Lecture 9 - Selection 2 and Maintenance of Genetic variation 1

Question 1

What is the formula for selection against a recessive phenotype when selection is mild?

Answer 1

Δ q = -spq^2
s = small when selection is milk
When p is small, Δq = -sp (slow)Wh
When q is small, Δq= -sq^2 (very slow)

Question 2

What is the rate of selection against a recessive allele?

Answer 2

Selection against a recessive allele is slowest when the allele is rare, slow when it is very common and fastest at intermediate allele freqencies

Question 3

What is the rate of selection against a dominant allele?

Answer 3

-there is no equilibrium of allele frequencies, the allele will disappear

Question 4

What is the tempo of evolution dependent on?

Answer 4

initial allele frequencies
dominance of the allele
strength of selection

Question 5

What are the genotype-specific fitness’ for Incomplete dominance when there is selection against ‘a’, with incomplete dominance?

Answer 5

wAA: 1
wAa: 1-s
waa: 1-s

Question 6

What is h?

Answer 6

h: degree of dominance
h = 0: A dominant, a recessive
h = 1: A recessive, a dominant
h = 0.5: additive gene action, co-dominance

Question 7

How is genetic variation maintained?

Answer 7

Through a mutation selection balance

• Interaction of two processes: mutation and selection
• Likely to be important in rare conditions that are likely to be deleterious

Question 8

What is a practical use of the mutation selection balance?

Answer 8

Estimate mutation rates in human populations

Question 9

Give 3 examples of an Autosomal dominant disease and their frequency

Answer 9

Achondroplasia (5 X 10^-5)
Retinoblastoma (5 X 10^-5)
Huntington's chorea (5 X 10^-4)
Lower frequency than recessive diseases
-selection acts more often

Question 10

Give 4 examples of an Autosomal dominant disease and their frequency

Answer 10

Albinism (3 X 10^-3)
Phenylketonuria (7 X 10^-3)
Tay-Sachs syndrom (1 X 10^-3)
Xeroderma pigmentosum (2 X10^-3)
Higher frequency than dominant disorders

Question 11

What are the symbols: μ, p, q and s in the mutation selection balance for recessive alleles?

Answer 11

μ: mutation rate from A to a

p: frequency of the dominant allele A
q: frequency of the recessive allele a
s: selection coefficient against the recessive genotype

Question 12

What are the features of p, q and the change in q in the mutation selection balance for recessive alleles?

Answer 12

q: very small
p: very close to one
Therefore change in q = -spq^2 = -sq^2

Question 13

What is the equilibrium frequency of a in the mutation selection balance for recessive alleles?

Answer 13

a: qhat = SQRT(μ/s)

Question 14

What is the Equilibrium frequency of A in the mutation selection balance for Dominant alleles?

Answer 14

q: frequency of the dominant allele
μ: mutation rate from a to A
s: selection coefficient against the dominant phenotype
qhat = μ/s

Question 15

What is the equilibrium frequency of A in the mutation selection balance for partially dominant alleles?

Answer 15

μ: mutation rate from the beneficial to the deleterious allele
A: qhat = μ/hs (h: degree of dominance)

Question 16

What is the difference in equilibrium frequencies of the mutation selection balance for Recessive and Dominant?

Answer 16

Recessive equilibrium is higher than the dominant equilibrium
When selection is less strong, the equilibrium of q is higher

Question 17

What is Huntington’s disease?

Answer 17

• a neuromusclar disease

- only partially dominant

Question 18

What are the fitness estimates for Huntingtons disease and how do you calculate the mutation rate?

Answer 18

waa = 1
wAa = 1 - hs = 0.81
wAA genotypes are too rare
allele frequency => q = 5 X 10^-5
Mutation rate:
9.5 X 10^-6
μ = allelefrequency*hs = 5 X 10^ -5 X 0.19 = 95 X 10^-6

Relatively high mutation rate or other factors maintaining in population

Question 19

How can selection maintain genetic variation?

Answer 19

Balancing selection - selection can lead to stable allele frequencies
Through heterozygote advantage

Question 20

What is heterozygote advantage?

Answer 20

assumes heterozygotes have higher fitness than homoxygotes

Question 21

Give an example of heterozygote advantage

Answer 21

Warfarin resitance in rats
-Warfarin reduces coagulation and leads to blood thinning and internal bleeding
Vitamin K requiements(μ/100g bodyweight/day)
S = susceptible allele R = resistance allele
SS: Male (0.05), Female (

Question 22

What are the selection coefficients for heterozygous advantage?

Answer 22

wAA: 1-s
wAa: 1
waa: 1-s1(t)

s1 = may be different levels of selection advantage/dis

Question 23

How do you calculate the equilibrium allele frequencies for heterozygote advantage?

Answer 23

At equilibrium Δq = 0

equilibrium of q = s/(s+t)

Question 24

How does directional selection act?

Answer 24

Eliminates alleles from a population and is most efficient at intermediate allele frequencies

Question 25

How is genetic variation maintained?

Answer 25

• mutation selection balance for rare deleterious alleles

- balancing selection: homozygote advantage - maintain both alleles at substantial quantities in the env