ConceptS and Connections 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

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.]

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

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

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

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

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 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.

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

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

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.

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.

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.

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.

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

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

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

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

Figure 17.10 (part 2) Edward East conducted an early statistical study of the inheritance of flower length in tobacco.

Figure 17.10 (part 3) Edward East conducted an early statistical study of the inheritance of flower length in tobacco.

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

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

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.

Figure 17.12 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.]

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.

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

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