Chapter 19 - The Inheritance of Complex Traits

• Genes play a role in determining our heights.
• Genes and environmental factors contribute to an individual phenotype.
• Mendel’s laws are applicable to continuous traits.
• Traits such as height that show a continuous range of variation and do not behave in a simple Mendelian fashion are known as quantitative or complex traits.
• The term complex trait is often preferred because variation for such traits is governed by a “complex” of genetic and environmental factors.
• A group known as the biometricians discovered that there are correlations between relatives for continuous traits such that tall parents tend to have tall children.
• The biometricians saw no evidence that such traits followed Mendel’s laws.
• Some adherents of Mendelism thought continuous variation was unimportant and could be ignored when studying inheritance.
• The multifactorial hypothesis proposed that continuous traits are governed by a combination of multiple Mendelian loci, each with a small effect on the trait, and environmental factors.
• Using this model, we will then show how quantitative geneticists partition the phenotypic variation in a population into the parts that are due to genetic and environmental factors.
• Complex traits are of paramount importance in medical and agricultural genetics.
• In this chapter, we will explore the inheritance of complex traits.
• We will review the methods used by plant and animal breeders to predict the phenotype of offspring from the phenotype of their parents.
• For crop plants, yield, resistance to pathogens, ability to tolerate drought stress, efficiency of fertilizer uptake, and even flavor are all complex traits.
• Next, we will develop the mathematical model used to connect the action of genes inside the cell with the phenotypes we observe at the level of the whole organism.
• The phenotypic deviation ( x ) is the sum of the genotypic ( g ) and environmental ( e ) deviations, so we can substitute ( g + e ) for x and obtain V X = E ( g 2 ) + E ( e 2 ).
• For independent traits, the covariance will be zero.
• The first term E ( g ) is the genetic variance, the middle term [ E ( e )] is the environmental variance, and the last term is twice the covariance between genotype and environment E ( ge ).
• Variances work only when genotype and environment are uncorrelated.
• In controlled experiments, different genotypes are placed into different environments at random, making genotype and environment independent.
• If there is no association between X and Y, then the covariance will be zero.
• In the main text, we saw that the variance is the expected value of the squared deviations: V X = E ( x 2 ).
• The phenotypic variance is the sum of the variance due to the different genotypes in the population and the variance due to the different environments within which the organisms are reared.
• Notice that trait values for the BC plants are intermediate between the two parents as expected but closer to the Beefmaster value because this is a BC population and Beefmaster was the backcross parent.
• In the BC generation, there is a continuous range of fruit sizes.
• Table 19-6 shows part of such a data set for just 20 plants and 5 marker loci that are linked on a single chromosome.
• In Table 19-6, you can see the positions of crossovers between the marker loci that occurred during meiosis in the F parent.
• We grow several hundred BC plants to maturity and measure the weight of the fruit on each.
• Although the multifactorial hypothesis provided a sensible explanation for continuous variation, classic Mendelian analysis is inadequate for the study of complex traits.
• The additive genetic variance (Va) is the variance of the additive deviations or the variance of the breeding values.
• The simple model for decomposing traits into genetic and environmental deviations, x = g + e, assumes that there is no genotype–environment interaction.
• Genetic deviation (g) is partitioned into the additive (a) and dominance (d) deviations.
• If one factor alters the effect of another factor, then there is an interaction.
• The variance of the genetic deviation (Vg) is the sum of the additive genetic variance (Va) and the dominance variance (Vd).
• The difference in the trait value between the two inbreds may be different in different environments, and so the difference between the lines averaged over the two environments may not accurately reflect their genetic difference.
• A genotype–environment interaction occurs when the performance of different genotypes is unequally affected by a change in the environment.
• The dominance variance (Vd) is the variance of the dominance deviations.
• The phenotypic variance (V) is the variance of the environmental variance (Ve) assuming that the additive and dominance components are not correlated with the environmental effects.
• The field of quantitative genetics aims to define the genetic architecture of complex traits.
• Genetic architecture is a description of all of the genetic factors that influence a trait.