Lecture 2 - Discontinuous variation and genotype frequencies

Question 1

What is the purpose of the Hardy Weinberg equilibrium?

Answer 1

• Null model
• For estimating genotype frequencies in a population when given allele frequencies
• based on mendelian genetics
• deviations suggest something interesting is going on

Question 2

What is discontinuous variation?

Answer 2

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

Question 3

What is the purpose of a null model?

Answer 3

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

Question 4

What are the features of using punnett sqaures to estimate expected frequencies?

Answer 4

Relies upon:

• random mating between individuals
• knowing the proportion of alleles in the population

Question 5

How are allele frequencies symbolised?

Answer 5

p and q

• always add up to 1
• p+q = 1

Question 6

How are the frequency of genotypes symbolised?

Answer 6

fGG
fGg
-Gg

Question 7

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

Answer 7

In diploid population, individuals have 2 alleles

-N alleles = 2N individuals

Question 8

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

Answer 8

p= ((2N(GG) + N(Gg))/2N

q=p-1

Question 9

How many degrees of freedom are there for the hardy weinberg test?

Answer 9

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

Question 10

What do deviations from the Hardy Weinberg equilibrium suggest?

Answer 10

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

Question 11

What are the assumptions of the Hardy Weinberg equilibrium?

Answer 11

• infinite population size
• Diploid organisms
• Random mating
• No mutations
• No selection
• No population structure
• Non-overlapping generations
• Equal allele frequencies among males and females

Question 12

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

Answer 12

the allele frequencies in a population remain contant over time

Question 13

Why is the Hardy Weinberg equilibrium useful if so many unrealistic assumptions need to be fulfilled?

Answer 13

• describes the behaviour of alleles in ‘ideal’ populations
• Many populations are in hardy weinberg
• null hypothesis that can be tested

Question 14

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

Answer 14

GG = p2
Gg = 2pq
gg = q2

Question 15

What is the Hardy-Weinberg equilibrium?

Answer 15

1908

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

Question 16

What are the gentype frequences in Hardy Weinberg?

Answer 16

p1:pq:q2

Question 17

Why is the Hardy Weinberg equilibrium important?

Answer 17

• 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

Question 18

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

Answer 18

Statistics chi-squared test

Question 19

What is the formula for the chi-squared test?

Answer 19

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

Question 20

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

Answer 20

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