Download Chapter 19 - The inheritance of complex traits

Chapter 19
The Inheritance of Complex Traits
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
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Quantitative traits
Statistics
Phenotypic distributions
Reaction norms
Broad-sense heritability
Narrow-sense heritability
QTL mapping
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
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
Populations can be described
mathematically
-Mean of a population X= 1/n ΣX
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.
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.
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?
Quantitative traits described by
frequency distribution
few
genes?
# genes partly
determines
curve“smoothness”
several
genes?
many genes?
Reaction norm relates environment
and phenotype
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”
Reaction norm shows no genotype
“best” for all environments
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
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)
Monozygotic twins are
genetically identical
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
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
The difference between
additive and dominant
gene action
Estimating components of genetic
variance
• allows us to use h2 to predict effects of
artificial selection
• Animal and plant breeders use h2
Different populations have
different heritabilities for traits
QTL Mapping
Mapping quantitative loci requires:
1. Setting up a cross that will result in a segregating population
2. Developing assays for a large number of molecular markers in the organism
3. Assaying for correlation between the trait in question and the molecular
markers in offspring
Association mapping finds a gene for body size in dogs
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