Flashcards in Topic 2 - Quantative Genetics Deck (11):
What is a quantitive trait?
A quantitative trait is a measurable trait that shows continuous variation. Quantitative traits are sometimes called complex traits or polygenic traits because their variation can be attributed to the cumulative effect of many genes.
Explain variance in terms of quantitive traits.
A quantifiable measure of the dispersion or spread of a trait about the mean (i.e. the amount of variation)
Why is it important to know how much of the variation in a trait is inherited to understand how natural selection is operating on the trait?
Because only heritable variation responds to selection. Variation that is attributable solely to environmental factors is generally not passed on from parents to offspring, so it cannot be included in any calculation of the strength of selection acting on a trait.
Explain heritability and how it is calculated.
Denoted by h2, the amount of the variance in a trait that is attributable to genetic factors. Often used in reference to the resemblance between parents and their offspring.
H2 = genetic variance divided by total phenotypic variation.
What is total phenotypic variation?
Total phenotypic variance (VP) is the sum of the variance due to genetic causes (VG) plus the variance due to environmental causes (VE). The relationship can be expressed as:
VP = VG + VE
What are the three types of natural selection which can occur?
1) directional selection
2) disruptive selection
3) stabilising selection
population geneticists measure the strength of selection as the selection coefficient, s. How is s defined?
It is defined as the intensity of selection against the less fit genotype or, put another way, the amount by which the fitness of a genotype is reduced relative to the most fit genotype in the population.
What is the selection differential?
the selection differential (S), is defined as the difference between the mean value of the trait for the population and the mean value of the trait for those individuals of that population that reproduce.
What does response to selection depend upon?
the extent to which phenotypic variation in a trait is heritable. If h2 = 0, then the population will not evolve. However, if h2 = 1 and all phenotypic variation is due to genetic differences among individuals, then the response to selection equals the selection differential, S.
Explain phenotype plasticity
Sensitivity of a phenotype produced by a single genotype to differences in the environment.