Population Genetics 2/3

Question 1

diploid (2)

Answer 1

• when an organism has two copies of each gene

- the vast majority of multi-cellular plants and animals are diploid during most of their life cycle

Question 2

what is an important characteristic of haploid models that differ from diploid models (2)

Answer 2

• haploid models “breed true” whether they produce sexually or asexually (A-bearing parent -> A-bearing offspring)
• only diploids that reproduce asexually breed true (Aa-bearing parents -> Aa-bearing offspring); with sexual reproduction, diploid models do not breed true and will produce a variety of offspring (Aa-bearing parents -> AA/Aa/aa-bearing offspring)

Question 3

what are the characteristics of diploid selection in asexuals (4)

• stages
• stage for natural selection
• frequencies tracked
• fitness values tracked

Answer 3

• two stages: diploid and asexual reproduction
• natural selection acts during the diploid stage
• track 3 frequencies: xAA + xAa + xaa = 1
• track 3 fitness values: W(AA), W(Aa), W(aa)

Question 4

mean fitness for diploid selection in asexuals (3)

• symbol
• formula
• change over time

Answer 4

• symbol: Wbar[t]
• W(AA)xAA[t] + W(Aa)xAa[t] + W(aa)*xaa[t]
• in asexual populations, the mean fitness Wbar, increases over time (or stays the same)

Question 5

how does the frequency of alleles change in diploid selection of asexuals if there is a heterozygous advantage for the allele?

Answer 5

• the population will experience a dramatic change in allele frequencies where Aa will increase greatly and the other alleles will decrease

Question 6

how does the frequency of alleles change in diploid selection of sexuals if there is a heterozygous advantage for the allele?

Answer 6

• the change due to selection is the same as in asexuals, but meiosis breaks apart and reassorts the diploid genotypes
• Key Point: sex (segregation) can undo genetic associations built by selection

Question 7

what are the two processes we need to model for diploid sexuals? (2)

Answer 7

• meiosis segregating alleles to create haploid gametes

- gamete union bringing alleles back together in diploids

Question 8

meiosis segregates alleles to create haploid gametes model

Answer 8

• AA, Aa and aa frequencies are divided into the total frequencies of either A or a alleles

Question 9

gamete union brings alleles back together in diploids

Answer 9

• A and a allele frequencies in both egg and sperm meet to create AA, Aa, and aa frequencies

Question 10

probability tree diagrams (3)

Answer 10

• help calculate chance of different events
• all options from one node sum to one
• multiply all probabilities along a path from start to finish to calculate probability of any one path

Question 11

frequency of A gamete in diploid life cycle after meiosis

Answer 11

p = xAA + (1/2)*xAa

Question 12

frequency of a gamete in diploid life cycle after meiosis

Answer 12

q = (1/2)*xAa + xaa

Question 13

what is the frequency of the AA diploid after gamete union assuming random mating among gametes

Answer 13

xAA = p^2

Question 14

what is the frequency of the Aa diploid after gamete union assuming random mating among gametes

Answer 14

2pq

Question 15

what is the frequency of the aa diploid after gamete union assuming random mating among gametes

Answer 15

q^2

Question 16

Hardy-Weinberg proportions (2)

Answer 16

p^2 + 2pq + q^2
- occurs during random rating and no selection: allele frequencies do not change after each generation and diploids are immediately in Hardy-Weinberg proportions

Question 17

what are the assumptions are the Hardy-Weinberg equation (4)

Answer 17

1. random combination of gametes from the gamete pool
2. no differences in fitness among genotypes
3. a very large populations (no chance effects changing genotype frequencies at any step)
4. no mutations or migration altering in population

Question 18

how do we apply the Hardy-Weinberg proportions?

Answer 18

• if populations of adults are not at Hardy-Weinberg proportions, it indicates that one of our assumptions is violated (there may be selection or non-random mating, etc)

Question 19

diploid sexuals with selection

Answer 19

• with random mating and selection, allele frequencies do change
• while diploids are at Hardy-Weinberg proportions at birth, they may not be after selection

Question 20

what happens to the allele A frequency if W(AA) > W(Aa) > W(aa) during long term natural selection (2)

Answer 20

p[t] -> 1

- “directional selection” favouring A

Question 21

what happens to the allele A frequency if W(AA) < W(Aa) < W(aa) during long term natural selection (2)

Answer 21

p[t] -> 0

- “directional selection” favouring a

Question 22

what happens to the allele A frequency if W(AA) < W(Aa) > W(aa) during long term natural selection (3)

Answer 22

p[t] -> phat

• “heterozygote advantage” or “overdominance” where population fixes on certain intermediate frequency value; polymorphism maintained
• stable equilibrium

Question 23

what happens to the allele A frequency if W(AA) > W(Aa) < W(aa) during long term natural selection (3)

Answer 23

p[t] -> 0 or 1; depending on starting condition (below, above or at phat)

• “heterozygote disadvantage” or “underdominance”
• unstable equilibrium

Question 24

equilibrium (3)

Answer 24

• a point of a system that when started at that point, the system no longer changes
• denoted with a caret on top (hat)
• may be stable (points nearby approach the equilibrium) or unstable (points nearby are repelled away from the equilibrium)

Question 25

when considering the spread of a new beneficial allele (A) in a population of wildtype alleles (a) for a sexual diploid population, how do we measure fitness?

Answer 25

- we typically measure fitness relative to the wildtype W(aa) = 1 W(Aa) = 1 + h*s W(AA) = 1 + s

Question 26

selection coefficient (2)

Answer 26

- symbol: s | - measures the fitness of one homozygote (AA) relative to the other (aa)

Question 27

dominance coefficient (2)

Answer 27

- symbol: h | - measures how dominant A is with respect to fitness

Question 28

for diploid organisms, do we use the absolute or relative values of fitness to determine allele frequencies?

Answer 28

- relative fitness values

Question 29

how do different values of dominance coefficient (5)

Answer 29

- A is recessive to a when h = 0 - A is partially recessive when 0 < h < 0.5 - A is additive with a when h = 0.5 - A is partially dominant when 0.5 < h < 1 - A is dominant to a when h = 1

Question 30

what occurs when h=large

Answer 30

- allele A is more dominant | - the frequency of allele A rises faster at first, but slows down when it becomes more frequent

Question 31

how does the value of s affect the allele frequency change

Answer 31

- if s is 10x smaller, it takes 10x longer to observe the same amount of frequency change

Question 32

notes on dominance (3)

Answer 32

- dominance is NOT a characteristic of an allele, but reflects the interaction between two alleles - allele A might be dominant with respect to a, but recessive with respect to another allele a' - dominance depends on the phenotype being measured

Question 33

sickle-cell anemia (4) - definition - cause

Answer 33

- human disease affecting the shape and flexibility of RBCs; mutant form of hemoglobin (S) tends to crystallize and form chains, causing distortions in the RBCs - caused by mutation in the sixth amino acid of the chain of hemoglobin - where malaria is common, adults are more often heterozygous than predicted by the HW proportions - due to the heterozygote advantage of sickle-cell anemia, the frequency of the A allele will settle at a certain phat

Question 34

sickle-cell anemia and diploid allele combinations (3)

Answer 34

- homozygous SS experience the greatest degree of sickling and tend to suffer severe anemic attack - heterozygous AS also suffer from sickling of RBCs, but to a lesser degree - homozygous AA do not experience sickle-cell anemia

Question 35

malaria and diploid allele combinations (3)

Answer 35

- heterozygous AS are less likely to die from malaria as RBCs infected with parasites causing malaria tend to sickle and be destroyed - homozygous SS do not die from malaria due to the sickling of the cells - homozygous AA are likely to die from malaria

Question 36

mean fitness in diploid sexuals

Answer 36

- mean fitness always increases (or stays the same) | - deltaWbar > 0