Genetics - Lecture 19: Population Genetics I

Chi-Squre Goodness-Of-Fit-Test

Statistical test used to evaluate how well a set of observed values fir the expected values

• the probability associated with a calculated chi-square value is the probability that the difference between the observed and the expected values is due to chance (determines statistically whether something is different by chance or actually significantly different)

• Chi square = (the sum of the observed values - the expected value)2 / divided by the sum of the expected value

• Will observed ratios = expected outcomes?

degrees of freedom

• n-1 (n = number of different expected phenotypes)

• the number of ways the expected classes are free to vary

• used to determine the p-value (probability)

population genetics

stay of genetic composition of populations and how that composition changes geographically and with the passage of time (evolution)

Mendelian Population

a group of interbreeding, sexually reproducing individuals that have a common set of genes

• in genetics, population means a group of the same genes

gene pool

set of all genes/alleles in a population

genetic variation

the diversity on DNA sequences among individual within a population or species

• a measure of the genetic difference that exist within a population

• variation of alleles and genotypes occurring within and between groups of a single species

variation

• obvious feature of life, just about everyone/everything is different

• much of phenotype variation is hereditary

  • genetic variation is the basis of all evolution

  • the extent of genetic variation within a population affects its potential to adapt to environmental change

• leads to natural selection

natural selection

the fundamental mechanism of evolution where organisms better adapted for their environment tend to survive and produce more offspring

• actor of heritable phenotypic variations, causing advantaged traits to become more common overtime

• driven by the differential survival and reproduction of individuals

Describe genetic structure in a population by…

enumerating the types and frequencies of genotypes and alleles in that population

frequency (f)

simply a proportion or a percentage, usually expressed by a decimal fraction

genotypic frequency

proportion of a particular genotype within a population

• number of individuals possessing a genotype divided by the total number of indivudlas in the sample (N)

• all frequencies added together should equal 100 al

To calculate Genotypic frequencies

• count the number of people with each genotype and divide by the total

  • sum of all genotypic frequencies should equal 1

Allelic frequency (*do questions from slide 15-24)

proportion of a particular allele within a population

• often more informative than genotypic frequencies

• in a sexually reproducing population, the genotypes are only temporary assemblages of the alleles

Two ways to allelic frequencies:

1. The number of genotypes

• count the number of copies of a particular allele present ay the locus in a sample and divide by the total number of alleles in the sample

2. The frequencies of the genotypes

• use the genotypes (which are easier to count) to infer allelic frequencies

Calculating allelic frequency

2 times the homozygous (because they carry two alleles) + the number of heterozygotes / 2 times the number of individuals (because each individual carries two alleles)

Hardy-Weinberg Law (*do questions from slide 30-37)

explains the genetic behaviour of large populations according to Mendels Laws, and more specifically, it predicts allelic and genotypic frequencies in those populations

• formulated independently by G. H. Hardy and Wilhelm Weinberg in 1908

• Mathematical model that evaluates the effect of reproduction on the genotypic and allelic frequencies of a population

• explains genetic behaviour or large populations according to Mendel’s Laws

Hardy-Weinberg assumption and predictions

• when assumption are met , reproduction alone does not alter allelic or genotypic frequencies, and the allelic frequencies determine the frequencies of genotypes

Assumption - If a population is large, randomly mating, and not affected by mutations, migration, or natural selection…

Predictions -

1. The allelic frequencies of a population do not change

2. The genotypic frequencies stabilize (will not change) after one generation in the proportions p2 (the frequency of AA), pq (the frequency of Aa), and q2 (the frequency of aa), where p equals the frequency of allele A and q equals the frequency of allele a

Hardy Weinberg Equilibrium

When genotypes are in the expected proportions of p2, 2pq, and q2

• also when the promotions have stabilized

• if predicted genotypes don’t match the observed values the population is not in HWE

• All genotypic frequencies should equal 1

2 equations used to describe a population

1. p² + 2pq + q² = 1

• The distribution of all genotypes in the population

2. p + q = 1

• The total of all alleles in the population

Implications of the Hardy-Weinberg Law

1. A population cannot evolve if it meets the Hardy-Weinberg assumptions, because evolution consists of change on the allelic frequencies of a population

• Populations are static and stable

• Sexual reproduction alone will not bring about evolution

2. When a population is in HWE, the genotypic frequencies are detrained by the allele frequencies

• use the formula

• when not in HWE, we have no basis for predicting the genotypic frequencies

• When allele frequencies for two alleles are perfectly balanced (0.5 each), the heterozygous will be the most popular genotype

• One an allele is favoured in a population, the homozygous for that allele will become more common

• once an allele is rare, most of that allele will be found in heterozygous and not in homozygous

Estimating Allelic frequencies with HWL

• When populations are in HWE, allele frequencies allow you to predict genotypic frequencies (and vice versa)

• Sometimes, we don’t know hoe much of the population is heterozygotic and how much is homozygous dominant

  • What if we want to know how much of the population is a disease carrier? (Ex: Cystic Fibrosis)

  • Hardy-Weinberg Law lets us calculate these genotypic rates