Flashcards in Lecture 9 - Selection 2 and Maintenance of Genetic variation 1 Deck (25)
What is the formula for selection against a recessive phenotype when selection is mild?
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)
What is the rate of selection against a recessive allele?
Selection against a recessive allele is slowest when the allele is rare, slow when it is very common and fastest at intermediate allele freqencies
What is the rate of selection against a dominant allele?
-there is no equilibrium of allele frequencies, the allele will disappear
What is the tempo of evolution dependent on?
initial allele frequencies
dominance of the allele
strength of selection
What are the genotype-specific fitness' for Incomplete dominance when there is selection against 'a', with incomplete dominance?
What is h?
h: degree of dominance
h = 0: A dominant, a recessive
h = 1: A recessive, a dominant
h = 0.5: additive gene action, co-dominance
How is genetic variation maintained?
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
What is a practical use of the mutation selection balance?
Estimate mutation rates in human populations
Give 3 examples of an Autosomal dominant disease and their frequency
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
Give 4 examples of an Autosomal dominant disease and their frequency
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
What are the symbols: μ, p, q and s in the mutation selection balance for recessive alleles?
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
What are the features of p, q and the change in q in the mutation selection balance for recessive alleles?
q: very small
p: very close to one
Therefore change in q = -spq^2 = -sq^2
What is the equilibrium frequency of a in the mutation selection balance for recessive alleles?
a: qhat = SQRT(μ/s)
What is the Equilibrium frequency of A in the mutation selection balance for Dominant alleles?
q: frequency of the dominant allele
μ: mutation rate from a to A
s: selection coefficient against the dominant phenotype
qhat = μ/s
What is the equilibrium frequency of A in the mutation selection balance for partially dominant alleles?
μ: mutation rate from the beneficial to the deleterious allele
A: qhat = μ/hs (h: degree of dominance)
What is the difference in equilibrium frequencies of the mutation selection balance for Recessive and Dominant?
Recessive equilibrium is higher than the dominant equilibrium
When selection is less strong, the equilibrium of q is higher
What is Huntington's disease?
-a neuromusclar disease
-only partially dominant
What are the fitness estimates for Huntingtons disease and how do you calculate the mutation rate?
waa = 1
wAa = 1 - hs = 0.81
wAA genotypes are too rare
allele frequency => q = 5 X 10^-5
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
How can selection maintain genetic variation?
Balancing selection - selection can lead to stable allele frequencies
Through heterozygote advantage
What is heterozygote advantage?
assumes heterozygotes have higher fitness than homoxygotes
Give an example of heterozygote advantage
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 (
What are the selection coefficients for heterozygous advantage?
s1 = may be different levels of selection advantage/dis
How do you calculate the equilibrium allele frequencies for heterozygote advantage?
At equilibrium Δq = 0
equilibrium of q = s/(s+t)
How does directional selection act?
Eliminates alleles from a population and is most efficient at intermediate allele frequencies