Lecture 17 – Quantitative Genetics and Heritability
- Before we made a simplification that phenotypes fall into discrete categories, determined
strictly by genotypes.
- HOWEVER, it is not so simple – there is not a one to one ratio of gene to phenotype
- Because of transcription from genotype to phenotype into protein
- This is also affected by biotic and abiotic factors.
- Before we talked about qualitative traits
- Now talk about quantitative traits
- Quantitative traits are characteristics for which phenotypes show continuous variation among
individuals (eg: the endless possibility of height)
- These traits are typically determined by the environment and genes working together to form
- We will focus on the product of phenotype and heritability
- Statistical properties of population traits such as mean and variance
Separate the idea of PHENOTYPE and HERITIBILITY from the GENETIC BASIS of the trait.
Experiment- Yarrow (Achilea)
- They took plants of a range of known genotypes and cloned then and reared then in different
environments (elevations in a mountain)
- Image: plants clones of genotype 4 reared at high elevation, medium elevation, and low
- What they found was that depending on the elevation that the plant clone was raised at, they
developed quite DIFFERENTLY (phenotypically) even though they are genetically identical
(clones). It was affected by the environment
- They repeated the experiment for different genotypes of the plant and they found that within
EACH ENVIRONMENT there was variation in what the plants look like. So there is an affect of the
genes of the plant to what it will look like.
- SO both genes and the environment affect the phenotype of the organism.
The typical distribution that would be seen is the Normal Distribution
Problem – we don’t usually know which loci (which genes) actually control the quantitative traits.
- This is done using quantitative Genetics
- Using heritability and intensity of selection
The common use of heritability asks questions such as: “is this person 5’ tall due to genes or the
environment? In Evolutionary biology, HERITIBILTY asks question such as: “ Why is the shortest student shorter than
the tallest student?”
- The difference between individuals and the cause of these differences
- The answer to both questions is: Both genes and the environment affect heritability
EVOLUTIONARY BIOLOGY definition of HERITIBILITY: figure out the extent to which phenotypic
differences are due to differences in genes underlying those traits.
- The bigger the relative effect on genes on a given trait, the stronger the response to selection.
At the population level, what fraction of the variation in phenotype is due to genes and what fraction is
due to the environment?
Broad Sense Heritability: the fraction of total variation in trait that is due to variation in genes
Variation – the difference between the max and min of the trait
Is the trait heritable?
- Example: finches with different beak sizes and there is an event that affects the survival of
certain beak sized finches then it is important to know how much of that trait is due to
heritability and how much is due to the environment how much of this problem will also
affect the nex