EXAM 2 FLASHCARDS
What is meant when we say the Hardy-Weinberg model serves as a null model?
It is the null hypothesis for population genetics for what happens to alleles/genotypes frequencies when drivers of evolutionary change (disturbances) ARE NOT present in a population
- No evolutionary change occurring
Define evolution from a population genetics perspective.
Change in allele/genotype frequencies over time. Population genetics is interested in evolution within populations.
What are the assumptions that must be met in order for the Hardy-Weinberg model to predict allele and genotype frequencies?
- NO SELECTION (no selective advantage for being homozygous dominant or heterozygous)
- NO MUTATION
- NO MIGRATION (b/c if individuals move in or out of a pop. it could remove or introduce genetic diversity -> evolution WE DON’T WANT FOR H-W)
- LARGE POPULATIONS (if small then genetic drift or random chance will and can alter the way populations change WE DON’T WANT)
- RANDOM MATING (no individuals are more likely to mate with each other than with others there is an equal chance of mating with everyone else)
If a population is not in Hardy-Weinberg equilibrium, what can be inferred? Why?
If NOT in H-W equilibrium it can be inferred that the population is evolving because the Hardy-Weinberg model provides a baseline (null) model for comparison which suggests no evolution is occurring
what is the probability rule for “AA” individuals and “aa” individuals
If event 1 and event 2 are independent events, then the probability of both occurring given by product of their probabilities
AA: probability of sperm with “A” and probability of egg with “A”. this indicates p*p = p2 which represents homozygous dominant genotype
aa: probability of sperm with “a” and probability of egg with “a”. this indicates q*q = q2 which represent homozygous recessive
what is the probability rule for “Aa” individuals
If event 1 and event 2 are mutually exclusive events, then the probability that either event 1 or event 2 occurs is given by the sum of their probabilities (two ways to get Aa: sperm with “A” + egg with “a” OR sperm with “a” + egg with “A”
Aa: probability sperm with “A” * probability of egg with “a” + probability of sperm with “a” * probability of egg with “A”. This indicates pq + qp = 2pq which represents heterozygous genotypes
Imagine a locus that has two possible alleles, B and b. In a hypothetical population the frequency of B is 0.1 and the frequency of b is 0.9 What are the expected genotypic frequencies if the population is in H-W equilibrium? First use the H-W equation to calculate. Now try using a modified Punnett square to visualize.
Allele frequency: p + q = 1 (0.1) + (0.9) = 1
Genotypic frequency: p2 + 2pq + q2 = 1
p2 = (0.1)^2 = 0.01 = BB homo dom
2pq = 2(0.1)(0.9) = 0.18 = Bb hetero
q2 = (0.9)^2 = 0.81 = bb homo recess
what is the equation for finding allele frequency? what do the letter indicate?
p + q = 1
p = dominant allele
q = recessive allele
what is the equation for genotypic frequency? what does each letter represent?
p2 + 2pq + q2 = 1
p2 = homozygous dominant genotype
2pq = homozygous genotype
q2 = homozygous recessive genotype
what are the equations to use to find allele frequency when genotype frequency is present?
p = f[AA] + 1/2 f[Aa]
- homozygous dom frequency and heterozygous
q = f[aa] + 1/2 f[Aa]
- homozygous recess and heterozygous
what is a population
group of individuals of the same species at the same time and in the same place. Those in sexually reproducing population that have the potential to interbreed
what is population genetics
looking at how genotype frequencies in a population related to genotype frequencies in parental population
what is allele frequency
The relative frequency of an allele at a particular locus in a population, expressed as a fraction or percentage
- there are two alleles “A” and “a”
- each allele has a frequency that = 1
- the two alleles are called “p” and “q”
what is genotypic frequency
The relative frequency of a genotype at a particular locus in a population, expressed as a fraction or percentage
- three possible phenotypes “AA”, “Aa”, “aa” (homo dom, hetero, homo recess)
- each genotype frequency = 1 (like allele frequency)
what is a mutation
change in the DNA sequence that is the primary source for genetic variation (NEW ALLELES ARE INTRODUCED INTO THE POPULATION)
what is an allele
alternative form of a gene in diploid individuals. Cells have 2 for each. When working with genetic problems we often designate alleles by single letter (EX: A or a)
what is a genotype
the set of alleles that an individual possess for that gene. Diploid genotype for particular gene typically written as a series of 2 letters (EX: AA, Aa, or aa)
Mendel’s seminal work on pea plants provided what insights into the nature of inheritance? Check all that apply.
A.DNA is the molecule of inheritance
B. Law of Segregation
C. The units of inheritance for two traits get sorted into gametes independently of one another
D. Central dogma
E. He refuted the idea of blending inheritance
F. Concept of recessive and dominant traits
B. Law of Segregation
C. The units of inheritance for two traits get sorted into gametes independently of one another
E. He refuted the idea of blending inheritance
F. Concept of recessive and dominant traits
What do we mean when we say that the Hardy–Weinberg model provides a “null model” for population genetics?
A. It models a population in which the mechanisms of inheritance reduce variation from one generation to the next until only one allele remains in the population
B. It models a population in which drivers of evolutionary change are not acting
C. It models a population that does not obey Mendel’s laws of inheritance
D. It models a population in which evolutionary change is occurring due to natural selection
B. It models a population in which drivers of evolutionary change are not acting
T/F: The Hardy-Weinberg equations can be used to predict genotype frequencies in any population.
FALSE (population has to be under H-W assumptions)
When H-W assumptions are met in a population, allele and genotype frequencies will
A. Stay the same from generation to generation
B. Change over time
A. Stay the same from generation to generation
Which of the following is an assumption of Hardy-Weinberg?
A. Population reproduces asexually
B. There is no mutation or migration
C. Population size is small
D. There is assortative mating with respect to the genetic locus in question
B. There is no mutation or migration
The relative frequency of an allele at a particular locus in a population, expressed as a fraction or percentage is a great definition of
A. Allele frequency
B. Genotypic frequency
C. Population frequency
A. Allele frequency
T/F: Genotype frequencies, like allele frequencies, in a population should sum to 1.
TRUE
In a population of turtles, the allele for ninja skills (S) is dominant over the allele for normal turtle skills (s). If the allele frequency of “S” in the population is 0.2, what is the probability that an egg cell from this population will have a “s” allele?
A. 0.4
B. 0.16
C. 0.8
D. 0.2
C. 0.8
0.8 + 0.2 = 1
In a population of cats, the allele frequency of a normal ZRS is 0.6 and the frequency of the mutated ZRS is 0.4. Assuming Hardy-Weinberg equilibrium assumptions are met, what is the frequency of genotypes that are homozygous for the mutation?
A. 0.4
B. 0.6
C. 0.36
D. 0.16
D. 0.16
q2 = (0.4)^2 = 0.16
Which of the following is true of a population that is in Hardy-Weinberg equilibrium for a particular locus?
A. For a locus with two alleles of frequencies p and q, p=q=0.5 at equilibrium
B. Allele frequencies change from one generation to the next, but these changes have no effect on genotype frequencies
C. For a locus with two alleles of frequencies p and q, heterozygous genotypes are expected to occur at frequency of 2pq
D. Genotype frequencies at the locus will change from one generation to the next
C. For a locus with two alleles of frequencies p and q, heterozygous genotypes are expected to occur at frequency of 2pq
T/F: If you know the genotype frequencies in a population, you can ALWAYS calculate the allele frequencies.
TRUE (you use the equations p = f[AA] + ½ f[Aa] and q = f[aa] + ½ [Aa] )
what are the misconceptions of Mendel’s Work
- dominant alleles would become more numerous over time and ultimately recessive alleles would disappear (NOT TRUE; selection chooses traits that are best for survival/reproduction whether that be dominant or recessive trait)
- dominant phenotypes should comprise 75% (3/4) of population
what is important for the year 1908
G.H. Hardy and Wilhelm Weinberg independently developed model that refuted both misconceptions that lent more credence to Mendel’s work
what are four key points of the Hardy-Weinberg model
- states that, under certain assumptions, genetic variation in a population will remain constant over time when NO evolutionary forces are present
- allows prediction of allele and genotype frequencies in a population under those assumptions
- provides baseline (null) model for comparison (no evolution)
- good starting point for understanding population genetics
what is transmission genetics
looks at how genotype of individual offspring related to genotypes of parents (individual level)
population genetics is interested in what?
the evolution WITHIN POPULATIONS
could the population that the Hardy Weinberg model wants ever exist?
NO the model is just a null hypothesis
what are 2 real world examples of pool of gametes presented in the lecture
- Barrel sponge tossing out gametes with the intent to combine with other organism’s gametes
- In pollinated plants the wind knocks out pollen into the air hoping to land on a receptive plant
if the Hardy-Weinberg assumptions are met (evolution NOT occurring) what can we calculate?
genotypic frequencies given known allele frequencies in parent population using Hardy Weinberg equations
T/F: genotypes vary depending on allele frequency
TRUE
Within a population of butterflies, color is controlled by a single gene. The allele for the color brown (B) is dominant over the allele for the color white (b). If 4% of the butterflies are white, and we assume that population is in H-W equilibrium, calculate
a) allele frequencies for brown and white alleles (p and q, respectively)
b) genotype frequencies of individuals that are homozygous dominant, heterozygous, and homozygous recessive when population is in H-W equilibrium.
a) p = 0.8 and q = 0.2
- square root of 0.04 = 0.2 = q. 1 - 0.2 = 0.8 = p
b) p2 + 2pq + q2
- p2 = (0.8)^2 = 0.64 = BB
- 2pq = 2(0.8)(0.2) = 0.32 Bb
- q2 = 0.04 (given to us in the question as 4%)
what happens if a population’s allele and genotype frequencies different from those predicted by HW model?
this means that one or more of the assumptions has been violated indicating evolutionary forces are acting upon population
what are the 4 ways to test whether a population is at or near Hardy Weinberg equilibrium
- use observed genotype frequencies to calculate the observed allele frequencies
- calculate expected H-W genotype frequencies based on observed allele frequencies
- compare observed to expected genotype frequencies. Are they the same or different (SAME = EQUILIB; DIFF = NOT EQUILIB)
- statistical test to evaluate difference = chi square test
what is a gene pool (“pool of gametes”)
Under the Hardy-Weinberg assumptions, we can calculate the frequencies of each genotype by imaging that all parents contribute their gametes to a hypothetical gene pool from which gametes are paired at random to produce offspring.
what does it mean when there is H-W equilibrium
when genotypic and allele frequencies remain constant when there is no evolution occurring in a population
- When the observed and expected genotypic frequencies match
- HW equilibrium: given a set of allele frequencies, the expected set of genotype frequencies will be observed under the HW model.
What are factors that can cause allele and/or genotype frequencies to differ from those predicted by Hardy-Weinberg equilibrium?
Mutation: random change in an organism’s DNA
AND
Selection: alleles and genotypes that carry a fitness advantage will become more common in a population over time when natural selection is operating
what are the different types of selection?
- frequency-independent selection
- directional
- stabilizing
- disruptive
- overdominance
- underdominance - frequency-dependent selection
- positive frequency-dependent
- negative frequency-dependent
what is mutation-selection balance and when does it occur
When deleterious alleles (with lower fitness) remain at low, steady numbers in a population (equilibrium)
OCCURS WHEN:
rate at which deleterious alleles are created by mutation = rate at which deleterious alleles are eliminated by selection
what are 3 key points about mutation-selection balance
- allele frequencies remain constant
- mutation and selection operate at the same times
- One of the reasons why we still see deleterious alleles remaining in a population
what is an example of mutation-selection balance in humans
FAMILIAL ADENOMATOUS POLYPOSIS (FAP): inherited condition which causes polyps to form in large intestine (can become cancerous)
what are key points for FAP
- Commonly caused by autosomal dominant mutation in APC tumor suppressor gene (selection selects AGAINST these)
- Although mutation rate is high, selection keeps removing alleles from population (selecting against mutation because they are passing away before being able to pass along offspring)
- caused by nonsense mutation (results in premature stop codon)
How can mutation affect allele and genotype frequencies?
Introduces new alleles into a population where natural selection will select FOR or AGAINST random mutations which can lead to a great change in population = evolution
What is meant when we say that mutation is the raw material for evolution?
It does NOT drive evolution into a certain direction or another but it instead generates the genetic diversity in order for natural selection to select FOR or AGAINST mutations that will eventually become more or less common in a population which then leads to evolution.
What is expected to happen to allele frequencies under directional selection
allele frequencies are driven in single direction
- Favored allele becomes “fixed” in a population
what occurs to A1 during directional selection
- A1 reaches fixation most rapidly when it is incompletely dominant (heterozygous) with A2
- A1 recessive = longer to increase in frequency but once common it goes to fixation quickly
- A1 dominant = initial increase frequency most rapid but slows down once common
What is expected to happen to allele frequencies under overdominance
balanced polymorphism will occur = stable equilibrium when BOTH alleles present
what occurs to A1 during overdominance
- A1 start high freq. = freq. will decline
- A1 start low freq. = freq increases
- A1 eventually reaches intermediate frequency that does NOT depend on the initial frequencies as long as BOTH ALLELES are present in population
What is expected to happen to allele frequencies under underdominance
over time, trajectory for one allele to become fixed (depends on starting pt)
- allele start at high freq. = will be fixed
- allele start at low freq. = lost in pop.
how are positive and negative-frequency dependent selection different
Positive Frequency-Dependent Selection:
- Mechanism: The fitness of a phenotype
increases as it becomes more common.
- Effect on Genetic Variance: It tends to decrease genetic variance because common phenotypes are favored and rare ones are selected against.
Negative Frequency-Dependent Selection:
- Mechanism: The fitness of a phenotype
decreases as it becomes more common
(more rare = better)
- Effect on Genetic Variance: It increases genetic variance by favoring rare phenotypes, which helps maintain multiple phenotypes in the population. - Example: Pathogen resistance in plants, where rare resistance alleles are favored because pathogens are less likely to have adapted to them.
what was the example discussed in the lecture for directional selection
Bistun betularia
1. Prior to the industrial revolution light (“typica”) morph was very common. As the revolution picked up there was environmental pollution which created soot in the environment leading to a carbonaria morph
- In 1848: “unusual” black morph seen in Manchester, England
- By end of century, the carbonaria morph outnumbered the “typica” (light) by 90% in some regions
- Selection was favoring dark colored morph because selection pressure was bird predation and dark moths were more camouflage
- Around the 1970’s rules were imposed on how much pollution you could dump into the environment leading to a change in frequency in peppered moth morphs (decline freq. in carbonaria and increase freq. back to typica)
what were the 2 examples discussed in the lecture for stabilizing selection
- Clutch Size in Robins
- Robins typically lay four eggs because there are tradeoffs with having more or less eggs. Larger clutches may result in malnourished chicks (disadvantage) while smaller clutches may result in no viable offspring (fewer genes are passed on) - Human Birth Weight
- Birth weight follows a normal distribution, that mortality for newborns is greater for those either under- or overweight, and that the mean birth weight (7 lbs) coincides with that showing minimum mortality
what was the examples discussed in the lecture for disruptive selection
MALE PHENOTYPES IN SALMON - SNEAKERS AND FIGHTERS
- threshold in male size in which female proximity is best gained by fighting, and by sneaking.
- High proximity to females in both small fish (sneaker) and large fish (fighter). Intermediates not good at fighting or sneaking = no advantage
what is the strategy for fighters and sneakers in male salmon (example: disruptive selection)
Fighters = fight off males to gain access to females (develops kype and hump on back and nose = bigger salmon).
Sneakers = does NOT develop kype or hunch on back = looks like female and goes undetected by fighter males because it knows it will lose battle (sneaks by fighters looking like female to mate with females)
what was the example discussed in the lecture for overdominance
Sickle Cell Anemia
- In malaria prone regions, the individuals which are heterozygous for this trait (one normal hemoglobin and one sickle cell) have survival advantage (more resistant to malaria) over either homozygotes.
what was the example discussed in the lecture for underdominance
Pseudacraea eurytus butterflies
- Homozygotes have different phenotypes, each mimicking different toxic butterflies. The heterozygote has an intermediate phenotype and experiences increased predation
what was the example discussed in the lecture for positive frequency-dependent selection
WARNING COLORATION IN BUTTERFLIES
- When there was a high frequency of warning coloration at a site, the lower the predation was for that particular phenotype
what is the example discussed in the lecture for negative frequency-dependent selection
SIDE-BLOTCHED LIZARDS
- Male lizards are found in 3 forms: orange throated/blue/yellow and the most rare phenotype (correlates to behavior strategy) is favored by females in each generation
what were the strategies for the 3 side blotched lizards and what colors beat out each other? (example for negative frequency-dependent)
Blue = monogamous: put all focus on 1 female and aren’t competitive with other blues but instead cooperate with them, would die for them, and warn blue neighbors
Orange = bigger/stronger and set up territories with lots of female and defend them aggressively
Yellow = sneaky and hide and dart into site as often as they can for a chance to mate with unguarded female
ORANGE BEATS BLUE: BIGGER AND AGGRESSIVE
BLUE BEATS YELLOW: BLUE IS VIGILANT YOU CANT FOOL THEM
YELLOW BEATS ORANGE: ORANGE CANT KEEP TRACK OF ALL THOSE FEMALES SO YELLOW CAN EASILY SNEAK BY
how did selection drive the evolution of rock pocket mice populations?
Researchers found light colored mice are more common in desert sands and dark mice are more common on lava flow substrate
Depending on the environment of the mouse, the fitness of the phenotype will be better or worse and selection will make that phenotype more common
- Light = fit in light (desert sand substrates)
- Dark = fit in dark lava rock substrate
why did dark mice not have dark underbelly?
dark mice with white underbelly because selection didn’t favor dark underbelly because predators come from above
what is a mutation
random change in an organism’s DNA (copy errors in the DNA and can be sometimes useful and sometimes not)
what is fitness
the expected reproductive success of individual who has trait/allele relative to other members of population (measured by the number of offspring an individual has)
what is directional selection
one extreme phenotype is favored over all others (allele frequencies are driven in a single direction)
what is stabilizing selection
intermediate phenotypes are more fit than extreme ones (can lead to LESS phenotypic and genotypic diversity over time)
what is disruptive (diversifying) selection
two or more extreme phenotypes are more fit than intermediate phenotypes. Increased diversity and could lead to such strong differences that new species form (if environment is stable and there is very strong selection)
what is overdominance
heterozygote advantage; heterozygote has a higher fitness than either homozygotes
what is underdominance
heterozygous disadvantage; heterozygote has lower fitness than either homozygote genotype.
what is allele fixation
occurs when one allele replaces ALL other alternative alleles at same locus (directional selection)
what is balanced polymorphism
population will reach a stable equilibrium where BOTH ALLELES are present
