Module 13-15 Alteration of Genetic Equilibrium Flashcards
These are processes that cause a change in gene frequency magnitude and degree.
Systematic processes (Ex. Mutation, selection and migration)
These are processes that change the magnitude but not the direction
Dispersive Processes (Genetic Drift and nonpanmictic/nonrandom mating)
This is the ultimate source of genetic variation inclusive of all genetic changes.
Mutation
T/F Mutation occurs at a generally very low rate per generation.
T, 10^-5 or 10^-6 per generation in most loci of most organisms
Two types of mutation and differentiate
Non-recurrent: Rare, small chance of survival
Recurrent: Consequence of altering gene frequencies
If p0 is the initial frequency of gene A and it mutates continuously to a at a rate, and the reverse mutation does not occur, A after n generations becomes pn. How is pn calculated?
pn=po*(1-u)^n
where:
pn = freq of A after n generations
po = initial freq of A
u = rate of mutation
n = number of generations
Given initial A and a frequencies p and q, how will you calculate new frequencies with forward and backward mutations after 1 generation?
freq (A) = p1 = p + (vq) - (up)
freq (a) = q1 = q + (up) - (vq)
where:
u is the forwards rate
v is the reverse rate
How do you calculate for p and q at mutational equilibrium?
At mutational equilibrium, up=vq therefore
p = v/(u+v) and q = u/(u+v)
Using mutational equilibrium, how do you calculate the value of q after any specified generation time with both forward and backward mutation?
(u+v)n=ln((q-qe)/(qn-qe))
where:
q = initial q value
qe = q freq at mutational equilibrium
qn = q after n generations
Algebra go brrrr
Refers to fluctuations in allele frequency that occur by chance, particularly in small populations, as a result of random sampling among gametes.
Random Genetic Drift
Two Causes of Random Genetic Drift
Mendelian Segregation
Finite population size
How to calculate variance in allelic frequency among populations under random genetic drift? What about sampling error?
(sp)^2 = (pq)/2N
where:
N = total population size
p and q = allelic freqs of p and q
(sp)^2 = Variance
To get sampling error/standard dev, get square root of Variance.
The number of breeding individuals in an idealized population that would show the same amount of dispersion of allele frequencies under random genetic drift or the same amount of inbreeding as the population under consideration
Effective Population Size
Causes of genetic drift (3)
Population size reduction by environmental factors
Founder Effect
Genetic Bottleneck
Occurs when a population is established by a small number of individuals resulting in significantly less genetic diversity
Founder Effect
Occurs when a population undergoes a drastic reduction in population size. Emphasis on drastic
Genetic Bottleneck
Four Main Aspects of Random Genetic Drift
▪direction is unpredictable
▪magnitude depends on population size
▪long-term effect is to reduce genetic variation within a population
▪causes populations to diverge
Composite of the forces that limit the reproductive success of a genotype
Selection
Comparative ability of a genotype to withstand selection
Fitness
The extent to which a genotype contributes to the offspring of the next generation relative to the other genotypes in a given environment
Adaptive Value (W)
Also known as fitness or Selective Value
This is the percentage reduction in fitness.
Selection Coefficient (s)
What is the relationship between Adaptive Value (W) and selection coefficient (s)?
W=1-s or s=1-W
In calculating the frequency of genotypes before and after selection, assuming Hardy-Weinberg proportions before selection, what are the steps?
- Get frequency before selection p^2, 2pq, and q^2
- Multiply by their respective selection coefficients to get their weighted contributions
- Sum all weighted contributions to get the weighted total
- Divide the weighted contributions by the weighted total to get the frequency after selection
Calculation of Change in Frequency (Delta q) of Gene a when |s|= 1 (homozygous recessive is lethal) for Homozygous Recessive Genotype
Delta q = -q^2/(1+q)
Applicable when aa is lethal
Calculation of Change in Frequency of a (Delta q) for a Gene with Varying Degrees of Deleterious Effects (0<s<1)
Delta q = ((-sq^2)(1-q))/(1-(q^2 * s))
Applicable when 0<s<1
For summarized formulas, just check slide 11 from module 14
Calculation of Change in Frequency of A (Delta p) for a Gene with Varying Degrees of Deleterious Effects on the Dominant Allele. Let t be the selection coefficient for p
Delta p = (-tp(1-p)^2)/(1-tp(2-p))
For summarized formulas, just check slide 11 from module 14
Calculation in Change in Frequency of Gene a (Delta q) in One Generation of Selection in which the Heterozygotes have Adaptive Value of 1 and the Homozygous Selected Against by t and s, respectively.
Delta q = (pq(pt-qs))/(1-(tp^2)-(sq^2)
For summarized formulas, just check slide 11 from module 14
How to calculate for change in q (Delta q) with complete dominance (selection against a)?
Delta q = ((-sq^2)(1-q))/(1-sq^2)
Slide 15 on Module 14
How to calculate for change in q (Delta q) with complete dominance (selection favoring a)?
Delta q = ((sq^2)(1-q))/((1-s) (1-q^2))
Slide 16 on Module 14
Let A1 and A2 donate two alleles for 1 gene. How do you calculate for change in q (Delta q) with no dominance (selection against A2).
Delta q = (0.5*sq(1-q))/(1-sq)
Slide 17 on Module 14
Let A1 and A2 donate two alleles for 1 gene. How do you calculate for change in q (Delta q) with no dominance (selection favoring A2).
Delta q = (0.5*sq(1-q))/(1-s+qs)
Slide 17 on Module 14
How do you calculate for the allele frequency of q after n amount of generations assuming s=1 and there is no mutation?
qt = q/(1+tq)
Where:
q = initial q frequency
t = number of generations to change q to qt
qt = you, jk, its the final q allele frequency
Note: Only use when s=1 (Selection is against homozygous recessive, no mutation)
At equilibrium the change due to mutation will be equal and opposite to the change due to selection (Delta q mutation = - Delta q selection)
At mutation/selection equilibrium, q can be calculated how?
qe=sqrt(u/s)
Where:
u = mutation rate
s = selection coefficient.
What are the three types of selection and what do they select against/for
Stabilizing Selection - Selects against extremes, selects for intermediate phenotype
Directional Selection - Selects for a particular genotype
Disruptive Selection - Selects against the intermediate phenotype, selects for the extremes.
What are the implications of stabilizing selection
Increased fitness of intermediate phenotype favors adaptation to existing environmental conditions but reduces phenotypic and genotypic diversity
What are the implications of Disruptive selection
Increased fitness of extreme phenotypes results in higher phenotypic and genetic diversity resulting in a population with a bimodel distribution
Can be thought of as two special kinds of natural selection, competition and preference
Sexual Selection
Offers explanations for the evolution of large human brains and behaviors such as humor, music, and poetry that do not have obvious survival value
“mating-mind” hypothesis
Technical term for altruistic behavior that has been shown, or is theoretically supposed, to be explained by kin selection or self sacrifice methods
Kin Altruism
Nepotism or favor towards own kin is needed for kin-selection to occur
This is the mathematical explanation of kin selection. What is the name and formula?
Hamilton’s Rule
rB>C
Where:
r = Genetic relatedness of recipient to actor
B = Additional reproductive benefit gained by recipient
C = Reproductive cost to actor
Also known as gene flow, this is the influx of genes from other populations leading to decreased genetic divergence and increased genetic variation between and within populations
Migration
In a population of 600 natives with a q frequency of q_o=0.2, 400 immigrants with a q frequency of q_m=0.6 were assimilated. What is the frequency of q in the mixed population (q1) and the change in frequency?
Given:
400 immigrants, 600 natives
q_m = 0.6, q_o = 0.2
Required: q1 and Delta q
Solution:
Get m, the proportion of immigrants in the mixed.
Use the formula
q1 = mq_m+(1-m)q_o
m=400/(600+400)=0.4
q1 = 0.4(0.6)+(1-0.4)0.2
q1= 0.36
Delta q = q1-q_o = 0.36-0.2 = 0.16
The value of q after n generations of migration is provided by the formula
q_n - q_m = (1-m)^n (q_o - q_m)
Algebra go brrr
Percentage of alleles contributed by the migrants (or proportion of gene flow/admixture) can be calculated by this formula:
m = (q_o-q1)/(q_o-q_m)
Matings between closely related individuals that leads to an increase in the proportion of homozygotes and a decrease in the proportion of heterozygotes
Inbreeding
This is the avoidance of mating between related individuals
Outcrossing
Result from increased homozygosity for heterozygous alleles and is associated with reduced fitness and lower survival rates among the offspring
Inbreeding Depression
Measures the extent of inbreeding occurring in a population and the probability that two alleles of a given gene in an individual are derived from a common ancestral allele or are identical by descent.
Inbreeding Coefficient (F)
if 2 alleles in an inbred individual are identical by descent, the genotype at the locus is said to be (a) otherwise it is said to be (b).
(a) Autozygous
(b) Allozygous
When inbreeding occurs, the heterozygote frequency is _______ at each generation
halved
At HWE, how do you solve for the inbreeding coefficient F?
F= 1- (Ho/He)
which can be further expanded to
F=1-(Ho/2pq) = (2pq-Ho)/2pq
Genotype frequencies when inbreeding occurs can be calculated by
AA:p^2 (1-F) + pF or p^2 +pqF
Aa:2pq(1-F)
aa:q^2 (1-F) + qF or q^2 +pqF
Xeroderma pigmentosum (XP) is an often-fatal skin cancer resulting from a recessive mutant allele that affects DNA repair. In the United States, the frequency of homozygous- recessive affected people is approx. 1 in 250,000.
What is the expected frequency of XP among the offspring of first-cousin matings?
Given: q^2 = 1/250000, First Cousins
Required: f(aa)
Solution:
First Cousins: F=1/16
q^2 = 0.000004->q=0.002
p=1-0.002=0.998
f(aa) = q^2 +pqF = 0.000004 + (0.998)(0.002)(1/16)
f(aa) = 0.000129
Xeroderma pigmentosum (XP) is an often-fatal skin cancer resulting from a recessive mutant allele that affects DNA repair. In the United States, the frequency of homozygous- recessive affected people is approximately 1 in 250,000. It was found that frequency of XP among offspring of first-cousin matings was f(aa) = 0.000129
What is the ratio of XP among the offspring of first- cousin matings to that among the offspring of nonrelatives?
Given:
Inbred f(aa) = 0.000129
Outbred f(aa) = 0.000004
Required: inbred/outbred
Solution:
0.000129/0.000004 = 32
Mating pattern in which similar phenotypes mate with one another more frequently than what is expected.
Assortative Mating (Homogamy)
A_ x A_ and aa x aa
Mating Pattern where in dissimilar phenotypes mate with one another more frequently than what is to be expected
Disassortative Mating (Heterogamy)
A_ x aa
