Genetics 3301

Chapter 20: Quantitative Genetics

Phenotypic variance (20-1):

Ä In nature, phenotypic variability is essentially continuous because 1) each genotype does not produce a single phenotype (20-5; 20-6) and 2) many genes contribute to a given phenotype.

Ä Describing quantitative variation requires measuring statistical distributions; frequency histograms and population size (20-4); Measuring phenotypic variance by calculating the mean (central tendency) and the variance.

Ä Effect of environment on phenotypic variation; Norm of reaction is the way in which the environmental distribution is transformed into the phenotypic distribution (20-8); effect of temperature on bristle number (20-7); superior genotypes of domesticated animals and cultivated plants ≠ generalists vs. specialists (20-9).

Quantifying heritability:

Ä V_phenotypic = V_genetic + V_{environmental} + V _{genetic x environmental interactions}; remove environmental variability by experimenting under controlled conditions (20-5; 20-6); The quantitative measure of heritability of a character is that part of the total phenotypic variance that is due to genetic variance is called broad heritability (H²).

Ä Components of genetic variance are V_A = additive genetic variance, V_D = dominance genetic variance, V_I = epistatic genetic variance and V_M = maternal genetic variance; V_A is the most important component because it predicts how well phenotype goes from parent to offspring; additive variance and codominance (7^th edition figure 4-3).

Ä Narrow heritability (h²) is the proportion of phenotypic variance that is attributable to additive genetic variance (V_A/V_P); estimating h² by plotting phenotypes of offspring against the average phenotypes of the two parents (20-11); the higher the h² value, the more selection can change the phenotype.

Key terms: Know the following terms: additive effect, additive genetic variance, broad heritability, central tendency, correlation, dispersion, distribution function, dominance variance, frequency histogram, genetic variance, heritability in the narrow sense (h²), heritable, mean, norm of reaction, statistical distribution and variance.

Problems: 1, 2.