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BMS2042 > WEEK 12: HUMAN EVOLUTIONARY GENETICS > Flashcards

WEEK 12: HUMAN EVOLUTIONARY GENETICS Flashcards

1
Q

Does human evolution have a direct impact on disease.?

A

-yes

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2
Q

Can we use models to stimulate the outcomes of evolutionary processes?

A

-YES

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3
Q

What does fixation mean in a population?

A
  • 100% frequency allele
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4
Q

What does a loss in the population mean?

A
  • 100% lost allele
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5
Q

Is TIME to fixation dependent on the population size?

A
  • YES

- Genetic drift works much faster in small populations than large populations

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6
Q

Does genetic drift work much faster in small or large populations?-

A
  • SMALL
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7
Q

Do neutral alleles have to eventually become fixed or lost?

A
  • YES
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8
Q

What is the probability of fixation of a neutral INDEPENDENT on?

A
  • Population size
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9
Q

What is the probability of fixation of a neutral allele equal to?

A
  • Its current frequency
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10
Q

How do new neutral alleles arise?

A

-De Novo mutations.

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11
Q

Are some genetic variants more likely to be neutral than others?

A
  • YES
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12
Q

What can we assume most 4-fold redundant sites are?

A
  • Selective neutral
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13
Q

What is required for new variation in the population?

A
  • De novo (spontaneous) mutations
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14
Q

Does the rate at which we get new mutations into the population vary across each species?

A
  • YES
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15
Q

Why is a selection not effective in small populations?

A
  • Because the sampling error is very large due to randomness and this is stronger than selection.
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16
Q

Is there any recombination between lineages for cancer?

A
  • NO

- Because it is a somatic disease

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17
Q

What are cancer divisions called and why?

A
  • Called clonal expansion because they spread from mitotic divisions and shares characteristics with bacterial evolution
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18
Q

In the Tibetan case study, how were the 5 SNPs most likely introdcued into the population?

A
  • By gene flow
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19
Q

Can evolution happen WITHOUT genetic variation?

A
  • NO
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20
Q

How we can use simple models to study complex systems:

A
  • Models make studying populations easy to deal with
  • The models portray the idealised population
  • We COMPARE THIS to the “null” model to REALITY –> determine if the population we are observing is different from the null
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21
Q

What are the five assumptions of the Wright-Fisher model?

A

 Haploid (each individual has only one copy of its genetic information)
 Constant population size
 No mating (just asexual clonal reproduction)
 Discrete generations  an entire population is replaced by its offspring in a single generation
 Genes are transmitted to the next generation by sampling with replacement
 NO SELECTION OR MUTATION

22
Q

How do we stimulate the production of the next generation?

A

-Sampling with REPLACEMENT so that the Frequency of the distribution is unchanged

23
Q

What is the situation if the population is very SMALL????(frequency of distribution)

A
  • Individual sample could vary a lot vary a lot from its source population
  • Random chance will play a bigger role because of LESS sampling
24
Q

Have 10 Wright-Fisher simulations at different population sizes N=10, N=100 and N=1000.
For which population size was there the largest change in frequency b/w generations?

A

N=10

25
Q

Have 10 Wright-Fisher simulations at different population sizes N=10, N=100 and N=1000.At which population size did fixation or loss happen more quickly?

A

-N=10 because it has the LARGEST VARIATION

26
Q

Have 10 Wright-Fisher simulations at different population sizes N=10, N=100 and N=1000. Given a starting frequency of 0.5, is an allele more likely to fix in a small or large population?

A
  • NEITHER
  • Would fix more quickly in a small population however given enough TIME, any allele that is neutral will be fixed or lost
27
Q

What is time to fixation dependent on?

A
  • Population size –> genetic drift works much faster in small than large populations
28
Q

What is the probability of fixation of a new neutral allele?

A

-1/N

29
Q

As the population size (N) increases, the probability of a newly arising neutral allele fixing_______ proportionally.

A
  • Decreases proportionally
30
Q

Are missense mutations, frameshift mutations, and nonsense mutations of equal likelihood to occur?

A
  • NO
  • Missense mutations are NOT as likely to occur
  • Nonsense and frameshift mutations are much more likely to occur
31
Q

What is an example of a synonymous substitution in terms of nomenclature?

A
  • 2/4 possible substitutions at the third site are synonymous, therefore this is a 2 fold synonymous site
32
Q

What is the probability of fixation of a neutral allele independent on?

A

Population size –>equals the current frequency of that allele

33
Q

What do new mutations enter the population at?

A
  • mutation rate U
34
Q

The number of mutations entering a diploid population :

A
  • 2NU
35
Q

The chance that any of these will go to fixation is:

A

The chance that any of these will go to fixation is: ½ N

36
Q

So the EXPECTED number of mutations that we expect to FIX every generation is:

A

½ N x 2NU

37
Q

Because ½ x 2= 1 the expected number of mutations we expect to fix every generation= (also what is this the basis of)

A

mutation rate U

- The molecular clock

38
Q

What is the molecular clock basis?

A

-Can figure out how long two species have been different by counting the neutral differences b/w them (comparing DNA sequences)

39
Q

How does the molecular clock work? (3 steps)

A
  1. Get DNA sequences of the species that you want to compare
  2. Count the number of neutral differences between the two DNDA sequences
  3. Find a way to calibrate the number of mutations with time
    - 4-fold synonymous sites are our best guess about what a neutral mutation is
40
Q

What is fitness?

A
  • The capacity to contribute offspring to the next generation (DO NOT SAY SURVIVAL OF THE FITTEST)
    e. g. Individual with HIGH fitness contributes more offspring than average to the next generation
41
Q

Examples of how a beneficial allele spreads through a population: (3)

A
  • Peppered moth
  • Selective predation
  • Pesticide resistance in insects
42
Q

WHAT IS THE SELECTION COEFFICIENT (S)?

A

-The measure of the strength of selection acting on a genotype (aka the slope of a graph)

43
Q

What is the time to fixation proportional to ?

A
  • The strength of selection
44
Q

mutation with twice the “s” (2S) will approx.. take____

A

½ the time to fix!

45
Q

How do we calculate fitness?

A

-By measuring the CHANGE in frequency over a number of generations –>time to fixation is PROPORTIONAL to the strength of selection–> so mutation with twice the “s” will take half the time to fix

46
Q

Which factors can modify the environment thus reducing the action of selection?

A
  • Enzyme replacement (insulin for type I diabetes and clotting factors for Hemophilia)
  • Antibiotics
  • Sanitary living conditions
  • Detection of heart and vascular defects
  • Surgery
  • Fixing of optical disorders
47
Q

STEPS IN THE EVOLUTION OF CANCER: (4 STEPS)

A
  1. Growth of cells initially repressed. Driver mutation occurs in ONE cell that allows escape from the growth repressors
  2. Secondary mutations occur –> few will drive further adaptation (increase in growth rate or capacity to spread)
  3. Next, chemotherapy is applied to the cancer, drastically reducing population size
  4. Finally, in this case one cell carries a mutation that confers RESISTANCE to the drug used for chemotherapy
48
Q

What are the mutations that drive cancer evolution called?

A

“Driver” mutations

49
Q

How do we identify “driver” mutations in complex sequence data?

A
  • Whole genome (or WES) of cancers

- Genomes of tumours found to contain 10 000 mutations (10s of 1000s)  bc. the cancer cells have HIGH mutation rates

50
Q

What is parallel evolution in terms of cancers?

A
  • If you look at many tumours that have been treated with the SAME drug; you should see that the SAME mutations drive resistance
  • Also genes that are present in genomes of a cancer more often than expected by chance (BRCA1 higher mutation rate)
51
Q

What is the general definition of parallel evolution?

A

-The INDEPENDENT evolution of the SAME trait, or the fixation of mutations in the same gene (even if they were genetically different, they evolve from the SAME selective pressures)

52
Q

What do the Tibetans have that makes them better adapted to higher altitudes?

A
  • 5 SNPs in the EPAS1 gene causing REDUCED function of the EPAS1 protein (40% have it)

BMS2042 (11 decks)

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