1. The Inheritance of Complex Traits
Chapter 19
The Inheritance of Complex Traits

2. Quantitative Genetics
Former basketball star Wilt Chamberlain (7 feet, 1 inch tall) and former renowned jockey Willie Shoemaker (4 feet, 11 inches tall) show some of the extremes in human height—a quantitative trait
Quantitative traits
Statistics
Phenotypic distributions
Reaction norms
Broad-sense heritability
Narrow-sense heritability
QTL mapping

3. What is quantitative genetics?
Traits such as height that show a continuous range of variation and do not behave in a simple Mendelian fashion are known as quantitiative or complex traits
Calculation of a mean (average) value and variance/standard deviation
Quantitative trait loci (QTLs) produce continuously variable phenotypes

4. Basic statistical concepts
phenotypic variation in quantitative traits described statistically (frequency histogram)
features of statistical distributions:
central tendency (mean or average)—observed values around a particular point
dispersion (variance or standard deviation)—how much variation about the mean

5. Populations can be described mathematically
-Mean of a population X= 1/n ΣX

6. Populations can be described mathematically
-Variance of a population
Vx= 1/n Σ (X –X)2
The standard deviation is the square root of the variance
In a normal distribution, the standard deviation describes the distribution.

7. How can the relative contributions to Yao Ming’s height be determined?
If we could clone Yao Ming and raise his clones in different environments, we could determine the relative contributions of genetics and environment.

8. What is quantitative genetics?
Correlates phenotypic trait distributions with genotypes, environment
Some questions researchers ask:
What proportion of observed phenotypic variation is determined by genetic variation?
What proportion of observed phenotypic variation is determined by environmental variation?
Do different alleles for a gene produce different effects?
What phenotypes of the genotypes inhabit different environments?
How many loci are involved for a trait?
What offspring result from crosses of different quantitative phenotypes?

9. Quantitative traits described by frequency distribution
few
genes?
several
genes?
# genes partly determines curve“smoothness”
many genes?

10. Reaction norm relates environment and phenotype

11. Reaction norm relates environment and phenotype
reaction norm = relation between environment and phenotype for a particular genotype
can graph this
under a “distribution of environments”, any given genotype yields a “distribution of phenotypes”

12. Reaction norm shows no genotype “best” for all environments

13. Reaction norm shows no genotype “best” for all environments
few reaction norm studies to date on quantitative traits of wild species
many on domesticated crops (e.g., corn, strawberries)
no genotype consistently produces “superior” phenotypes across all environmental conditions

14. Broad-Sense Heritability: Nature Versus Nurture
If trait is heritable, we can quantify heritability
We can separate total phenotypic variation of population (VX) into genetic variance (Vg) and environmental variance (Ve)
Broad-sense heritability (H2) is defined as H2 = Vg/VX
H2 varies from 0 (all environment) to 1 (all genetic)

15. genetically identical
Monozygotic twins are
genetically identical

16. Narrow–Sense Heritability
understanding of broad-sense heritability (H2) useful, but may want to know more about genetic variance specifically
heterozygotes not exactly intermediate in phenotype between homozygotes (partial dominance)
difference in average effect between alleles is “additive effect”
accounts for some, but not all, variance in phenotype

17. Narrow-sense heritability
so, genetic variance (Vg) can be subdivided into additive genetic variation (Va) and dominance variance (Vd): Vg = Va + Vd
recall that total phenotypic variance (Vx) is
Vx = Vg + Ve = Va + Vd + Ve
so, narrow-sense heritability (h2) is defined as:
h2 = Va/Vx

18. The difference between additive and dominant gene action

19. Estimating components of genetic variance
allows us to use h2 to predict effects of artificial selection
Animal and plant breeders use h2

21. Different populations have different heritabilities for traits

22. QTL Mapping Mapping quantitative loci requires:
Setting up a cross that will result in a segregating population
Developing assays for a large number of molecular markers in the organism
Assaying for correlation between the trait in question and the molecular markers in offspring

23. Association mapping finds a gene for body size in dogs

24. Summary
reaction norm studies show that no single genotype “superior” over all environments
broad heritability (H2) separates genotypic from environmentally induced variance: H2 = Vg/Vx
narrow heritability (h2) subdivides genetic variance into additive and dominance variance: h2 = Va/Vx
can use h2 to predict effects of artificial selection
can use various two mapping approaches to determine the genetic basis of quantitative traits
many traits have many contributing loci, each usually providing small effects