Flashcards in Lecture 2 - Discontinuous variation and genotype frequencies Deck (20)
What is the purpose of the Hardy Weinberg equilibrium?
-For estimating genotype frequencies in a population when given allele frequencies
-based on mendelian genetics
-deviations suggest something interesting is going on
What is discontinuous variation?
-when a trait shows a limited number of distinct categories e.g. presence/absence of stripes, flower colour
What is the purpose of a null model?
Necessary as natural population have varying degrees of genotypes
-for determining whether observed frequencies are unusual in any way and whether an evolutionary process is occuring
What are the features of using punnett sqaures to estimate expected frequencies?
-random mating between individuals
-knowing the proportion of alleles in the population
How are allele frequencies symbolised?
p and q
-always add up to 1
-p+q = 1
How are the frequency of genotypes symbolised?
How does the number of alleles relate to the number of individuals?
In diploid population, individuals have 2 alleles
-N alleles = 2N individuals
What is the equation for working out the expected frequency of p (dominant) allele in hardy Weinberg equilibrium?
p= ((2N(GG) + N(Gg))/2N
How many degrees of freedom are there for the hardy weinberg test?
d.f = (# of categories in the data -1) - (# of parameters estimated from the data) e.g.
d.f. = (# phenotypic categories of the data - 1) - (# of allele frequencies estimated from the data)
d.f = (3-1)-1 = 1
estimated number of allele frequencies = 1 as only p needs to be estimated, q is then forced to be 1-p=q
What do deviations from the Hardy Weinberg equilibrium suggest?
that assumptions are not fulfilled
Hardy weinberg therefore serves as the null hypothesis to be tested
-Mating is not random
-The locus studied is under selection
-There is hidden population structure
-some kind of evolutionary process
What are the assumptions of the Hardy Weinberg equilibrium?
-infinite population size
-No population structure
-Equal allele frequencies among males and females
What occurs in a Hardy Weinbergy equilibrium if the assumptions are fulfilled?
the allele frequencies in a population remain contant over time
Why is the Hardy Weinberg equilibrium useful if so many unrealistic assumptions need to be fulfilled?
-describes the behaviour of alleles in 'ideal' populations
-Many populations are in hardy weinberg
-null hypothesis that can be tested
What are the symbols of GG, Gg and gg in hardy weinberg ?
GG = p2
Gg = 2pq
gg = q2
What is the Hardy-Weinberg equilibrium?
-A model that predicts allele and genotype frequencies across future generations if they stay constant
What are the gentype frequences in Hardy Weinberg?
Why is the Hardy Weinberg equilibrium important?
-provides a null model and expectation of what happens if no evoltionary processes are acting
-expectation of what genotype frequencies should be
-can be compared to actual genotype frequences
-if there is a difference suggest some interesting evolutionary processes are happening
How do we know whether observed and expected genotype frequencies are different in hardy weinberg?
Statistics chi-squared test
What is the formula for the chi-squared test?
sum of ((observed-expected)^2)/expected))