Lecture 8: Hardy-Weinberg Equilibrium

Question 1

What is the Hardy-Weinberg equilibrium

Answer 1

A principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors.
- Describes the expected relationship between genotype and allele frequencies

Question 2

What should the A allele frequency (p) and the B allele frequency equal?

Answer 2

p + q = 1

Question 3

What is the expected genotype frequencies for a population (given allele frequencies p and q for alleles A and B)?

Answer 3

AA (homozygote) = p^2 (p squared)
AB (heterozygote) = 2pq
BB (homozygote) = q^2 (q squared)

(This is the HW principle in a nutshell!)

Question 4

What is the HW equilibrium (mathematically)?

Answer 4

F(AA) = p^2
F(AB) = 2pq
F(BB) = q^2

The expected relationship between those genotype frequencies is:
p^2 + 2pq + q^2 = 1

Question 5

What is a shortcut we can use to figure out the HW expression for polyploid species?

Answer 5

Pascal’s triangle.
(SEE LECTURE 8 @ 25 MINS)

Question 6

How is the HW principle useful?

Answer 6

It allows us to ask “is evolution happening?” by asking if genotype frequencies are different to what we would expect under HW principle.
- Deviation from expected values indicates that evolution may be occurring

Question 7

When genotypes are said to be in Hardy-Weinberg equilibrium, what does this mean?

Answer 7

It means the observed values are the same as the expected values, thus indicates nothing interesting is happening (i.e., no evolution/selection occurring).

Question 8

How do we formally test deviation from expected values?

Answer 8

Pearson’s chi square test statistic.
(SEE LECTURE 8 @ 34:30 FOR FORMULA!!)

Question 9

What is the chi-square statistic that corresponds to the critical significance level of 0.05 (the p-value threshold)?

Answer 9

3.84 - meaning…
- If we get a chi-square statistic greater than 3.84, we reject the null hypothesis (meaning we conclude the genotype frequencies are deviating from HW equilibrium)

• If we get a chi-square statistic less than 3.84, we fail to reject the null hypothesis (meaning we conclude the genotype frequencies are in HW equilibrium, as stated in the null hypothesis)

Question 10

In order for a population to conform to HW principle, what do we assume?

Answer 10

The population should be behaving as an idealised population.

Question 11

What are the features of an idealised (Fisher-Wright) population?

Answer 11

1. No selection
2. No mutation
3. No migration
4. Infinite population size
5. Random mating

Question 12

What is meant by “population structure”?

Answer 12

A population where some kind of subdivision exists that is effecting the mating structure (i.e., the population is not behaving as one homogenous group).

Question 13

What is the Wahlund effect?

Answer 13

A characteristic lack of heterozygotes in the observed population compared to what is expected (seen in populations that exhibit population structure).