the simplest model:

- Assume quantitative trait controlled by two loci (A and B), each having two alleles (A1, A2 and B1, B2)
- There is no interaction or dominance
- The 1 alleles contribute 1 and the 2 alleles contribute 2
- The effects of each allele add up in each genotype to determine the phenotype, they are additive


A table with dominance will show

- Additive effects of genes
- A way to test dominance is to cross two traits together (aa and AA) if the Aa falls back a bit there must be additive inheritance


Epistasis in Danaus chyssipus, a butterfly:

- eg) forewing size is largely determined by 2 loci showing both dominance and epistasis
- B dominant to b but this depends on the C genotype
- BB Bb are smaller when heterozygous at the C locus


Measuring genetic diversity in quantitative traits:

- Heritability
- Narrow sense
- Broad sense


Narrow-sense heritability:

- Proportion of phenotypic variance passed on to relatives
- Indicative of response to selection


Broad-sense heritability:

- The degree of genetic determination of a trait
- h squared(broad) = Vg/(Vg + Ve)



Vp = (Va + Va + Vd + Vi) + Ve



- Varies among traits
- Measure the ability to respond to selection
- Higher genetic variance (h squared) allows persistence under stronger environmental change


Estimating heritability from family data:

- Plot family data on a scattergram of both parents
- We need the slop of the line of best fit (regression line)
- b = slope of the line of best fit (relates to heritability)
- b = h squared (mid parent offspring)
- b = 0.5h squared (one parent)
- Slope of 1 the trait is completely heritable