Chapter 21: Population Genetics Flashcards Preview

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Flashcards in Chapter 21: Population Genetics Deck (61):

Natural selection is a mechanism of adaptive evolution based on what three things?

1. phenotypic variation exists among individuals of spp.
2. differences are passed on to offspring
3. more offspring are born than survive in a pop^n


Phenotypic proportions change in the population after ?????

each generation
L> population allele frequency (Hz) also change.


Define Population in genetics terms ??

- group of interbreeding individuals of the same species that inhabit the same space at the same time


Define Gene pool!

- sum total of all alleles carried in all members of a population in one generation.


What is allelic frequency?

- proportion of all copies of a gene in a population that are of a given allele type.
L> Total # of gene copies= 2x individuals in pop
( ex: 16 in pop....2x16= 32 copies of CF gene)
L> heterozygotes contribute 1x for each allele
L> homozygotes contribute 2 x for desired allele.


For HIV what is the normal allele of the gene associated with its infection rate?
L> what about the mutated one?

L> CCR5-delta32


HIV Infection rates:
- what do those with the mutated CCR5 allele not possess?

- CCR5 receptors


HIV Infection rates:
- Two exposed but uninfected individuals were homozygous for CC-CCR5 mutation on Chr.3. What does CC-CCR5 encode for?

- it encodes for chemokine receptor protein CCR5. CCR5 is also a receptor for HIV-1 strains.


HIV Infection rates:
- HIV-1 infection, explain.

- begins with binding of env proteins to CD4 receptor on the host cell. This causes the env to change chap and form a second binding site. This second site binds to CCR5 which will initiate fusion of the viral protein coat with the host cell membrane.


HIV Infection rates:
- The mutant allele of the CCR5 gene has a 32bp _____ in the coding region. The CCR5 mutant protein does/does not reach the host cell membrane.

- deletion
- does not


HIV Infection rates:
- HIV-1 env proteins bind to the CD4 receptors. A second site binds to ___ which will initiate the fusion of the protein coat with the host cell membrane. The mutant allele of the CCR5 gene has a 32bp ____ in the coding region.

- CCR5
- deletion


HIV Infection rates:
- What genotype= resistant to infection
- What genotype = AIDS progresses slowly
- What genotype= susceptibility to HIV-1 strains

- delta32/delta32
- 1/delta32=
- 1/1 homozygous


Hardy Weinberg Law??? (explain it generally)

- it clarifies the relationship between genotype and allele Hz within a generation and from one generation to the next. Assumptions have to be made with respect to the nature of the population, individuals within it and genes carried by the individual.


What are the assumptions for Hardy Weinberg? (5)
L> what is said when all five are met?

1.infinitely large population
2. random mating
3. free from mutation
4. free from migration (no gene flow)
5. free from natural selection ( no genotype-phenotype differences in the ability to survive to reproductive age and pass on genes to the next generation)
*** when all five are met the population will maintain genetic equilibrium and the frequency of each allele is conserved and the allele frequencies can be predicted!


In an infinitely large, randomly mating population, free from mutation, migration and natural selection; the frequencies of the alleles do/do not change over time where p =__, q=___ and the genotypic frequencies remain in the proportions p^2 (____), 2pq(___) and q^2(____). The sum of the genotype frequencies equals___. The overall binomial equation representing hardy weinberg is???

- do not
---->The following are Hz of gametes
-p= frequency of A
- q= frequency of a
----> The following are Hz of genotypes after mating
- p^2= frequency of AA
- q^2= frequency of aa
- 2pq= frequency of Aa
- 1
- equation: p+2pq+q= 1
- square root of p^2= Hz of dominant allele


What five things cause changes in allele frequency?

2. random genetic drift
3. migration
4. non random mating
5. natural selection


Changes in allele frequency:
- migration and mutation increase/decrease genetic variation?

- increase


Changes in allele frequency:
- genetic drift increases/decreases genetic variation?

- decreases


Changes in allele frequency:
- Inbreeding leads to an increase/decrease in homozygosity and an increase/decrease in genetic variation ?

- decrease


Changes in allele frequency:
- Out breeding causes an increase/decrease in heterozygosity and an increase/decrease in genetic variation?

- increase
- increase


Changes in allele frequency:
- Natural selection can ____ genetic variation
L> OR through ____ and ____.

- increase and decrease genetic variation (one allele can be favoured over another and the other will be lost)
L> over dominance and balancing selection


Changes in Allele Hz:
Genetic Drift:
-increases/decreases allele Hz.
- it results from???
L>formula? (effective population)

- decreases
- results from random events due to small population size. A population may be large but due to the number of breeding individuals the effective population may be small.
L> Ne= 4 x (Nf x Nm)/ (Nf + Nm)
L> progeny of a small pop will not show HW proportions


Changes in Allele Hz:
Genetic Drift:
- This decrease in allele Hz is caused by a small population size over many generations:
L> Founder effect??
L>Bottleneck effect??

-FE: population is established by a small number of individuals
BE: population is dramatically reduced in size, genes lost from gene pool via chance. This reduction is due to tsunami, pandemic etc via natural disasters. The re-founding (FE) can occur only by the few that survive the BE .
*** The BE and FE are related! bottleneck occurs and then founder effect will remake population.


Changes in Allele Hz:
Effects of Genetic Drift?Explain.
L> also what does an Hz=0 and an Hz= 1 mean?

- causes changes in allele Hz over time in pop
- can cause loss of genetic variation in pop and reduces heterozygosity. Change in frequency over time is caused by chance in genetic drift, either increasing or decreasing.
L> 0= allele is lost
L> 1= allele has a fixed frequency


Changes in Allele Hz:
Genetic Drift:
- What does it mean for an allele to be fixed?

- no further change in Hz can occur unless the other is reintroduced ( migration or mutation). Fixation occurs randomly but rare alleles are more likely to be lost.


Changes in Allele Hz:
Genetic Drift:
- genetic drift + mutation =?

- via these heterozygosity decreases and the population has less genetic diversity.


Changes in Allele Hz:
Summatively Genetic Drift causes what three things?

1. loss of genetic variation within a pop
2. genetic divergence among populations
3. random fluctuation in the allele frequencies of a population over time.


Changes in Allele Hz:
Genetic Drift:
- probability of fixation by genetic drift =??

- current frequency of the allele.


Changes in Allele Hz:
- Mutation

- "A" mutates to "a" at a rate of u (mu) (forward )
- "a" mutates to "A" at a rate of v (nu) (reverse)
- The Hz of alleles in the pop is determined by interaction of mutation rates and natural selection


Changes in Allele Hz:
- Mutation
L> pn=?
L> qn=?

L> the Hz of A in generation n
L> the Hz of a in generation n


Changes in Allele Hz:
- Mutaiton
L> what are the new frequencies for qn?

L> qn+1
L> qn+1= qn+ upn- vqn
L> delta q( change in q?)= (qn+1) -qn = upn-vqn


Changes in Allele Hz:
- Mutation:
L>what is the equilibrium condition (Hz) for each allele?

1. q= u/u+v
2. p=v/v+u


Changes in Allele Hz:
- Mutation: continues to ad new variation to the pop ____ allele Hz.
- random genetic drift: net effect is to ___ variation in population.

- increase
- remove


Changes in Allele Hz:
- Migration?

- increases allele Hz. Causes gene flow, which is the exchange of genes with other populations adding new alleles to gene pool, altering Hz of existing alleles.


Changes in Allele Hz:
- Migration:
L> introduces two effects! Explain them.
hint for one: homogenizing

1. introduces new alleles, sources of genetic variation
2. changes the allele frequencies in the recipient population. The introduced allele frequencies change the allele frequencies of the population being migrated to, making them more similar and thus homogenizing the population which is the complete opposite of divergence ( the populations become more alike). Pop with homogeneity will lead to speciation as quickly as divergence , in this case a merging of species.


Changes in Allele Hz:
- Natural selection?
L> what is it

- differential reproduction of genotypes. Genes responsible for the increased survival and reproduction will increase in Hz in the next generation. Organisms adapt to their environment and that adaption arises from NS which shapes much of the phenotypic variation observed in nature.


Changes in Allele Hz:
- Natural selection
L> causes?(3)

- Environment
- diseases around
- genotypes present


Changes in Allele Hz:
- Natural selection
L> Effects of natural selection depend on the relative ??

fitness of the different genotypes


Changes in Allele Hz:
- Natural selection
L> Directional selection?

L> directional changes in allelic frequencies, disfavoured allele is eliminated from population
L> decreases allele frequencies


Changes in Allele Hz:
- Natural selection
L> Balancing selection?

- heterozygote superiority
L> maintenance of genetic variation in the population...increases allele Hz


Changes in Allele Hz:
- Natural selection
L> fitness??

L> relative reproductive ability go a genotype (W)... Genotype that produces the most offspring gets W=1
W= fitness


Changes in Allele Hz:
- Natural selection
L> selection against?

L> measure of relative intensity of selection against a phenotype
L> s= 1-W... IF low fitness (W), high selection against and no selection agains the genotype if fitness (W) = 1.


Changes in Allele Hz:
- Natural selection
L> Selection against ex:
B1B1= 7 offspring, W11=1
B1B2= 3 offspring, W12= 0.43
B2B2= 2 offspring, W22= 0.29

1. S=1-1 = 0
2. S= 1-0.43= 0.57 (SELECTION AGIANST)
3. S= 1-0.29= 0.71 (SELECTION AGAINST )


Changes in Allele Hz:
- Natural selection
L> Selection effect:
L> explain the six finiteness classes

A: all genotypes have equal fitness, no selection against)
B: NS operating against dominant alleles, if 1)
( W11= W12 W22 W11 W11 and W22 W12<(W11 and W22)=1


Heterozygote superiority:
L> Heterosis or over-dominance?

- an equilibrium of allele Hz arises when the heterozygote has a higher fitness than either homozygote.


Heterozygote superiority:
L> Heterosis or over-dominance
L> example of this from lecture?

- Sickle cell anemia...even with the medical problems associated with individuals with this disease, NS cannot eliminate the allele from the population. ...classE


What are the two types of assortative mating?

1. positive assortative mating
2. negative assortative mating


Positive assortative mating?

- individuals with similar phenotypes mate preferentially ( Males have a positive assortative mating for height...more than expected if by chance)


Negative assortative mating?

- phenotypically dissimilar individuals mate more often than randomly chosen individuals.


Inbreeding decreases/increases allele Hz?



Inbreeding involves what?

- preferential mating between close relatives and can tend to occur in small populations even if there was no tendency to choose relatives as mates.


The amount of inbreeding in a population is expressed as what?

F= coefficient of inbreeding
L> the probability that two alleles of a given gene are identical because they are descended from the same single copy of the allele of an ancestor.


With inbreeding:
L> recessive homozygous alleles?

- they are deleterious


Inbreeding increases the proportion of ____ in a population and ____ fitness.

- reduces


Inbreeding depression??
L>hint: think of lethal alleles as well

- a measure of loss of fitness caused by inbreeding. Each of us carries ( on average) 4 lethal alleles (lethal when homo), inbreeding can cause death in small population ( high probability of getting two lethal alleles), decreases heterozygotes in population.


Benefits of inbreeding (huh?)
L> It is an important tool for breeders of domesticated animals and plants because it increases what?

the proportion of individuals carrying desirable traits.


Benefits of inbreeding (huh?)
L> Hybrid vigour?

- when members of two inbred lines are mated with each other, causing the hybrid offspring to be more vigorous in desirable alleles than in either parental line.


Benefits of inbreeding (huh?)
L> What are the two explanations for hybrid vigour?

1. Dominance Hypothesis
2. Over-dominance


Benefits of inbreeding (huh?)
L> Hybrid Vigour Explained:
1.Dominance Hypothesis

1. F1 hybrids are heterozygous at all loci, recessive alleles are masked by the more favourable dominate alleles in the hybrid


Benefits of inbreeding (huh?)
L> Hybrid Vigour Explained:
2. Over-dominance

2. heterozygote is superior to either homozygote, two forms of a gene product may be present leading to biochemical diversity



alike but not related