1. ConceptS and Connections
Benjamin A. Pierce
GENETICS ESSENTIALS
ConceptS and Connections
SECOND EDITION
CHAPTER 17
Quantitative Genetics
© 2012 W. H. Freeman and Company

2. 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

3. Methods of quantitative genetics coupled with molecular techniques have been used to identify a gene that determines oil content in corn. [Walter Bibikow/Getty Images.]

4. 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

5. Figure 17.1a Discontinuous and continuous characteristics differ in the number of phenotypes exhibited.

6. Figure 17.1b Discontinuous and continuous characteristics differ in the number of phenotypes exhibited.

7. 16.1 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

8. 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

9. The Relationship Between Genotype and Phenotype
More loci more complex relationships
Environment can influence the phenotypes
So, many overlapping phenotypes are observed
Figure 17.2 For a quantitative characteristic, each genotype can produce a range of possible phenotypes. In this hypothetical example, the phenotypes produced by genotypes AA, Aa, and aa overlap.

10. Types of Quantitative Characteristics Meristic characteristics
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

11. 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

12. 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

13. It is basically dihybrid cross but with two loci affecting same trait
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
Figure 17.4 Nilsson-Ehle demonstrated that kernel color in wheat is inherited according to Mendelian principles. The ratio of phenotypes in the F2 can be determined by breaking the dihybrid cross into two simple single- locus crosses and combining the results by using the multiplication rule.

14. Figure 17.4 (part 1) Nilsson-Ehle demonstrated that kernel color in wheat is inherited according to Mendelian principles. The ratio of phenotypes in the F2 can be determined by breaking the dihybrid cross into two simple single-locus crosses and combining the results by using the multiplication rule.

15. Figure 17.4 (part 2) Nilsson-Ehle demonstrated that kernel color in wheat is inherited according to Mendelian principles. The ratio of phenotypes in the F2 can be determined by breaking the dihybrid cross into two simple single-locus crosses and combining the results by using the multiplication rule.

16. Figure 17.4 (part 3) Nilsson-Ehle demonstrated that kernel color in wheat is inherited according to Mendelian principles. The ratio of phenotypes in the F2 can be determined by breaking the dihybrid cross into two simple single-locus crosses and combining the results by using the multiplication rule.

17. Figure 17.5 The results of crossing individuals heterozygous for different numbers of loci affecting a characteristic.

18. 17.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Distribution
Frequency distribution
Normal distribution: a symmetrical (bell-shaped) curve

19. Figure 17.7a Distributions of phenotypes can assume several different shapes.

20. 24.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Mean: the average
Variance: the variability of a group of measurements

21. 17.2 Statistical Methods Are Required for Analyzing Quantitative Characteristics
Apply Statistics to the Study of a Polygenic Characteristic

22. Figure (part 2) Edward East conducted an early statistical study of the inheritance of flower length in tobacco.

23. Figure (part 3) Edward East conducted an early statistical study of the inheritance of flower length in tobacco.

24. 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

25. Phenotypic variance: Vp
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
additive genetic variance: VA
dominance genetic variance: VD
genic interaction variance: VI
Summary: Vp = VA + VD + VI + VE + VGE

26. 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.

27. Figure The heritability of shell breadth in snails can be determined by regression of the phenotype of offspring against the mean phenotype of the parents. [From L. M. Cook, Evolution 19:86–94, 1965.]

28. The Limitations of Heritability
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.

29. Locating Genes That Affect Quantitative Characteristics
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

31. 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

32. 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= =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 ( =38.1)
Estimating heritability from response to selection
H2 = R/S; realized heritability

33. 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

34. 17.4 Genetically Variable Traits Change in Response to Selection
Limits to Selection Response
Response may level off after many generations