Transcript Pierce5e_ch24_lecturePPT

Benjamin A. Pierce
GENETICS
A Conceptual Approach
FIFTH EDITION
CHAPTER 24
Quantitative Genetics
© 2014 W. H. Freeman and Company
24.1 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
• Determining Gene Number for a Polygenic Characteristic
24.1 Quantitative Characteristics Vary Continuously
and Many Are Influenced by Alleles at Multiple Loci
The Relationship Between Genotype and
Phenotype
•
Quantitative characteristics
− Exhibit complex relationship between genotype
and phenotype
− May be polygenic
− May have environmental influences
− Phenotypic ranges may overlap
− Cannot use standard methods to analyze
24.1 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
24.1 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.
24.1 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
24.1 Quantitative Characteristics Vary Continuously
and Many Are Influenced by Alleles at Multiple Loci
Determining Gene Number For a Polygenic
Characteristic
• (1/4)n = number of individuals in the F2 progeny
that resemble each of the homozygous parents.
• n = number of loci with a segregating pair of
alleles that affects the characteristic
24.2 Statistical Methods Are Required for
Analyzing Quantitative Characteristics
• Distribution
− Frequency distribution
− Normal distribution: a symmetrical (bellshaped) curve.
• Samples and populations
− Population: group of interested individuals
− Sample: small collection of individuals from
the population
Concept Check 1
A geneticist is interested in whether asthma is caused by
a mutation in DS112 gene. The geneticist collects DNA
from 120 people with asthma and 100 healthy people and
sequenced their DNA. She finds that 35 of the people with
asthma have a mutation in the DS112 gene, and none of
the healthy people have a mutation in the DS112 gene.
What is the population in this study?
a.
b.
c.
d.
the 120 people with asthma
the 100 healthy people
the 35 people with a mutation in their gene
all people with asthma
Concept Check 1
A geneticist is interested in whether asthma is caused by
a mutation in DS112 gene. The geneticist collects DNA
from 120 people with asthma and 100 healthy people and
sequenced their DNA. She finds that 35 of the people with
asthma have a mutation in the DS112 gene, and none of
the healthy people have a mutation in the DS112 gene.
What is the population in this study?
a.
b.
c.
d.
the 120 people with asthma
the 100 healthy people
the 35 people with a mutation in their gene
all people with asthma
24.2 Statistical Methods Are Required for
Analyzing Quantitative Characteristics
• The Mean: the average
• The Variation and Standard Deviation
− Variance: the variability of a group of
measurements
− Standard deviation: the square root of the
variance
Concept Check 2
The measurements of a distribution with a
higher
will be more spread out.
a.
b.
c.
d.
mean
variance
standard deviation
both b and c
Concept Check 2
The measurements of a distribution with a
higher
will be more spread out.
a.
b.
c.
d.
mean
variance
standard deviation
both b and c
24.2 Statistical Methods Are Required for
Analyzing Quantitative Characteristics
• Correlation: when two characteristics are correlated, a
change in one characteristic is likely to be associated
with a change in the other.
• Correlation coefficient: measures the strength of their
association.
– Correlation doesn’t imply a cause-and-effect relation. It simply
means that a change in a variable is associated with a
proportional change in the other variable.
24.2 Statistical Methods Are Required for
Analyzing Quantitative Characteristics
• Regression: predicting the value of one
variable, if the value of the other is given.
− Regression coefficient: represents the slope
of the regression line, indicating how much
one value changes on average per increase
in the value of another variable.
Concept Check 3
In Lubbock, Texas, rainfall and temperature
exhibit a significant correlation of -0.7. Which
conclusion is correct?
a. There is usually more rainfall when the
temperature is high.
b. There is usually more rainfall when the
temperature is low.
c. Rainfall is equally likely when the temperature is
high or low.
Concept Check 3
In Lubbock, Texas, rainfall and temperature
exhibit a significant correlation of -0.7. Which
conclusion is correct?
a. There is usually more rainfall when the
temperature is high.
b. There is usually more rainfall when the
temperature is low.
c. Rainfall is equally likely when the temperature is
high or low.
Applying Statistics to the Study of a
Polygenic Characteristic
24.3 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
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
− additive genetic variance: VA
− dominance genetic variance: VD
− genic interaction variance: VI
Summary: Vp = VA + VD + VI + VE + VGE
24.3 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
• Heritability by elimination of variance components
– (VP – VE = VG)
• Heritability by parent-offspring regression
– (h2= b or h2= 2b)
• Heritability and degrees of relatedness
- H2 = 2(rMZ - rPZ)
Concept Check 4
If the environmental variance (VE) increases and
all other variance components remain the same,
what will the effect be?
a.
b.
c.
d.
Broad-sense heritability will decrease.
Broad-sense heritability will increase.
Narrow-sense heritability will increase.
Broad-sense heritability will increase, and narrowsense heritability will increase.
Concept Check 4
If the environmental variance (VE) increases and
all other variance components remain the same,
what will the effect be?
a.
b.
c.
d.
Broad-sense heritability will decrease.
Broad-sense heritability will increase.
Narrow-sense heritability will increase.
Broad-sense heritability will increase, and narrowsense heritability will increase.
24.3 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.
• An individual does not have heritability.
• There is no universal heritability for a characteristic
• Even when heritability is high, environmental factors
may influence a characteristic.
• Heritability indicates nothing about the nature of
population differences in a characteristic.
24.3 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
24.4 Genetically Variable Traits Change
in Response to Selection
• Natural selection arises through the differential
reproduction of individuals with different
genotypes.
• Artificial selection: selection by promoting the
reproduction of organisms with traits perceived
as desirable.
24.4 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
− Selection differential
• Calculation of response to selection
− R=h2 x S
• Estimating heritability from response to selection
− H2=R/S; realized heritability
24.4 Genetically Variable Traits Change
in Response to Selection
• Limits to Selection Response
− Response may level off after many generations
• Correlated Responses
– Phenotypic correlation
– Genetic correlation