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Chapter 22 Notes.docx

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Robert Boulianne

Chapter 22 – Quantitative Genetics and Multifactorial Traits Discontinuous Traits • Most of the traits that we have examined so far have been discontinuous traits. – Either yellow or green – Either tall or short • Each trait was markedly different from one another. – Single identifiable phenotype observed Continuous Traits • Many traits exhibit a wide range of possible phenotypes. – Eg. Human birth weight, adult height and number of eggs laid by fruit flies. • Traits like these are called continuous traits. – Since these traits are usually described by quantitative measure (eg how tall, how many etc.), these traits are also know as quantitative traits. Polygene Hypothesis for Quantitative Inheritance • A trait may have a range of phenotypes because of environment influence. – Same genotype may produce range of phenotypes. – Multiple genotypes may produce same phenotype. • Work in the early 20th century indicated that both the environment and genotype influence quantitative traits. – These are multi-factorial traits. – Phenotype cannot be explained by a single locus. • Explanation for these traits is the polygene or multiple-gene hypothesis for quantitative inheritance. • Meristic Traits – Phenotypes described by whole numbers • Eg. Seeds in a pod or eggs laid by a chicken • Threshold Traits – Small number of discrete classes – Found in number of human diseases Eg. Type II diabetes Polygene Hypothesis for Kernel Colour • The work of Hermann Nilsson-Ehle in 1909. • Crossed true breeding red kernel to white kernel plants. – F1all were all the same intermediate shade between red and white…pink. – F2kernels were 15 red(all shades) to 1 white. • When he categorized the red shades he found a phenotypic ratio of 1:4:6:4:1 • Did not observe 9:3:3:1 ratio, but a 1:4:6:4:1 ratio. • Alleles here can either be contributing alleles or noncontributing alleles. – Each contributing allele will allow for the synthesis of some pigment. Kernel coloration is dependent on the number of R or C alleles (the r and c alleles are noncontributing). • Eg. RRCC dark red, rrcc white. Polygenes • Multiple gene hypothesis fits some observations of quantitative inheritance. • May be explained by the action and segregation of allelic pairs at a number of different loci called polygenes. Each polygene has a small effect on the overall phenotype. Calculating the Number of Polygenes • 1/4 = ratio of F 2ndividuals expressing either extreme phenotype n • Eg. 1/4 = 1/16 n 4 = 16 n ln 4 = ln 16 n = (ln 16)/ (ln 4) n = 2 genes Stats in Genetics • Quantitative genetics addresses this question of nature vs nuture, or the relative roles of genes vs the environment. Variation • Variation in the phenotype of a population is V P – The portion of this variation that is genetic variation is V , while the variation G from the environment is V . E V P V +GV E • How do we partition the phenotypic and genetic components? We need to understand some statistical tools first. The Variance and Standard Deviation • Variance is a measure of how much the individual measurements spread out around the mean. – How variable the individuals and their measurements are. • Two distributions can have the same mean, but have dramatically different distributions or variances. 2 • The sample variance is symbolized as s , is defined as the average squared deviation from the mean. 2 2 • Variance = s = ∑ (x -ix) /n – 1 • Standard Deviation = s = s 2 Some Generalizations to Quantitative Inheritance Studies • Mean of F 1 is usually intermediate between means of parents. • Mean of F i2 usually approximately equal to F . 1 • F2almost always shows more variability around the mean than F . 1 The extreme values in the F exte2d closer to the two parental values than do the extreme values of the F . 1 Heritability • Proportion of a population’s phenotypic variation that is attributable to genetic factors. • Important to know genetic contribution of traits. – Eg In agriculture such as weight gain in cattle, milk production in cows and # of eggs laid by chickens. – Eg. In natural populations such as body size, fecundity and developmental rates can help us understand natural selection and evolution. Phenotypic Variance • Phenotypic variance (V ) Ps a measure of all variability for a trait. – Genetic contribution to the phenotypic variance is called genetic variance (V ) G – Environmental contribution to the phenotypic variance is called environmental variance (V E • Includes any nongenetic variation such as temperature, nutrition and parental care. VP= V G V E Covariance (COV )G,E • 100% of the variation among individuals is accounted for by genetic and environmental factors. • However, the sum of genetically and environmentally caused variance may not add to the total phenotypic variance. – Here the genetically and environmentally caused variance covary. – Eg. Milking cows known to produce more milk are given greater feed than those that are poor milking cows. • Individuals of above average genetic quality receive above average resources leading to a covariance between genotype and environment. Genotype-by-Environment Interaction (V GxE) • Relative effects of genotypes differ among environments. – Eg. In cold environment AA plants are 40cm tall on average, while aa plants are 35cm tall. – In warm environments AA plants are 50cm, while aa plants are 60cm. • Both plants grow taller in warm environment so there is an environmental component. • There is also a genetic effect with the relative performance of the genotypes differ in the two temperatures. – While both genetic and environmental differences contribute to the phenotypic variance, the effects of the genotype and environment cannot simply be added together.
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