Lecture 2 - Discontinuous variation and genotype frequencies Flashcards Preview

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Flashcards in Lecture 2 - Discontinuous variation and genotype frequencies Deck (20)
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1

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

2

What is discontinuous variation?

-when a trait shows a limited number of distinct categories e.g. presence/absence of stripes, flower colour

3

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

4

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

5

How are allele frequencies symbolised?

p and q
-always add up to 1
-p+q = 1

6

How are the frequency of genotypes symbolised?

fGG
fGg
-Gg

7

How does the number of alleles relate to the number of individuals?

In diploid population, individuals have 2 alleles
-N alleles = 2N individuals

8

What is the equation for working out the expected frequency of p (dominant) allele in hardy Weinberg equilibrium?

p= ((2N(GG) + N(Gg))/2N
q=p-1

9

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

10

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
Normally:
-Mating is not random
-The locus studied is under selection
-There is hidden population structure
-some kind of evolutionary process

11

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

12

What occurs in a Hardy Weinbergy equilibrium if the assumptions are fulfilled?

the allele frequencies in a population remain contant over time

13

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

14

What are the symbols of GG, Gg and gg in hardy weinberg ?

GG = p2
Gg = 2pq
gg = q2

15

What is the Hardy-Weinberg equilibrium?

1908
-A model that predicts allele and genotype frequencies across future generations if they stay constant

16

What are the gentype frequences in Hardy Weinberg?

p1:pq:q2

17

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

18

How do we know whether observed and expected genotype frequencies are different in hardy weinberg?

Statistics chi-squared test

19

What is the formula for the chi-squared test?

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

20

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

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