Population Genetics - Study Guide
What are the five conditions that must be met in order for validation of the Hardy-Weinberg law? What are each of these assumptions violated by in natural populations?
1) Infinite population size - absence of fluctuations in gene frequency due to shear chance because population size is infinite or effectively infinite. *violated by genetic drift (sampling error in finite population) 2) Random mating - absence of any degree of preferential matings because individuals mate with each other at random (panmixia). *violated by non-random mating (assortative mating, inbreeding and sexual selection) 3) No natural selection - absence of differential mortality or fertility because all alleles are equally competent at making copies of themselves, which enter the gene pool in gametes. *violated by natural selection (differential replacement rates of alleles) 4) No mutation - absence of input of new copies of allele because there are not recurrent mutations *violated by mutation (transformation of one allele into another) 5) No immigration - absence of input or loss of genes to or from another population because immigration and emigration of individuals does not occur *violated by gene flow (migration of individuals)
What are Mendel's two laws (list)?
1) Law of segregation 2) Law of independent assortment
What are five ways in which the Hardy-Weinberg law is heuristically significant?
1) assumptions under the expression psquared+2pq...etc... give the conditions under which genotypic frequencies do not change, which in a panmictic population are segregation and random assortment 2) law is entirely theoretical 3) represents a static situation wherein genetic structure (composition) of the population does not change 4) describes situation in which there is no evolution 5) by revealing the conditions under which evolutionary change cannot occur, it shows the forces that could operate to cause change in the genetic composition of a population
What are the two steps of calculating gene frequencies based upon genotypic frequencies?
1) calculate relative genotypic frequencies 2) calculate gene frequencies using relative genotypic frequencies
In what two ways is the Hardy-Weinberg Law historically significant?
1) demonstrates that Mendelian inheritance does allow variation to be maintained in populations, as opposed to the diluting effect of blending inheritance 2) demonstrates that genetic processes of Mendelian inheritance do not themselves result in changes in the genetic structure of populations (i.e., gene frequencies)
What two factors describe genetic structure of populations?
1) gene frequency 2) genotypic frequency
Analytically, what two hypotheses can the Hardy-Weinberg law be used to test?
1) panmixia 2) evolutionary stasis
What are the two most significant features of the gene frequency concept?
1) provides quantitative description of genetic structure of a deme 2) represents the relationship between generations of populations (Gene frequency is the genetic characteristic of interest in the transmission of genes in a local population)
What two things is genotypic equilibrium based on? Why?
1) stable gene frequencies 2) random mating Why? Because genotypic frequencies depend on genetic equilibrium
What are the two main points of the Hardy-Weinberg Law?
1) the initial gene frequencies do not change 2) genotypic frequencies come to the equilibrium value determined by the initial gene frequencies in just one generation
What two features describe the gene pool for each locus?
1) the kinds of alleles present 2) the frequencies of alleles
Define evolution
A change in gene frequencies in populations over time.
Define deme
A deme is a local, randomly interbreeding population. Isolated or semi-isolated group of individuals of one species whose members interbreed in a manner which approximates panmixia.
What happens to the alleles in the gene pool during meiosis?
Alleles in the gene pool are shuffled and distributed to the gametes during meiosis each new generation.
What is the primary assertion of the Hardy-Weinberg law with regard to genotypic frequencies after one generation of random mating (with all other assumptions also met)?
Asserts that any arbitraty initial gentypic frequencies of D(AA), H(Aa) and R(aa) will equal the values D=psquared, H=2pq and R=qsquared in one generation and thereafter remain constant at those values.
Why can gene frequencies be calculated from genotypic frequencies, but genotypic frequencies cannot be calculated from gene frequencies?
Because selection can change genotypic frequencies so they cannot be calculated by multiplying allele frequencies.
What changes does the HW law allow us to concentrate on?
Changes in gene frequencies between zygotes and adult stage of generation t+1 because there is no change in gene frequency between adult stage at generation t and zygote stage of generation t+1
How can the Hardy-Weinbergy law be used to test hypotheses about whether or not evolution is occurring within a population?
Compare expected frequencies with observed frequencies using a chi-square test to determine whether observed frequencies are significantly different from expected frequencies. If they are, we conclude that the population is not in Hardy-Weinberg equilibrium and evolution is occurring.
How is gene frequency represented?
For any one allele, it is its fractional representation among all alleles at that locus in the population.
Frequencies of _______ in the gene pool of a population determine the composition of _______ produced in the new generation.
Frequencies of ALLELES in the gene pool of a population determine the composition of GENOTYPES (genotype frequencies) produced in the new generation.
What do the frequencies represent?
Frequencies represent the probabilities of occurrence of the respective alleles in the gene pool.
_____ frequencies are inherited by a population.
Gene frequencies
________ of a population can be calculated from _________ but _________ cannot be calculated from __________.
Gene frequencies of a population can be calculated from genotype frequencies, but genotype frequencies cannot be calculated from gene frequencies.
What do gene frequencies describes?
Gene frequency describes genetic variation in the entire gene pool.
Hardy-Weinberg Law states that, in an evolutionary static situation, a system of random mating leads to a condition of _______ ___________.
Genetic equilibrium
What is the idea of blending inheritance?
Genetic traits are not particulate. Every generation, variation is blended out as natural selection acts on traits.
What does genotypic frequency describes?
Genotypic frequency describes how variation is organized into genotypes.
State the Hardy-Weinberg Law
In the absence of disrupting factors (unbalanced mutation, unbalanced gene flow, natural selection, and genetic drift), the allelic and genotypic frequencies at a locus in a randomly interbreeding population (of diploids) will be repeated faithfully from generation to generation. Should the frequencies be perturbed for any reason, they will come to expected equilibrium frequencies after one generation of random mating (that is if the parents were not in genetic equilibrium the Hardy-Weinberg equilibrium will be restored in one generation).
Law of Segregation
Independence of alleles at one locus allows us to model the distribution of alleles using laws of probability
How does the Law of Independent Assortment influence population genetics?
Independence of loci (genes) tells us that we can model one locus independently of other loci.
Is the Hardy-Weinberg law only applicable to loci in which only two alleles occur?
No. The Hardy-Weinberg Law has been extended to cases of multiple alleles, multiple genes (loci), linked genes, sex-linked genes, and haploid organisms.
What reproductive characteristic of a population in Hardy-Weinberg equilibrium is assumed to allow for random mixing of the gametes?
Population is assumed to be panmictic
What does the law predict concerning genetic equilibrium?
Predicts that genetic equilibrium exists and that the equilibrium is reached very rapidly.
In genetic equilibrium, proportions of _____ remain unchanged from generation to generation.
Proportions of genotypes remain unchanged from generation to generation.
Define panmixia
Random interbreeding with each individual having an equal chance of mating with any member of the opposite sex. In panmixia, individuals share a common set of genes called a gene pool.
The Hardy-Weinberg law represents a _____ ______ against which deviations in _______ frequencies can be compared.
The HW law represents a null hypothesis against which deviations in genotypic frequencies can be compared.
What basic theorem is employed in order to demonstrate Hardy-Weinberg law?
The basic theorem of probability
The composition of __________ in a population determines the frequencies of __________ in the gene pool.
The composition of GENOTYPES in a population determines the frequencies of ALLELES in the gene pool.
What happens to the gene pool each new generation?
The gene pool is reconstituted with each new generation.
What is the basic theorem of probability? What does this mean in terms of calculating probabilities of different single-locus genotypes in progeny?
The probability of co-occurence of any two (or more) independent events equals the product of their independent probabilities. Gene frequencies are multiplied to get probabilities of different single-locus genotypes in progeny because two alleles per locus occur in each gamete.
Define gene frequency
The proportion of different alleles of a gene at that locus in the population.
Define population genetics
The study of genetic structure (genetic constitution) of populations, including changes within individual populations from generation to generation, and changes among populations as a result of evolutionary processes.
Define gene pool
The sum of genotypes of all individuals in a deme or the sum total of genes in gametes of a population (two copies per individual, which may or may not be the same allele).
Provide an example of a case of multiple alleles where the Hardy-Weinberg equilibrium has been applied?
Three-allele ABO blood types in a population with gene frequencies p, q, r (respectively).
What is the life cycle of a hypothetical population?
adult t --> zygote t+1 --> adult t+1
