Chapter 18 – Complex TraitsI. Most traits are complex & the product of more than 1 gene and are influenced by the environmentII. When multiple genes are involved, the effect of anindividual gene on phenotype can be obscured. III. Different genotypes can have similar phenotypesA. Fig 18.4: Multiple genes contributing to acomplex trait. Three unlinked genes, each with 2alleles, influence intensity of red in wheat. Littleeffect of environment. Each dominant gene formincreases the amount of red pigment in a wheat seed.1. Wheat can be from white to dark red2. Each parent is intermediate in color. Colordistribution in normal (bell shaped)B. General conclusion: no environment is best for allgenotypesIV. Understanding the effect of a genotype requiresunderstanding the effect of the environment A. Fig. 18.8 – Height data1. Shows regression towards the mean2. Regression towards mean is shown becausecombinations of genes that result in extremephenotypes tend to be broken up during meiosis3. Environmental influence on parents’ phenotypess not reproduced in offspringV. Heritability : the proportion of total variation due to genetic differences in a particular population (not traditional definition) (Fig. 18.9)A. Fig 18.9: height differences are 100% due to heritability, because the environment is constantB. If human height was 100% heritable, the offspring mean would equal the parental mean (back to fig. 18.8). However, it’s influenced by environmentC. Magnitude of heritability is not an intrinsiccharacteristic of a trait1. Applies to the trait in a particularpopulation across the range ofenvironments that exist at that particular timeVI. Twin StudiesA. Twin studies suggest strong effect ofenvironment on behavioral disorders (eg.alcoholism), but little effect on otherdisorders, such as autism, etc. (fig. 18.11)B.VII. Most common diseases are affected by manygenes; each on its own causes a small effect (fig.18.13)Typical differences between:Prokaryotes EukaryotesCell size (big or small) Small (1-10um) Big (10-100 um)# of genes (few or many) Few (500 – 4000) Many (6,000 – 30,000)Gene structure (simple or complex)Simple (no introns) Complex (introns)Cell structure (simple or complex)Simple: no membrane bound organellesComplex: membrane bound
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