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Lesson 13: Polygenic Inheritance • The very first personality trait that was ascribed to a known gene is “novelty seeking”. People can be high novelty seekers – they like bungee jumping and risky behaviors, or they can be low novelty seekers – they like reading and solving Sodoku puzzles. While as many as 10 genes are estimated to play a role in this trait, the first one identified was the gene that encodes a dopamine receptor. DRD2 (sometimes referred to as D2DR) was the second dopamine receptor identified and is found in the nucleus accumbens among other places. The idea is that the rewarding feeling of reading is enough for some people, but others need more stimulation. Lesson 13: Polygenic Inheritance • appreciate how most behavioral traits, including response to drugs, are attributable to many genes; • learn how to make predictions about probabilities for given polygenic phenotypes; • understand the principles of independent assortment that underlie these predictions; and • appreciate that polygenic traits are “quantitative” and bell-curve distributions of phenotypes are generally observed in a population and even in a mating What do we mean by polygenic traits? “Brain Storm” by alexiuus.deviantart.com What happens to traits controlled by genes on different chromosomes? Chromosomes move to gametes INDEPENDENTLY How do gametes get random combinations of chromosomes? W w w W S s S s What do you think? For genes on DIFFERENT chromosomes, it is RANDOM which of each type of chromosome an egg or sperm receives ws WS Ws wS How do we predict for multiple genes? Ww x Ww W w WW Ww Can make a BIG S= Bent square pinkiefors= straight pinkie Punnett w Ww independent genes…. ww WwSs x WwSs W = widow’s peak w=none W S s S SS Ss s Ss ss Ss x Ss “Doc Linda’s Rule” Aka Boolean logic: Activity 2 AND OR Versus Using the Punnet Squares you’ve created, answer these Qs: What proportion of kids will be WW AND ss? ¼ WW x ¼ ss = 1/16 MULTIPLY your fractions What is the proportion of kids with the dominant trait for W? ¼ WW + ½ Ww = ¾ ADD your fractions Led to second Thing Mendel Didn’t Know A trait can be controlled by MANY genes example – height, skin color, novelty seeking RR Rr rr Most work by incomplete dominance Imagine several genes influence a trait such as height Each gene has two type of allele, one that adds inches and one that doesn’t Contributing allele Non-contributing allele Try it with alcohol response genes: AADd x AaDd AA x Aa Dd x Dd ADH ALDH Fraction of kids with ANY TWO (AA or DD) contributing alleles (good metab. of alcohol) = ? A AA x Aa A AA A Aa d D DD Dd d ADH AA Aa D Dd x Dd a Dd dd ALDH Alcohol response gene problems: A AA x Aa A AA A Dd x Dd a Aa AA Aa D d D DD Dd d Dd dd Fraction of kids with ALL contributing alleles (great metabolism of alcohol) = ? More Alcohol response genes problems A AA x Aa A AA A Aa d D DD Dd d Fraction of kids with 2 and only 2 contributing alleles (medium metabolism of alcohol) = ? AA Aa D Dd x Dd a Dd dd AAdd, OR AaDd, OR aaDD – all equally medium in alcohol metabolism. Imagine that two genes control hair color: (mom)AaDd x AaDd (dad) What are the probable ratios of their children? 1. 2. 3. 4. 5. 6. 1/8 are AAdd + 1/4 are AaDd 1/4 are AAdd + 1/4 are AaDd 1/4 are AAdd + 1/2 are AaDd 1/8 are AAdd + 1/16 are aaDD 1/4 are AaDd + 1/16 are aaDD None of the kids has just two contributing alleles Bell Curves and Polygenic Inheritance LOTS of contributing alleles FEW contributing alleles Why are there more people with an intermediate phenotype (for polygenic traits?) 0 xxyyzz 1 2 3 4 5 6 XXYYZZ XXyyzz = XxYyzz = XxyyZz = xxYYzz = xxYyZz = xxyyZZ Doesn’t matter which two alleles are contributing (capitals) The Enormous Pumpkin Problem Y Y X x X XX Xx Y YYYY Z ZZ ZZ x Xx xx y YyYy z Zz Zz XX = ¼ YY = ½ ZZ = ½ What is the probability of getting a 6 pumpkin? Chance of getting a 5? XXYYZz OR XXYyZZ OR XxYYZZ 1/16 1/16 1/8 Therefore: • 1) drug response genes make proteins whose “dosage” can be counted. • 2) Use small/easy Punnett squares and Boolean logic to compute probabilities. • 3) Polygenic trait extremes are represented by fewer genotypes and often least frequent • Now you are ready for the LAB! What happens to traits controlled by genes on different chromosomes? Chromosomes move to gametes INDEPENDENTLY Part III of Lab: • Example of branching diagram: 12-RrTt x RrTt ¼ TT ¼ RR ½ Tt ¼ tt ½ Rr ¼ rr ¼ TT ½ Tt ¼ tt Etc. What is probability of having a tongue rolling, non- taster? Check w/#4 Lab Questions: • Why are phenotypes distributed in the manner your graph shows for polygenic traits? • How will you remember when to add and when to multiply fractions? • Pick up LAB HW when done Jacobw.com