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Quantitative Traits
Characteristics of quantitative traits
- polygenic
- may show either a continuous distribution or have
discrete classes (meristic)
- continuous - height, weight, intelligence, etc.
- discrete - fertile/sterile, diseased/normal, # of
eggs, etc. Despite the discrete character the trait is
determined by an underlying continuous distribution
- affected by environmental factors
- a genetic model: additive genes
- multiple genes effect the trait
- each gene has two alleles
- one allele adds to the trait
- the other allele does not add to the trait
- alleles from different genes have the same effect
on the trait
- the amount of the trait is determined by the total
number of additive alleles at all of the different
genes plus a contribution from the envirnoment
- simple example, red coloring of wheat kernals
- simple because their is very little
environmental effect
- two genes, A and B with two alleles each, A, a,
B, b
- cross AABB (red) with aabb (white) and get AaBb
(pink)
- cross F1 (AaBb x AaBb) and get 1/16
AABB (red), 4/16 aABB, AaBB, AAbB, AABb (all medium
red), 6/16 aaBB, AAbb, AaBb, aAbB, AabB, aABb (all
pink), 4/16 Aabb, aAbb, aaBb, aabB (all light pink)
and 1/16 aabb (white).
Analysis of quantitative traits
If there are many genes involved and there is much
environmental effect then it becomes much harder to figure
out.
- Determing the number of genes
- Cross two extremes and the ratio of either extreme
in the F2 should equal 1/4n,
where n is the number of genes
- a quantitative, polygenic trait can be characterized
by two numbers; the mean (X) and the variance
(s2)
- the mean phenotype for a trait in a population is the
sum of the genetic and environmental contributions to the
trait
- the variation in the trait is the sum of the genetic
variability and the environmental variability
- the fraction of the variation caused by genetic
variation is called the heritability of the trait,
H2 = Vg/Vp
- For breeding or selection experiments the narrow
heritability, h2 = Va/Vp
is more useful
How to measure heritability?
- Reduce genetic differences (inbreeding, siblings,
twins);
Vp = Ve (Vg = 0)
- Reduce environmental differences (greenhouse, matched
populations);
Vp = Vg (Ve = 0)
- Measure the regression of the phenotype between
relatives, this takes advantage of the fact that for the
parent-offspring covariance, covOP =
1/2(VA)
- Selection experiments, if S is the difference between
the mean of the selected population and the starting
population (the "selection differential") and R is the
mean for the next generation (the "response") then
h2 = R/S
- Typical h2 values are 0.1 to 0.5, this
seems high
Additional information can be found at
Quantitative
Genetic Resources
Bell
CSU Chico
Library
This document is copyright of
Jeff
Bell
Last Update: Friday, September 11, 1998
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