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Chapter 5 Notes.doc

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University of Toronto Mississauga
Fiona Rawle

Chapter 5 - Quantitative Genetics PRINCIPAL POINTS: • Discontinuous traits - exhibit few distinct phenotypes - continuous or quantitative traits display a range of phenotypes • Continuous traits - have a range of phenotypes - many loci contribute to phenotype - environmental factors influence the phenotype produced by a genotype - can be studied by using samples of populati hygons & statistics such as (1) Mean (2) Variance (3) Correlations between characters - combined with analysis of variance & regression analysis • variation can be partitioned into genetic & environmental components • broad-sense heritability of a trait is the proportion of the phenotypic variance resulting from genetic differences among individuals - narrow-sense heritability is proportion of the phenotypic variance due only to additive genetic variance - both measures depend on particular population in certain environment • amount that a trait changes in one generation as a result of selection on the trait is called the response to selection - magnitude of the response to the selection depends on the selection differential & narrow-sense heritability • genetic correlations arise when 2 traits are influence by the same genes or linked genes - trait is selected, genetically correlated traits also exhibit a response to selection • Quantitative trait loci determine continuous traits can be identified through marker-based mapping - QTL mapping provides estimate of the number & relative importance of genes influencing quantitative genetic variation The Nature of Continuous Traits 1 • Most traits studied have been characterized by presence of only few distinct phenotypes - seed coats of pea plants, either grey or white, seedpods either green or yellow & plants either tall or short - in each trait phenotypes were markedly different & each phenotype is easily separated from all other phenotypes - traits with few distinct phenotypes are called discontinuous traits • Discontinuous traits - simple relationship exists between genotype & phenotype - when dominance occurs the same phenotype is produced by 2 different genotypes - single genotypes can give rise to range of phenotypes as the genotype interacts with variable environments during development to give rise to norm of reaction - traits exhibit only few distinct phenotypes - described in qualitative terms • Continuous traits - there are not many traits with phenotypes that fall into a few distinct categories - traits like human birth weight, adult height, protein content in corn, & number of eggs laid by Drosophila exhibit wide range of possible phenotypes - traits with a continuous distribution of phenotypes - phenotypes of continuous traits must be described in quantitative measures which are known as Quantitative Traits - field of quantitative genetics studies inheritance of these traits Questions Studied in Quantitative Genetics • great deal of genetic variation - amount of variation & how its distributed determines population’s genetic structure - quantitative genetics plays important role in understanding of evolution, conservation, & complex human traits - especially important in agricultural genetics, where traits such as crop yield, rate of weight gain, milk production, & fat content are all studied - psychology uses it to study IQ, learning ability, & personality - human geneticists use it to study traits such as blood pressure, antibody titer, fingerprint patterns & birth weight - individuals different tin the quantity of a trait - quantitative geneticists would ask: (1) To what degree does the observed variation in phenotype result from differences in genotype & to what degree does this variation reflect the influence of different environments? (2) How many genes determine the phenotype? 2 (3) Are the contributions of the determining genes equal? (4) Are the effects of alleles additive? (5) When selection occurs for a particular phenotype, how rapidly can the trait change? (6) What is the best method for selecting & mating individuals to produce desired phenotypes in the progeny? The Inheritance of Continuous Traits • Francis Galton & Karl Pearson demonstrated that many traits in humans such as weight, height, & mental traits, the phenotypes of parents & their offspring are statistically associated - Concluded that traits are inherited - but not sure how genetic transmission occurs Polygene Hypothesis for Quantitative Inheritance • trait may have range of phenotypes b/c environmental factors affect the trait - same genotype may produce a range of phenotypes (norm of reaction) or multiple genotypes may produce the same phenotype - Wilhelm Johansen published study showing quantitative variation in seed weight in bean had both environmental & genetic determinants - He recognized that both environment & genotype influence some quantitative traits - These traits are called Multifactorial Traits - Inheritance of quantitative traits can be explained as being controlled by many genes - This explanation is called the Polygene or Multiple-gene hypothesis for quantitative inheritance Polygene Hypothesis for Wheat Kernel Colour • Polygene hypothesis can be dated back to when Hermann studied the colour of wheat kernels - started crossing true-breeding lines of red kernel plants & white kernel plants - F1 had grains that were all the same shade of intermediate colour between red & white - Interbred F1’s, F2 progeny showed kernels that were white & many shades of red in a ratio of ~15 red (all shades) : 1 white kernels - 15 : 1 ratio is modified ratio of 3 : 1 for expected monohybrid cross - saw there were 4 discrete shades of red among the progeny - counted relative number of each class & got a ratio of 1:4:6:4:1 phenotypic ratio of plants with dark red, medium red, intermediate red, light red, & white kernels 3 - 15 : 1 ratio results from the interaction of products of 2 genes that affect the same trait - Hypothesize that there are 2 independently segregating loci that control the production of pigment: red locus with alleles R/r & crimson locus with alleles C/c - If R & C have simple dominance to r & c then a 9:3:3:1 phenotypic ratio should result - From kernel phenotypic ratio however, dominance is not the simple answer b/c the observed phenotypes fell into 5 classes with a ratio approximating to 1:4:6:4:1 • Alleles can be classified as either functional (contributing alleles) or not functional (noncontributing alleles) in pigment production - alternative explanation for 1:4:6:4:1 ratio - each contributing allele allows for the synthesis of a certain amount of pigment - intensity of kernel coloration is a function of the number of R or C alleles in the genotype (1) RR CC would be dark red (2) rr cc would be white - inheritance of red kernel colour in wheat is an example of a polygene series of 2 loci with as many as 4 contributing alleles Table 5.1 Genetic Explanation for the Number & Proportions of F2 Phenotypes for the Quantitative Trait Red Kernel Colour in Wheat Genotype Number of Phenotype Fraction of F2 Contributing Alleles for Red RR CC 4 Dark red 1/16 RR Cc, Rr CC 3 Medium red 4/16 RR cc, rr CC, Rr Cc 2 Intermediate red 6/16 rr Cc, Rr cc 1 Light red 4/16 rr cc 0 White 1/16 • Multiple-gene hypothesis proposes that quantitative inheritance can be explained by action & segregation of allelic pairs at a number of loci called polygenes each with a small effect on the overall phenotype Statistical Tools • Quantitative geneticists applied statistical & analytical procedures to understand the overall influence of genes on continuous traits 4 - question addressed: how much of the variation that exists among individuals in populations is genetically determined & how much is environmentally induced? - Rephrased the question to how much variation in some aspect of the phenotype (Vp) results from genetic variation (Vg) & how much from environmental variation (Ve) Vp (phenotype) = Vg (genetics) + Ve (environment) - need to deal with some statistical methodology, developed to deal with quantitative genetics which are the follow listed below Samples and Populations • some aspect of a trait for a large group of individuals - i.e. average birth weight of infants born in New York City (1987) - ways to collect data (1) collect weight of each of the 1000’s of babies born that year in that city (2) collect these data from subset of the group, say birth weights of 100 infants born in New York city (1987) & use the average obtained from this subset as an estimate of the average for all the infants from the entire city - group of interest in the study is called the Population (infants born in 1987, in New York City) - subset used to give us an estimate of the population is called the Sample (all other infants from entire city) - rules to follow for statistical tool to be successful: (1) sample must be enough that changes differences between the sample & population are not misleading (2) sample must be a random subset of the population - many errors made b/c data are not collected randomly Distributions • one means of summarizing the phenotypes of a continuous trait is with the frequency distribution - summary of a group in terms of the proportion of individuals that fall within a certain phenotypic range • guidelines to make frequency distribution (1) classes are constructed that consist of a specified range of phenotypic measure (2) number of individuals in each class is counted - data from study of inheritance of seed weight in the bean P. vulgaris - weighed 5 494 beans from F2 progeny of a cross & classified them into 9 classes - each class covered a 100-mg range of weight 5 - data can be displayed in a frequency histogram - phenotypic classes = horizontal axis & number present in each class = vertical axis - there are certain shapes of frequency distributions that correspond to probability distributions - many continuous phenotypes show a symmetrical, bell-shaped distribution similar to curve surrounding the data - this is referred to as a normal distribution Normal Distribution: common probability distribution that exhibits a bell-shaped curve when plotted graphically - data that conforms to normal distribution can be accurately described by few statistics, the mean & the variance The Mean • sample mean (x) is also known as the average - tells us where the centre of the distribution of the phenotypes from a sample is located - calculated by adding up all individual measurements & dividing by the total number of measurements that were added (n) - used to characterize the phenotypes of a group of individuals - i.e. inheritance of flower length using a cross between a long-flowered & short-flowered strain of tobacco - in each strain, flower length varied so mean phenotype of short strain was 40.4mm & 93.1 mm for long strain - f1 progeny consisted of 173 plants,
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