lecture 9 - genetic drift

Question 1

what is genetic drift?

Answer 1

Process of change in genetic composition in a population due to a chance or a
random event but not due to natural selection

Question 2

what does genetic drift lead to

Answer 2

Leads to change in allele frequencies
○ Remvoes genetic variation from population

Question 3

How is genetic variation removed with drift? which populations are more effected?

Answer 3

rate is inversely proportional to population size so genetic drift is a very
weak dispersive force → affects smaller population more

Question 4

2 Main types of genetic drift?

Answer 4

bottle neck
founder effect

Question 5

describe bottleneck

Answer 5

Type of genetic drift that happens when population size is severely
reduced
- Can happen from natural events and leaving a small random
number of people and killing most
- The people left behind are not alive because they are
fittest, but because they are lucky

Question 6

ex of bottleneck?

Answer 6

eg:) Elephant Seals
- Too much hunting in the 1800s
- Surviving population had different allele frequencies and
little genetic diversity

Question 7

describe founder effect

Answer 7

Reduction in genomic variability that occurred when small group of
individuals become separated from a larger population
- They started their own population but not reflecting the original
population

Question 8

exs of founder effect?

Answer 8

eg:) Old order amish population of pennsylvania
- Kids born mainly with 6 fingers/toes
- Amish population got separated from the rest and caused
inbreeding
- eg.) Desert BIg Horn Sheep
- Increased in size from 20 founders in 1975 to 650 in 1999

Question 9

does genetic drift have natural selection?

Answer 9

Genetic drift has no natural selection involved and it is also random

Question 10

what is fixation?

Answer 10

In the context of genetic drift, when an allele reaches a
frequency of p = 1.0

Question 11

what does probability theory show

Answer 11

Probability theory shows that, with enough given time, genetic drift will
always result in fixation or extinction.

Question 12

With smaller populations, we will
see these 2 things (allele freq of them?):

Answer 12

a. Fixation: allele frequency of B is fixed at p=1.0
- The probability of fixation of an allele is equal to the initial
frequency of the allele
b. Extinction: allele frequency is at p=0

Question 13

what do we see with extinction in larger populations

Answer 13

n larger populations, there is not that much change visible
for B
- Extinction are irreversible events
- The probability of extinction is 1 minus the probability of
fixation

Question 14

describe latency. ex?

Answer 14

atent state: no disease sympoms are showing
- Eg: tuberculosis
- Bacteria attaackd mainly lungs but can also attack other
organs like kidneys, brain, etc.
- 1 in 3 of us might have it
- TB in latency state so no symptoms are showing
- If a person is in latency stage, they aren’t infectious ye

Question 15

what is a stochastic process?

Answer 15

a process where individual outcomes are dictated by chance but the average of a
large number of outcomes can be described as a probability distribution based on
initial conditions

Question 16

describe markov chain

Answer 16

usually discrete stochastic process … in which the probabilities of
occurrence of various future states depend only on the present state of
the system or on the immediately preceding state and not on the path by
which the present state was achieved
■ Given the present, the future is independent of the past

Question 17

describe neutral theory

Answer 17

suggests that most of the genetic variation in populations is the result of mutation
and genetic drift and not selection
○ if a population carries several different alleles of a particular gene, odds are that
each of those alleles is equally good at performing its job
■ variation is neutral: whether you carry allele A or allele B does not affect
your fitness

Question 18

what does the wright fisher model describe

Answer 18

Describes the sampling of alleles in a population with no selection, no migration,
no overlapping generations time, no mutation and have random mating