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qtl - Mapping genes for complex traits/diseases Most...

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Mapping genes for complex traits/diseases Most interesting traits are quantitative (high, weight, milk yield, etc.) and controlled by a number of genes and also strongly influenced by the environment No single gene shows perfect segregation with the trait Generally, Multiple loci contribute Environmental effects are also important Complex traits (cont.) Even if only a single gene underlies the trait, genetics can still be complex incomplete penetrance, variable expressivity Phenocopies -- individuals showing the trait for strictly environmental reasons. Even traits that appear binary (disease is present/absent) can result from a number of interacting genes Here the quantitative trait is the risk (penetrance) given a particular genotype The Genetics of Human Disease Can score individuals as affected / unaffected Alternatively, if known, the appropriate underlying physiological variable can be used as a quantitative character Blood pressure vs. hypertension (affected / normal) In most cases, don't know underlying biochemical variables. How to we find and map such underlying genes? Mapping genes underlying complex traits QTLs ( Quantitative Trait loci ) -- those underlying quantitative traits DS ( Disease susceptibility ) genes -- those influencing an individual's susceptibility to a particular disease (e.g., heart disease, Alzheimer's). Note that DS genes are simply QTLs that influence a disease.
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Approach 1 for mapping: Candidate genes If one has some biochemical/developmental understanding of the trait/disease, then one can test to see if specific genes in known pathways are involved Variation seen in the population can map to allelic (i.e., DNA sequence) variance in particular candidate genes . Note that just because a gene is involved in a pathway leading to the trait does not mean that trait variation is due to genetic variation in that trait. One looks for associations between the trait and the average values for particular alleles at the candidate gene Example: Apo E (Apolioprotein E) ApoE gene known to be involved in cholesterol metabolism Each copy of allele e 4 found to increases cholesterol level by 5-10 Each copy of allele e 2
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