Population genetics Flashcards
Population genetics
take into account the way allele frequency changes over time and genotype frequencies
Allele frequency
number of alleles X in a population/number of total alleles in a population
Genotype frequency
number of genotype X in a population/ number of total genotypes
Hardy- Weinberg equilibrium equation
p2 + 2pq + q2= 1
What are the assumptions of the Hardy- Weinberg equilibrium?
- Big population
- Random mating
- Absence of migratory flux
- Absence of selective pressure on genotypes
- Absence of new mutations
Inbreeding coefficient
F = Σ (0.5^n) (1+Fn)
How often does alleles mutate?
u= 10^-5 - 10^-6
Equation for the variation of allelic frequency due to spontaneous mutations
(1-u)^t =pt/po
Genetic drift
Random change in allele frequency
Causes of genetic drift
- Sampling errors: smaller populations have a larger sampling variance
- Founder effect
- Bottleneck effect
Founder effect
A population is established from a small number of breeding individuals
Bottleneck effect
A population is drastically reduced in size
What does it mean that an allele is fixed?
When the allele frequency reaches a value of 0.0 or 1.0 and there is no change in allele frequency unless another allele is reintroduced through mutation/migration
Fixation probability
1/2N
Elimination probability
1- 1/2N
Migration equation
Δpc = m( pD- pR)
Δpc= allelic frequency variation
pD= donor allelic frequency
pR= recipient allelic frequency
m= migration fraction
Monodirectional gene flux
one population moves into another
Bidirectional gene flux
2 populations migrate to form a new one
Factors of natural selection
- survival and adaptation
- efficiency of mating
- fecundity
Directional selection
An extreme is favoured
Stabilising selection
Intermediates are favoured
Disruptive selection
The 2 extremes are favored
Fitness
s= 1-w
s: selection coefficient
When is the sum of the Hardy-Weinberg equation not equal to 1?
In a changing population because our population is not fit in terms of the selection applied
Hardy-Weinberg with fitness
p2WAA/ W + 2pqWAa/W + q2Waa/ W= 1
Hardy-Weinberg with fitness frequency of allele A
p2WAA/ W + pqWAa/W = freq A
Hardy-Weinberg with fitness frequency of allele a
pqWAa/W + q2Waa/ W= freq a
Cases of overdominance
Sickle cell anemia
Phenylketonuria
CF
How is phenylketonuria an example of overdominance?
The heterozygous state can protect against the toxic effects of ochratoxin A
How is CF an example of overdominance?
Carriers of CF gene may have resistance to bacterial toxin-mediated diarrhoea
