Lab 3 Population Genetics

ANTH 110L LAB # 3: GENETICS PART 2 28 February 2024 Kenya DE Loza Anthropology 102 – LABORATORY EXPERIENCE 3 1

GENETICS PART 2: POPULATION GENETICS THE THEORY AND PRACTICE OF THE HARDY-WEINBERG EQUILIBRIUM MODEL Lab report begins on page 13. In 1908, Godfrey Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, simultaneously published papers dealing with a misconception was that dominant genes would eventually replace recessive genes. Both Hardy and Weinberg demonstrated mathematically that gene frequencies , or the percent of genes in a breeding population , would not change from one generation to another if the following conditions were met : (1) random mating across the population (panmixia) (2) equal number of males and females (3) the population is infinitely large (4) there is no “in” or “out” migration (5) natural selection, mutation, and genetic drift are not acting upon the population This is the Hardy-Weinberg Equilibrium Model. To conceptualize this model, it is easiest to think of it as the Mendelian genetics of breeding. Using a Punnett square it is possible to calculate genotype frequencies or the percent of a certain genotype (dominant homozygotes, heterozygotes, recessive heterozygotes) in a breeding population if the genotypes of both parents are known. The study of genetics of breeding populations is called population genetics and the Hardy-Weinberg Equilibrium Model is central to understanding and doing research in population genetics. DETERMINING GENE FREQUENCIES Determining gene frequencies is not very difficult. One method simply involves counting identifiable genes. For example, in the human MN blood-group system (this is similar to the ABO system) there are three identifiable genotypes and three identifiable phenotypes because the MN system is a co-dominant system. If two alleles are co-dominant both will be expressed when both are present. One allele is not dominant and the other is not recessive. In a co-dominant system the heterozygote is distinguishable from the dominant homozygote. The three identifiable phenotypes are M , MN , and N . These are identifiable phenotypes so it is less difficult to determine genotypes. Phenotype M has the genotype MM 2

or two copies of the M allele; phenotype MN has the genotype MN because both alleles are expressed; and phenotype N has the genotype NN , or two copies of the N allele. Example 1: PHENOTYPE NUMBER OF INDIVIDUALS GENOTYPE NUMBER OF M ALLELES NUMBER OF N ALLELES M 600 MM 1200 0 MN 300 MN 300 300 N 100 NN 0 200 1000 individuals 1500 genes 500 genes By doing a census in this hypothetical population of 1000 , it was determined that 600 individuals were blood type M , 300 were blood type MN , and 100 were blood type N . Using this information, gene frequencies of the percent of genes at this locus that are M and the percent that are N can be determined. Remember that each individual has two genes at each locus, so for every 1000 individuals there will be 2000 genes ( 1000 x 2 = 2000 ). If 600 individuals are blood type M , then each has two copies of M allele or 1200 total M alleles. If 300 individuals are blood type MN , then each has one copy, or 300 total copies of the M allele and one copy or 300 total copies of the N allele. If 100 individuals are blood type N , then each has two copies of the N allele or 200 total N alleles. Using this information, it is then a simple matter to determine the frequency of the M allele and the frequency of the N allele in this population. The frequency of the M allele in this population equals 1500/2000, which equals 0.75 or 75 percent . The total number of M alleles is 1500 and the total number of M and N alleles is 2000 . The frequency of the N allele in this population equals 500/2000, which equals 0.25 or ( 25 percent) . Notice that the frequency of the M plus the frequency of N equals 1.0 or ( 100 percent) . It is impossible to have more than 100 percent of the alleles at a given gene locus. Example 2: 3

PHENOTYPE NUMBER OF INDIVIDUALS GENOTYPE NUMBER OF M ALLELES NUMBER OF N ALLELES M 75 MM 150 0 MN 75 MN 75 75 N 50 NN 0 100 200 individuals 225 genes 175 genes The frequency of M in this population equals 225/400, which equals 0.56 (or 56 percent). The frequency of the N allele equals 175/400, which equals 0.44 (or 44 percent). RANDOM AND NON-RANDOM MATING There is much interest in defining the genetic population because this is a convenient way to describe a population that shares a gene pool. In order to share a gene pool equally, mating must be random. Random mating in a sexually reproducing species, such as humans, means that the statistical chance of mating with any individual of a particular genotype is proportional to the frequency of that genotype among members of the opposite sex. For example, blood groups are not visible characteristics so they should not play any role in mate selection. To use Example 1 above, the likelihood that a woman will mate with a man who is blood type M is 60 percent because 60 percent of the male breeding population is blood type M . If mating is non-random, then women of blood type M will mate with men of blood type M either more or less than 60 percent of the time. If this occurs then it is necessary to look for the reasons for the departure from random mating. Non-random mating is frequently the result of social factors, especially for traits that are outwardly visible such as skin color; mating among humans is not random with respect to skin color. Another term for non-random mating is assortative mating. Assortative mating with respect to one trait usually leaves mating with respect to other traits essentially random. For example, tall men may mate with tall women but since stature is not related to blood type, mating with respect to the ABO and MN blood systems should be random. There are two types of assortative mating: positive and negative . Positive assortative mating refers to preferential matings between like genotypes as reflected through their phenotypes 4

while negative assortative mating refers to preferential mating between different or opposite genotypes as reflected through their phenotypes. A consistent pattern of assortative mating can lead to changes in genotype and ultimately gene frequencies. THE HARDY-WEINBERG EQUILIBRIUM FORMULA If random mating is assumed, then there are certain conclusions that can be drawn about the distribution of genotypes. The most basic is that the proportion of genotypes in a population is basically stable. Once equilibrium has been reached, this means that gene and genotype frequencies do not change, they will not change unless some force is applied to the system. This is the entire idea behind the Hardy-Weinberg Equilibrium Model. (1) The Hardy-Weinberg Equilibrium Formula is used to determine whether or not evolution is taking place in a given population with respect to a particular gene locus. It tests whether or not the gene and genotype frequencies are changing from one generation to the next; so it tests whether or not the genetic make-up of a population is stable. (2) The Hardy-Weinberg Equilibrium Formula is only a simple, basic model demonstrating idealized gene behavior; it does not tell us why change is occurring – only that change is or is not occurring. (3) Any population, until proven otherwise, is assumed to be in Hardy-Weinberg Equilibrium . For a population to be in Hardy-Weinberg equilibrium we assume the following : (a) mating is random; (b) equal numbers of males and females, (c) the population is infinitely large, and (d) no migration “in” or “out” of the population; and (e) no forces of evolution (natural selection, mutation, genetic drift, gene flow) are acting on the population. (4) Hardy-Weinberg demonstrates two important concepts: (a) genes do not change in and of themselves; there must be some pressure or force to produce the change (four forces of evolution and non-random mating) (b) there is a consistency within the system; without some type of pressure being applied the system will remain stable and static – it will remain in a state of equilibrium (5) Four practical uses of the Hardy-Weinberg Equilibrium Formula are to: 5