Transcript V p

Benjamin A. Pierce
•GENETICS ESSENTIALS
•ConceptS and Connections
• SECOND EDITION
CHAPTER 17
Quantitative Genetics
© 2012 W. H. Freeman and Company
CHAPTER 16 OUTLINE
• 17.1 Quantitative Characteristics Vary
Continuously and Many Are Influenced by Alleles
at Multiple Loci, 438
• 17.2 Statistical Methods Are Required for
Analyzing Quantitative Characteristics, 443
• 17.3 Heritability Is Used to Estimate the Proportion
of Variation in a Trait That Is Genetic, 445
• 17.4 Genetically Variable Traits Change in
Response to Selection, 451
17.1 QUANTITATIVE CHARACTERISTICS VARY
CONTINUOUSLY AND MANY ARE INFLUENCED
BY ALLELES AT MULTIPLE LOCI
• Discontinuous (qualitative) traits possess only a few
phenotypes.
• Continuous (quantitative) characteristics vary along a
scale of measurement with many overlapping
phenotypes.
• The Relationship Between Genotype and Phenotype
• Types of Quantitative Characteristics
• Polygenic Inheritance
• Kernel Color in Wheat
16.1 QUANTITATIVE CHARACTERISTICS VARY
CONTINUOUSLY AND MANY ARE INFLUENCED
BY ALLELES AT MULTIPLE LOCI
The Relationship Between Genotype and Phenotype
Quantitative characteristics
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Exhibit complex relationship between genotype and phenotype
May be polygenic
May have environmental influences
Phenotypic ranges may overlap
Cannot use standard methods to analyze
The Relationship Between Genotype and
Phenotype
Hypothetical: Three loci determine plant’s
height; each with two alleles;
• A+; B+; C+ are producing growth hormone
• A-; B-; C- are not producing growth hormone
• For A the possible genotypes are
• A+A+; A+A-; A-A-
• So for all three loci there are 27 combinations
(33) but only 7 phenotypes
The Relationship Between Genotype and Phenotype
More loci more complex relationships
Environment can influence the phenotypes
• So, many overlapping phenotypes are observed
17.1 QUANTITATIVE CHARACTERISTICS VARY
CONTINUOUSLY AND MANY ARE INFLUENCED
BY ALLELES AT MULTIPLE LOCI
Types of Quantitative Characteristics
• Meristic characteristics
• Determined by multiple genetic and environmental factors, and
can be measured in whole numbers
• Animal litter size
• Threshold characteristics
• Measured by presence or absence
• Susceptibility to disease
17.1 QUANTITATIVE CHARACTERISTICS VARY
CONTINUOUSLY AND MANY ARE INFLUENCED
BY ALLELES AT MULTIPLE LOCI
Polygenic Inheritance
• Refers to quantitative characteristics controlled by cumulative
effects of many genes
• Each character still follows Mendel’s rules
• May be influenced by environmental factors
17.1 QUANTITATIVE CHARACTERISTICS VARY
CONTINUOUSLY AND MANY ARE INFLUENCED
BY ALLELES AT MULTIPLE LOCI
Kernel Color in Wheat
• Illustrates multiple genes acting
to produce continuous range of
phenotypes
• Nilsson-Ehle experiment
• Intensity of red pigmentation is
determined by three unlinked loci
• Number of phenotypic classes in
F2 increases with the number of
loci affecting a character
Kernel Color in Wheat
• It is basically dihybrid cross but with two loci affecting same trait
• All red possibilities
• Sum of all individual combinations
1/16 + 1/16 + ¼ = 6/16
17.2 STATISTICAL METHODS ARE REQUIRED
FOR ANALYZING QUANTITATIVE
CHARACTERISTICS
Distribution
• Frequency distribution
• Normal distribution: a symmetrical (bell-shaped) curve
24.2 STATISTICAL METHODS ARE REQUIRED
FOR ANALYZING QUANTITATIVE
CHARACTERISTICS
• Mean: the average
• Variance: the variability
of a group of
measurements
17.2 STATISTICAL METHODS ARE REQUIRED
FOR ANALYZING QUANTITATIVE
CHARACTERISTICS
Apply Statistics to the Study of a Polygenic
Characteristic
17.3 HERITABILITY IS USED TO ESTIMATE THE
PROPORTION OF VARIATION IN A TRAIT THAT
IS GENETIC
Heritability: the proportion of the total phenotypic
variation that is due to genetic difference
17.3 HERITABILITY IS USED TO ESTIMATE THE
PROPORTION OF VARIATION IN A TRAIT THAT IS
GENETIC
Phenotypic variance: Vp
• Components of phenotypic variance Vp = VG + VE + VGE
• genetic variance: VG
• environmental variance: VE
• genetic-environmental Interaction VGE
• Components of genetic variance: VG = VA + VD + VI
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additive genetic variance: VA
dominance genetic variance: VD
genic interaction variance: VI
Summary:
Vp = VA + VD + VI + VE + VGE
17.3 HERITABILITY IS USED TO ESTIMATE THE
PROPORTION OF VARIATION IN A TRAIT THAT IS
GENETIC
Types of Heritability
• Broad-sense heritability (h2 = VG/VP)
• Narrow-sense heritability (h2 = VA/VP)
Calculating Heritability
• Most methods compare the degree of resemblance
between related and unrelated individuals or between
individuals with different degrees of relatedness.
17.3 HERITABILITY IS USED TO ESTIMATE THE
PROPORTION OF VARIATION IN A TRAIT THAT IS
GENETIC
The Limitations of Heritability
• Heritability does not indicate the degree to which a
characteristic is genetically determined.
• Pure breed no polydactilly rabbits: still polydactilly can happen
• An individual does not have heritability.
• Narrow-sense heritability of 0.6 in population does not indicate that an
individual’s characteristic is 60% additive
• There is no universal heritability for a characteristic.
• Two populations will have different heritability due to environment
• Even when heritability is high, environmental factors
may influence a characteristic.
• Human height
• Heritability indicates nothing about the nature of
population differences in a characteristic.
17.3 HERITABILITY IS USED TO ESTIMATE THE
PROPORTION OF VARIATION IN A TRAIT THAT
IS GENETIC
Locating Genes That Affect Quantitative
Characteristics
• Mapping QTLs
• Genomewide association studies
17.4 GENETICALLY VARIABLE TRAITS CHANGE
IN RESPONSE TO SELECTION
• Natural selection: selection that arises through the
differential reproduction of individuals with different
genotypes
• Artificial selection: selection by promoting the
reproduction of organisms with traits perceived as
desirable.
• Response to selection
17.4 GENETICALLY VARIABLE TRAITS CHANGE
IN RESPONSE TO SELECTION
• Predicting the Response to Selection
The extent to which a characteristic subject to selection
changes in one generation
• Factors influencing response to selection
• Narrow sense heritability
• Selection differential (S= top-mean)
• Calculation of response to selection
• R = h2 × S
• h2=0.52; S=40.6-35.3=5.3; R=0.52 x 5.3= 2.8
• Expected progeny is to have 2.8 hairs more than the mean of
the previous generation (35.3+2.8=38.1)
• Estimating heritability from response to selection
• H2 = R/S; realized heritability
•17.4 GENETICALLY VARIABLE TRAITS
CHANGE IN RESPONSE TO SELECTION
• Estimating heritability from response to selection
• h2 = R/S; realized heritability
• Previous formula for narrow sense heritability
• h2= VA/VP
17.4 GENETICALLY VARIABLE TRAITS CHANGE
IN RESPONSE TO SELECTION
Limits to Selection Response
• Response may level off after many generations