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BIOL 336 > Population Genetics 2/3 > Flashcards

Population Genetics 2/3 Flashcards

1
Q

diploid (2)

A
  • 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

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2
Q

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

A
  • 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)
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3
Q

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

  • stages
  • stage for natural selection
  • frequencies tracked
  • fitness values tracked
A
  • 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)
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4
Q

mean fitness for diploid selection in asexuals (3)

  • symbol
  • formula
  • change over time
A
  • 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)
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5
Q

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

A
  • the population will experience a dramatic change in allele frequencies where Aa will increase greatly and the other alleles will decrease
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6
Q

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

A
  • 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
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7
Q

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

A
  • meiosis segregating alleles to create haploid gametes

- gamete union bringing alleles back together in diploids

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8
Q

meiosis segregates alleles to create haploid gametes model

A
  • AA, Aa and aa frequencies are divided into the total frequencies of either A or a alleles
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9
Q

gamete union brings alleles back together in diploids

A
  • A and a allele frequencies in both egg and sperm meet to create AA, Aa, and aa frequencies
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10
Q

probability tree diagrams (3)

A
  • 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
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11
Q

frequency of A gamete in diploid life cycle after meiosis

A

p = xAA + (1/2)*xAa

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12
Q

frequency of a gamete in diploid life cycle after meiosis

A

q = (1/2)*xAa + xaa

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13
Q

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

A

xAA = p^2

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14
Q

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

A

2pq

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15
Q

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

A

q^2

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16
Q

Hardy-Weinberg proportions (2)

A

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

17
Q

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

A
  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
18
Q

how do we apply the Hardy-Weinberg proportions?

A
  • 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)
19
Q

diploid sexuals with selection

A
  • with random mating and selection, allele frequencies do change
  • while diploids are at Hardy-Weinberg proportions at birth, they may not be after selection
20
Q

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

A

p[t] -> 1

- “directional selection” favouring A

21
Q

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

A

p[t] -> 0

- “directional selection” favouring a

22
Q

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

A

p[t] -> phat

  • “heterozygote advantage” or “overdominance” where population fixes on certain intermediate frequency value; polymorphism maintained
  • stable equilibrium
23
Q

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

A

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

  • “heterozygote disadvantage” or “underdominance”
  • unstable equilibrium
24
Q

equilibrium (3)

A
  • 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)
25
Q

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?

A
  • we typically measure fitness relative to the wildtype
    W(aa) = 1
    W(Aa) = 1 + h*s
    W(AA) = 1 + s
26
Q

selection coefficient (2)

A
  • symbol: s

- measures the fitness of one homozygote (AA) relative to the other (aa)

27
Q

dominance coefficient (2)

A
  • symbol: h

- measures how dominant A is with respect to fitness

28
Q

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

A
  • relative fitness values
29
Q

how do different values of dominance coefficient (5)

A
  • 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
30
Q

what occurs when h=large

A
  • allele A is more dominant

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

31
Q

how does the value of s affect the allele frequency change

A
  • if s is 10x smaller, it takes 10x longer to observe the same amount of frequency change
32
Q

notes on dominance (3)

A
  • 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
33
Q

sickle-cell anemia (4)

  • definition
  • cause
A
  • 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
34
Q

sickle-cell anemia and diploid allele combinations (3)

A
  • 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
35
Q

malaria and diploid allele combinations (3)

A
  • 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
36
Q

mean fitness in diploid sexuals

A
  • mean fitness always increases (or stays the same)

- deltaWbar > 0

BIOL 336 (13 decks)

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