I Most traits are complex the product of more than 1 gene and are influenced by the Chapter 18 Complex Traits environment II When multiple genes are involved the effect of an individual gene on phenotype can be obscured III Different genotypes can have similar phenotypes A Fig 18 4 Multiple genes contributing to a complex trait Three unlinked genes each with 2 alleles influence intensity of red in wheat Little effect of environment Each dominant gene form increases the amount of red pigment in a wheat seed 1 Wheat can be from white to dark red 2 Each parent is intermediate in color Color distribution in normal bell shaped B General conclusion no environment is best for all genotypes IV Understanding the effect of a genotype requires understanding the effect of the environment A Fig 18 8 Height data 1 Shows regression towards the mean 2 Regression towards mean is shown because combinations of genes that result in extreme phenotypes tend to be broken up during meiosis 3 Environmental influence on parents phenotypes s not reproduced in offspring V Heritability population not traditional definition Fig 18 9 the proportion of total variation due to genetic differences in a particular A Fig 18 9 height differences are 100 due to heritability because the environment is constant B If human height was 100 heritable the offspring mean would equal the parental mean back to fig 18 8 However it s influenced by environment time C Magnitude of heritability is not an intrinsic characteristic of a trait 1 Applies to the trait in a particular population across the range of environments that exist at that particular VI Twin Studies A Twin studies suggest strong effect of environment on behavioral disorders eg alcoholism but little effect on other disorders such as autism etc fig 18 11 B VII Most common diseases are affected by many genes each on its own causes a small effect fig 18 13 18 11 Typical differences between Cell size big or small of genes few or many Gene structure simple or complex Cell structure simple or complex Prokaryotes Eukaryotes Small 1 10um Few 500 4000 Simple no introns 18 13 Big 10 100 um Many 6 000 30 000 Complex introns Simple no membrane bound organelles Complex membrane bound organelles
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