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
• P = G + E + GE
• the variation in the trait is the sum of the genetic variability and the environmental variability
• Vp = Vg + Ve
• 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