Lecture 2 - Discontinuous variation and genotype frequencies Flashcards Preview

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What is the purpose of the Hardy Weinberg equilibrium?

-Null model
-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?

Relies upon:
-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
-Diploid organisms
-Random mating
-No mutations
-No selection
-No population structure
-Non-overlapping generations
-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?

Chi-squared =
sum of ((observed-expected)^2)/expected))


When do genotypes differ significantly from expected in Chi-squared?

If the observed chi squared value > than critical chi squared value (p=0.05)