8. Population Genetics

Question 1

What is the Hardy-Weinberg Equilibrium

Answer 1

• Serves as a model to demonstrate allelic and genotypic frequencies in the absence of evolution in a population and is based off the following assumption:
  1. random mating
  2. no natural selection
  3. no mutation
  4. no migration
  5. no genetic drift

Question 2

Calculating allelic frequencies

Answer 2

p + q = 1
copies of one allele/sum of alleles

Question 3

Calculating genotypic frequences

Answer 3

p^2 + 2pq + q^2 = 1
number of progeny of one genotype/total number of progeny

Question 4

What is important to note about the HW equilibrium

Answer 4

It’s only a way to predict, nothing ever fits perfectly in a model

Question 5

As P decreases, (HW equilibrium)

Answer 5

Q increases and vice versa

Question 6

What is the maxium freq. for heterozygotes (HW equilibrium)

Answer 6

50% (p=q=50%)

Question 7

Calculate the proportions of the genotypes when AA = 0.36, Aa = 0.48 and aa = 0.16

Answer 7

f(A) = 0.36AA + 1/2(0.48Aa) = 0.6
f(a) = 0.16Aaa + 1/2(0.48Aa) = 0.4

Question 8

Natural Selection and Population Fitness

Answer 8

because of natural selection causing evolution, this will change allelic frequencies in a population

Question 9

Differential reproductive fitness - natural selection

Answer 9

• individuals that leave more offspring distribute more copies of their alleles in the next generation
• FAVOURS THE MOST FIT
• traits passed to progeny from more successful reproducers
• traits are not present in individuals with lower fitness
• fitness measured at the individual level

Question 10

Relative fitness (selection coefficient) - natural selection

Answer 10

• quantifies the reproductive success of a genotype compared to the most favoured genotype in a population
• not measured on individuals
• genotypes with the greatest fitness have w=1
• genotypes less favoured w < 1
  > relative fitness is reduced by a selection coefficient (s)

Question 11

Directional natural selection

Answer 11

shifts the phenotypes in the population to the homozygous genotype
- higher relative fitness than other genotypes
- increases the allelic freq. of the favoured allele and decreases the freq. of the other one

Question 12

Balance polymorphism

Answer 12

• alleles reach an equilibrium
• selective pressure favours maintaining heterozygote but selects against homozygous recessive

Question 13

What is the heterozygote advantage in directional natural selection

Answer 13

Ex) hemoglobin
- variants in the beta globin proteins cause sickle-cell disease
- heterozygotes for the variants result in some deformed cells, but also some resistance to malaria

Question 14

Mutations in gene pools

Answer 14

• source of genetic variation
• changes in nucleotides change amino acid sequences, which change gene expression
• by itself, a slow evolutionary process since its effect on alleles in a population is small and gradual

Question 15

Why are mutations slow?

Answer 15

They affect an allele in 2 directions

Question 16

Forward mutation rate

Answer 16

creates new A2 alleles by mutating A1

Question 17

Reverse mutation rate

Answer 17

changes A2 alleles by mutation to A1
- can create a balanced equilibrium in the absence of other factors

Question 18

f(A1) = ?
f(A2) = ?
forward mutation: ?
reversion mutation: ?
change in q?

Answer 18

f(A1) = p
f(A2) = q
forward mutation: up
reversion mutation: vq
change in q: up - vq

Question 19

mutation-selection balance

Answer 19

natural selection removes the recessive trait, but mutation keeps it in the population - there is a balance between frequency of the recessive mutation and the forces of natural selection

Question 20

What are the mutations with significant evolutionary impact

Answer 20

point mutation - creates new alleles
chromosomal inversion - alleles inside inversion are transmitted together as a unit
gene duplication - redundant genes may acquire new functions through accumulation of additional mutations
genome duplication - may create new species, massive gene duplication

Question 21

gene flow/migrations

Answer 21

moves alleles into and out of populations
- introduction of novel alleles can increase allelic frequencies already present - moving out can reduce it too

Question 22

admixed populations

Answer 22

addition of new organisms into an existing population

Question 23

island model

Answer 23

one way flow of genes/individuals
1-m
pn = (1-m)(p1) +mpc)
pn = (1-0.8)(1.0) + (0.8)(0.5) = 0.6

Question 24

gene flow and moving alleles between populations

Answer 24

• increases genetic variation within populations - decreases divergence due to drift

Question 25

Low and high divergence for neutral alleles

Answer 25

• high n x m (>2 migrants per gen) = low divergence
• low n x m (1< migrants per gen) = high divergence

Question 26

how does genetic drift cause allele frequency changes

Answer 26

• by sampling error
• when a small portion of a population is removed from a larger one, not all the alleles are sampled in the same frequencies as the larger population
• random but one gen affects gamete availability in the next

Question 27

Founder effect

Answer 27

• a new population branches off a larger one
• random draw from larger population
• also a smaller population compared to original
• allelic freq. carried by founders may be greater or less than the original populations allelic freq.

Question 28

Example of the founder effect) old order amish

Answer 28

EVC syndrome
in pop, freq of the allele is 7.3%
in non-amish pop, less than 1%
- traced the origin back to 2 people who were both carriers for the recessive allele

Question 29

Genetic bottleneck

Answer 29

When a large population is drastically reduced to a small population
- catastrophes/natural disasters
- independent/natural selection
- the survivors have less genetic diversity due to the huge loss of alleles from the gene pool

Question 30

Ex of bottleneck) elephant seals

Answer 30

• can affect a single population or an entire species
• due to high hunting, numbers reduced
• remaining population established a new breeding grounds
• sequenced genes from dna show that the population has very little or no genetic variation in the population.

Question 31

interbreeding aka consanguineaous mating

Answer 31

• a form of non-random mating
• mating between related individuals and affects the entire genome
• does not change allelic frequencies - only redistributes the alleles into different genotypes
• no longer a 1:2:1 genotypic frequency

Question 32

What is the most extreme case of inbreeding?

Answer 32

• self fertilization
• proportion of heterozygotes is halved after each generation as homozygosity increases

Question 33

coefficient of inbreeding

Answer 33

• found by wright
• the probability that two alleles carried in an individual are homozygous identical by descent
• descended from the same copy of allele carried by the common ancestor

Question 34

READ: topic 8, page 52-53

Answer 34

yayy

Question 35

Inbreeding depression

Answer 35

increases the homozygosity within a population, which in small populations can reduce the overall fitness of the population/species

Question 36

example of inbreeding depression

Answer 36

cheetahs
- population decreased immensely almost extinct
- conservation efforts and captive breeding have increased the number of cheetahs
- but inbreeding depression has reduced the sperm quality of male cheetahs

Question 37

conservation genetics

Answer 37

looks to design, conduct, and manage captive breeding programs to increase genetic diversity

Question 38

assortative mating

Answer 38

• nonrandom mating
• sexual selection/ preference
  individuals choose certain phenotypes to mate with over others
• only affects genes associated with mate choice
• positive (like with like) : increases homozygosity, decreases genetic variation
• negative (opposites): decreases homozygosity, increases genetic variation

Question 39

see page 56, topic 8

Answer 39

topic 8 done!