CHAPTER 15 The Genetic Bases of Complex Inheritance Multifactorial genes have multiple genetic and environmental factors implicated in their causation and they are important in medical genetics and animal and plant breeding With a multifactorial trait a single genotype can have any one of many possible phenotypes and similar phenotypes can result from many different genotypes Multifactorial traits are often called complex traits because each factor that affects the trait contributes at most a modest amount to the total variation in the trait observed in the pop Most traits that vary in populations of humans and other organisms including common human diseases that have a genetic component are complex traits For a complex trait the genetic architecture consists of a description of all of the genetic and environmental factors that affect the trait along with the magnitudes of their individual effects and the magnitudes of interactions among the factors Environmental factors are much less easily partitioned into separate factors whose individual effects and interactions can be sorted out 15 1 Complex traits are determined by multiple genes and the environment Complex or multifactorial traits are often called quantitative traits because the phenotypes in a population differ in quantity rather than in type Ex height The opposite of this is a discrete trait which differs in kind such as eye color Quantitative traits are affected by genetic and environmental factors Studying inbred lines which are homozygous for most genes can eliminate variation in genotype Complete elimination of environmental variation is impossible Traits that are susceptible to small environmental effects will never be uniform even in inbred lines Continuous categorical and threshold traits are usually multifactorial Continuous traits have a continuous gradation from one phenotype to the next Ex weight Non continuous traits include categorical and threshold traits Categorical traits are traits in which the phenotype corresponds to any one of a number of discrete categories Ex kernels in corn eggs laid Threshold traits are traits what have only two or a few phenotypic classes but their inheritance is determined by the effects of multiple genes together with the environment Ex parthenogenesis in turkeys Many human diseases with a genetic component are threshold trait disorders and the phenotypic classes are affected versus not affected Threshold traits can be interpreted as continuous traits by imagining that each individual has an underlying risk or liability toward manifestation of the condition A liability above a certain cutoff or threshold results in expression of the condition a liability below the threshold results in normality The distribution of a trait in a population implies nothing about its inheritance The distribution of a trait in a population is a description of the population in terms of the proportion of individuals that have each of the possible phenotypes The mean or average is the peak of the distribution The variance is a measure of the spread of the distribution and is estimated in terms of the squared deviation difference of each observation from the mean The variance describes the extent to which the phenotypes are clustered around the mean Large spread out small clustered the standard deviation is the square root of the variance 68 95 99 7 15 2 Variation in a trait can be separated into genetic and environmental components Discrimination between genetic and environmental effects is not usually possible particularly in human quantitative genetics It can however be used to make comparisons in variance Four sources contribute to phenotypic variation genotypic variation environmental variation variation due to genotype by environment interaction and variation due to genotype by environment association The genotypic variance results from differences in genotype The variation in phenotype caused by differences in genotype among individuals is termed genotypic variance Even in the absence of environmental variation the distribution of phenotypes by itself provides no information about the number of genes influencing a trait and no information about the dominance relations of the alleles The number of genes influencing a quantitative trait is important in determining the potential for long term genetic improvement of a population by means of artificial selection The environmental variance results from differences in environment The distribution of a trait in a population provides no information about the relative importance of genotype and environment Variation in the trait can be entirely genetic entirely environmental or a combination of both influences When genetic and environmental effects contribute independently to phenotype the total variance equals the sum of the genotypic and environmental variance Genotype and environment can interact or they can be associated G E interaction can change the relative ranking of genotypes Genotype by sex interaction is where the same genotype results in a different phenotype according to the sex of the organism common in quantitative genetics When different genotypes in a population are not distributed at random among all the possible environments there is G E association There is no genotypic variance in a genetically homogenous population A genetically uniform population has a genetic variance of 0 The number of genes affecting a quantitative trait need not be large Additive the phenotype of the heterozygote is the average of the phenotypes of the corresponding homozygotes Traits determined by a small number of genes have a small potential for change Traits determined by a large number of genes have a large potential for improvement Selective breeding can create an improved population in which the value of every individual greatly exceeds that of the best individuals that existed in the original population The broad sense heritability includes all genetic effects combined Broad sense heritability is the ratio of the genotypic variance to the total phenotypic variance H genetic variance genotypic variance environmental variance Broad sense measures how much of the total variance in phenotype results from differences in genotype predicts outcome of selection among clones inbred lines or varieties Twin studies are often used to assess genetic effects on variation in a trait Potential sources of error in twins are G E interaction which increases the variance