BSC 219
Quantitative Genetics
11/27/12
24.1 Quantitative Characteristics Vary Continuously and Many
Are Influenced by Alleles at Multiple Loci
The
Relationship Between Genotype and Phenotype
For
continuous characteristics: several different genotypes produce same phenotype.
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
Types of Quantitative Characteristics
Polygenic
inheritance
Refers
to quantitative characteristics controlled by cumulative effects of many genes.
Each
character still follows MendelÕs rules.
Kernel color in
wheat.
Determining Gene Number For a Polygenic Characteristic
(1/4)n = number of individuals in the F2 progeny.
N = gene
number.
24.2 Statistical Methods Are Required for Analyzing
Quantitative Characteristics
Distribution
Frequency
distribution
Normal
distribution: a symmetrical (bell-shaped) curve.
Samples
and populations
Population:
group of interested individuals
Sample:
small collection of individuals from the poulation.
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.
One
standard deviation from mean covers 66% of area
Two
standard deviations from mean covers 95% of area
Three
standard deviations from mean covers 99% of area
Correlation
Correlation: when two characteristics are correlated, a change in one
characteristic is likely to be associated with a change in the other.
Correlation
Correlation
coefficient: measures the strength of their
association
Correlation
doesnÕt 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.
24.3 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.
Heritability
Phenotypic
variation : Vp
Genetic
variance: Vg
Va: additive genetic variance
Vi: genic interaction variance
Vd: dominance gentic variance
Heritability
Phenotypic
variation : Vp
Genetic
variance: Vg
Environmental
variance: Ve
Genetic-Environmental
Interaction Vge
Vp = Vg + Ve
+ Vge
Vp = Va
+ Vd + Vi + Ve + Vge
Heritability
Broad-Sense
Heritability
H2
= Vg/Vp
If H2
= 0, then none of the phenotypic variance is caused by genetic variance.
If H2
= 1, then the phenotypic variance is 100% caused by genetic variance.
Locating Genes That Affect Quantitative Characteristics
Mapping QTLs
24.4 Genetically Variable Traits Change in Response to
Selection
Response
to selection: the extent to which a
characteristic subjected to selection changes in one generation.
Artificial
selection: Selection by promoting the
reproduction of organisms with traits perceived as desirable.