Introduction to Quantitative Genetics

In principle:

We can extend single-locus  multilocus  quantitative models

1AA : 2Aa : 1 aa (1AnAn : 2Anan : 1anan)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 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 (2G) & environmental (2E) variance

phenotypic variance      2P  = 2G2E2GxE
heritability                        h2  = 2G  /  2A  = 2G / (2G2E)

"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 2GxE  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

Offspring / midparent correlation estimates heritability
Limits of prediction from correlation & regression

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      2P  = 2G
[ignore   2E , 2GxE]

Myth 2: That which is genetic is fixed & unchangeable

Ex.: IQ test scores in Homo: h2 0.7 within groups
Inter-group differences: environmental and/or genetic?
Heritable traits can be modified by environment2GxE is large

See:
Gray & Thompson. 2004. Neurobiology of intelligence: ethics and science,
Nature Reviews Neuroscience 5: 471-482.
Foster 2006. Science & Ethics in the Human Genome Project.
2001 NCBI International HapMap Project meeting
American Eugenics Archive
```All text material © 2013 by Steven M. Carr
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