In principle:

We can extend single-locus  multilocus  quantitative models

1AA : 2Aa : 1 aa (1A_nA_n : 2A_na_n : 1a_na_n)ⁿ normal distribution

Genotype / Phenotype correlation Heritability
    Genotypic expression depends on environment
    Heritability (h²) estimates proportion of phenotypic variation due to genetic variation  

Genotype / Environmental interaction is variable (& unpredictable)
       The Norm of Reaction describes this

Heritability is not inevitability
    Genetics is not destiny

Variation can be quantified (review)

      mean  standard deviation: 
      variance: ²
      coefficient of variation (CV)  =  (/) x 100

      CV removes effect of size  when comparing variance:
        Ex.: Suppose  X = whale length     Y =  tail width
                X = 100  1.0 versus Y = 1.0  0.1
                CV of X = 1%        CV of Y = 10%
                Y is more variable, though _X is larger

      Variation follows "normal distribution" (bell-curve) iff
              Multiple loci are involved  (quantitative)
              Each locus has about equal effect (additive)
              Each locus acts independently
                    [interaction variance (see below) is minimal]
              Ex.: Suppose a trait is influenced by 5 loci, each with two alleles A & a
                      A contributes 2 units to phenotype, a contributes 1 unit
                      Range of contributions = (2u aa : 3u Aa: 4u AA)⁵
                         mean = 30 units , range 20 ~ 40 units
                               (AaBbCcDdEe) vs (aabbccddeeff ~ AABBCCDDEEFF)
                         3⁵ = 243 genotype classes => variation continuous

Variation has two sources: genetic (²_G) & environmental (²_E) variance

      phenotypic variance      ²_P  = ²_G + ²_E+ ²_GxE
      additive variance            ²_A  = ²_G + ²_E 
      heritability                        h²  = ²_G  /  ²_A  = ²_G / (²_G + ²_E)

          "heritability in the narrow sense" additive variance
             heritability h² is the fraction due to variance in genotypes 
                  assumes genotype / phenotype relationship is independent of environment

                 ignores ²_GxE  interaction variance:
                       genotype / phenotype relationship differs  in different environments.
                       Ex.: same strain of corn produces different yields in different conditions

       Artificial breeding indicates that organismal variation is highly heritable

             Artificial selection on agricultural species
                  Commercially useful traits can be improved by selective breeding
             Common Garden experiments
             Correlation / Regression analysis shows association between variables

            For many traits in many organisms:
                    CV   =     5 ~ 10 %
                     h²    =     0.5 ~ 0.9

             The Norm of Reaction mediates genotype through environment to produce phenotype
                   single-genotype traits
                   two-genotype traits
                  

Genetics & Society

"Is it Genetic?"
       Myth 1: That which is heritable is purely genetic
                   phenotypic variance      ²_P  = ²_G
                                                  [ignore   ²_E , ²_GxE]

       Myth 2: That which is genetic is fixed & unchangeable

0.7

All text material © 2013 by Steven M. Carr