BIOL202 - Study Guide - Lecture 20 - Population Genetics Practice Problems with Solutions

BIOL202 - Study Guide - Lecture 20 - Population Genetics Practice Problems with Solutions

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POPULATION GENETICS MATH PROBLEMS Below are a series of math problems that require you to calculate allele and genotype frequencies in a population . Both the text and the material below will be useful in helping develop a strategy to solve these problems. Compare answers with somebody in class to make sure you are getting the right answers. A big, big note of caution: In the past couple of years, many students have put very little time or thought into learning how to do the population genetics problems. An inability to answer any such problems on the exam has had a strong negative effect on grades. These problems are not difficult, and with a little practice you should be able to do them with ease. Please take the time to learn how to do them and get help if and when needed. The Hardy-Weinberg Theorem tells us that: If a certain gene has two alleles (e.g., A and a or R and R' or T 1 and T 2, etc) and the population is in "equilibrium" for that gene, one can calculate the frequencies of the different genotypes in the population as follows: If we let p equal the frequency of one allele and let q equal the frequency of the other allele, then… p 2 will equal the frequency of one homozygote 2 pq will equal the frequency of the heterozygote q 2 will equal the frequency of the other homozygote Note: p + q must = 1 and p 2 + 2 pq + q 2 must = 1 Sample Worked Problems A. Say that you have a population of eagles. The eagles have a gene that affects talon size. There are two alleles for this gene, T1 and T2. There are three possible genotypes: T1T1, T1T2 , T2T2. Say that you determine the number of individuals to have each genotype as follows: T1T1 = 45 individuals T1T2 = 55 individuals T2T2 = 30 individuals What are the frequencies of the two alleles? Let’s use p to denote the frequency of T1 and q to denote the frequency of T2. The frequency of of the T1 allele, p , will equal the total number of T1 alleles in the population divided by the total number of alleles of any type. Each individual carries two alleles so the total number of alleles is 2N where N = population size. Here that is 2 times 45+55+30 = 2(130) = 260. How many of the 260 total alleles are T1? The total number of T1 alleles = 2(no. of T1T1 individuals) + 1(no. of T1T2 individuals) each carries two T1 alleles each carries one T1 allele Thus the total T1 alleles = 2(45) + 1(55) = 90 + 55 = 145
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The frequency or proportion of all alleles that are T1, p, is then calculated as follows: freq. of T1 = p = 2(45) + 1(55) = 90 + 55 = 145 = 0.558 (Meaning 55.8% of alleles in pop are T1) 2 (130) 260 260 What is the frequency of the T2 allele? There is an easy and a less easy way to figure this out. First, the easy way: Since : p + q = 1 we can say 0.558 + q = 1 or q = 1 - 0.558 = 0.442 Thus 44.2% of alleles are T2. Alternatively we could do this:
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This test prep was uploaded on 04/18/2008 for the course BIOL 202 taught by Professor Johnson during the Spring '05 term at Maryland.

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BIOL202 - Study Guide - Lecture 20 - Population Genetics Practice Problems with Solutions

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