Chapter18_SSM - 65781_CH18_347_369.qxd 8/1/08 1:24 PM Page...

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347 Chapter 18: The Genetic Basis of Complex Inheritance Chapter Summary Many traits that are important in agriculture and human genetics are determined by the effects of multiple genes and by the environment. Such traits are known as complex traits, and their inheritance is called com- plex inheritance. There are three types of complex traits: continuous, categorical, and threshold traits. Continuous traits (also called quantitative traits) are traits expressed according to a continuous scale of measurement, such as height. Categorical traits are traits expressed in whole numbers, such as the number of grains on an ear of corn. Threshold traits have an underlying risk and are either expressed or not expressed in each individual; an example is diabetes. The genes affecting complex traits are no different from those affecting simple Mendelian traits, and the genes can have multiple alleles, partial dominance, and so forth. When several genes affect a complex trait, the pattern of genetic transmission need not fit a simple Mendelian pattern, because the effects of one gene can be obscured by other genes or the environ- ment. However, the number of genes can be estimated, and many of them can be mapped using linkage to polymorphic DNA markers. Many continuous and categorical traits have a distribution that approximates the bell-shaped curve of a nor- mal distribution. A normal distribution can be completely described by two quantities: the mean and the variance. The standard deviation of a distribution is the square root of the variance. In a normal distribu- tion, approximately 68 percent of the individuals have a phenotype within 1 standard deviation of the mean, and approximately 95 percent of the individuals have a phenotype within 2 standard deviations of the mean. Variation in phenotype of multifactorial traits among individuals in a population derives from four principal sources: (1) variation in genotype, which is measured by the genotypic variance; (2) variation in environment, which is measured by the environmental variance; (3) variation resulting from the interaction between geno- type and environment (G–E interaction); and (4) variation resulting from nonrandom association of genotypes and environments (G–E association). The ratio of genotypic variance to the phenotypic variance of a trait is called the broad-sense heritability; this quantity is useful in predicting the outcome of artificial selection among clones or inbred lines. When artificial selection is carried out in a randomly mating population,
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348 artificial selection broad-sense heritability candidate gene categorical trait complex trait continuous trait correlated response correlation coefficient covariance distribution environmental variance fraternal twins genotype-by-environment association genotype-by-environment interaction genotype-by-sex interaction genotypic variance heterosis hybrid vigor identical twins inbreeding depression individual selection liability Keywords
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This note was uploaded on 09/29/2011 for the course GENETICS 380 taught by Professor Glodowski during the Spring '08 term at Rutgers.

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Chapter18_SSM - 65781_CH18_347_369.qxd 8/1/08 1:24 PM Page...

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