Chapter 18 13 – 18 April, 2005 Quantitative Genetics Continuous variation Overvi
Huffines, Brad, Meteorologist has reference to this Academic Journal, PHwiki organized this Journal Chapter 18 13 – 18 April, 2005 Quantitative Genetics Continuous variation Overview In nature, variation in most phenotypic characters is continuous. Such quantitative variation is studied with statistical techniques. Continuous variation may be the result of segregation of interacting alleles at several loci with cumulative effect on phenotype. Environmental interaction with genotype contributes to phenotypic variance. Heritability is a population trait, not an individual one.
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Quantitative genetics Genetics of continuously varying characters Attempts to determine genetic variation contributing to character number of loci with segregating alleles how genes interact with one another in addition to the environment (norm of reaction) How do loci interact (dominance, epistasis, etc) possible maternal effects Often requires statistical analysis Quantitative traits: small variation between genotypes, large variation within genotype.
Statistical measures Statistical distribution: description of set of quantitative measurements graphical representation: e.g, histogram distribution function: continuous curve Mean: measure of central tendency (average) mode: most frequent observation Variance: measure of dispersion about mean Correlation: relationship between two measured quantities Measures of central tendency Mode: most frequent class in sample bimodal: two frequent classes Mean: arithmetic average
Measures of dispersion Variance of sample: average squared deviation from mean St in addition to ard deviation
Measures of relationship (1) Correlation coefficient: relation between two variables, x in addition to y calculated from covariance of sample in addition to st in addition to ard deviations of x in addition to y R2=.82 R2=.99 Measures of relationship (2) Correlation is estimate of precision of relation between two variables; can not be used to predict value of one given the other. Regression describes relationship between two variables in addition to allows their prediction. linear regression:
Although most quantitative traits are polyfactorial, variation at a single locus can behave quantitatively.
Norm of reaction The relationship between environment in addition to phenotype as long as a given genotype Studied by subjecting homozygous lines to different environments replicated (e.g., cloned) genotypes inbred lines (e.g., from repeated sib mating) use of dominant markers in addition to crossover suppressors to make lines homozygous as long as particular chromosomes Studies indicate that phenotypic differences between genotypes are small
Heritability Important to know whether genes contribute to phenotypic variation of quantitative character Heritability is a population trait, not property of individual Not same as familial trait shared by members of a family Characters are heritable only if similarity arises from shared genotypes estimated from phenotypic similarity of relatives estimated by cosegregation of gene markers difficult to estimate in humans
H2: broad heritability s2p = total phenotypic variance in population s2g = genetic variance s2e = environmental variance cov = covariance between genotypic in addition to environmental effects s2p = s2g + s2e + 2 cov ge H2 tells what part of populations variation is attributable to genotypic variation. Intercross homozygotes in addition to measure variance within each heterozygous genotype. Average is s2e which can be subtracted from s2p to give s2g. Statistically estimate from genetic similarities between relatives, particularly using difference in phenotypic correlation between monozygotic in addition to dizygotic twins Estimating H2
Meaning of H2 It is a population parameter, with no application to particular individuals H2 > 0 means that genetic variance is present H2 = 0 means that there is no genetic variation; genes may still be relevant to trait (as in development) Value of H2 provides limited prediction about effect of environmental modification H2 is specific to environmental conditions under which it is measured Locating genes Difficult to identify genes as long as quantitative characters C in addition to idate gene: from prior biochemical or developmental knowledge Quantitative trait loci (QTL) may be located to regions of chromosomes by cosegregation with marker genes Dominance in addition to epistasis may be involved
Assignment: Concept map, Solved Problems 1-3, All Basic in addition to Challenging Problems.
Huffines, Brad Meteorologist
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