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Lecture 6

Lecture 6 - Population Genetics - Part 1

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Esteban Parra

Population Genetics 1 What is a Population? - Population can be defined as a group of interbreeding individuals of the same species sharing a common geographical area. - Gene pool is the sum of the alleles in a population at a particular time. - Evolution is the change of allele frequencies over time in a population. Important Things To Consider - An assumption usually made is that there is random mating among the individuals within a particular population. - However, remember that in many cases, there is no random mating among individuals living in the same geographical area, due to o Ethnic/Religious Isolation o Linguistic/Political Isolation o Other forms of isolation - In those cases, there is population subdivision, or in other words, population structure. - We won’t review the consequences of population structure in this course, but you should know that this happens quite often in human populations. Allele Frequency - What is the allele frequency of the yellow (Y) allele in this population? - Four Y alleles our of a total of ten = o 4/10 = 0.4 or 40% - Since there are only two alleles the frequency of the blue (B) allele is 100% - 40% = 60% Alleles are not Loose in the Population but are Paired Together in Individuals - These pairs are called genotypes - Homozygotes have two copies of the same allele - Heterozygotes have two different alleles Which Scenario is More Likely? - Inheritance works in randomly, where alleles combine randomly and the frequency will depend on how frequently the allele is present in the population - In the same way that the alleles are segregating independently in the formation of gametes, they are found independently assorted among individuals of a population. Hardy-Weinberg - This random assortment of alleles to form individuals in the population is key to understand the Hardy-Weinberg principle, or Hardy-Weinberg equilibrium. - In fact, the Hardy-Weinberg principle can be considered an extension of the Punnet square. - Let’s see why…. The Punnet Square - In the typical Punnet square, the genotype outcome of a cross is dependent on the possible combinations of gametes transmitted by each parent. - In this case, the probabilities of passing each allele are 0.5 (50%) if the parents are heterozygotes and 1 (100%) when they are homozygotes. Thus, the genotype probabilities of the resulting progeny are…. o BB:25% BY: 50% YY:25% o BB:50% BY:50% For Individuals to Populations… - In a population, when randomly selecting alleles from the gene pool, the probability of selecting a particular allele is equal to the frequency of that allele - Standard notation for allele frequencies: o p = freq(B); 0.6 = 60% in this population o q = freq(Y); 0.4 = 40% in this population HWE: Generalizing the Punnett Square for Population Genetics - In a population, the probability of individuals receiving each allele is equal to the allele frequency in the population (in this case p and q). p and q can vary between 0 and 1. - Note that the probability is not restricted to 0.5 or 1 as is the case when two persons mate. o pp + pq +qp + qq = 1 Hardy-Weinberg Equilibrium - pp + pq +qp + qq = 1 - pp + pq + pq + qq = 1 - p2 + 2pq + q2 = 1 - p + 2pq + q = 1 - p = frequency (A1) - 1 – p = q = frequency (A2) - p + 2pq + q = 1 The Importance of HWE - The HWE is very important because it states that under certain circumstances o There is a simple theoretical relationship between allele frequencies and genotype frequencies in a given population. o Allele frequencies will not change from generation to generation in a given population. - Thus, knowing genotype frequencies, we can easily estimate the allele frequencies in the population, and those allele frequencies can be used to predict the expected
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