Transcript Population Genetics

Ch 23
Population Genetics
Its importance
Its predictive value
Population? Define
- Group of interbreeding organisms (a species) Reside within a specified geographic region
Example: Humans in the Pacific northwest
Grey whales in Puget Sound
Genetically related
Gene pool =all the alleles within the population
Gene frequency in a population
p= frequency of dominant allele or f(dominant allele)
q= frequency of recessive allele or f(recessive allele)
1= p+ q
If p=0.3, then q=1-0.3=0.7
The frequency of every possible combination of a given
pair of alleles in a population can be determined
mathematically with:
1=(p+q)(p+q)
Expand:
1=p2+ 2pq +q2
F(homozygous dominants)
F(homozygous recessives)
F(heterozygotes)
If the frequency of cystic fibrosis, a recessive allele, is 0.4
in Europe,
1. what is the frequency of the normal allele?
Want to solve for p
p=1-0.4= 0.6
2. What is the frequency of people with the disease?
Want to solve for q2
q2=0.42=0.16
3.What is the frequency of carriers?
Want to solve for 2pq
2pq=2(0.4)(0.6)=0.48
4.If the population size is 20,000,
how many people have the disease?
q2(20,000)=3,200
Genotype of population-->phenotype
Some phenotypes adaptive (advantageous)-->
increased fitness (fertility)
Some:neutral --> fitness unchanged
Others: negative --> decrease fitness
Consider: frogs
adaptive traits
Ability to leap away from predators
neutral
Color of toenails
negative
Inability to live in arid environment
Genetic change--> phenotypic change-->impacts fitness
Evolutionary ecologists take all these factors into account.
Focus on:
Changes in a population’s gene pool
--> changes in gene frequency in a population
Microevolution
Hardy-Weinberg equilibrium
1= p+ q
1=p2+ 2pq +q2
Assumes: no change over time; no evolution from one generation
to the next
Equilibrium Conditions
1. No DNA mutation
2. No natural selection
3. No migration (no gene flow between populations)
4. Random mating
5. Large population
Hardy and Weinberg must be joking!
A static environment; no change!
I would question that.
When changes in allele frequencies occur, how
does one determine the cause? What significance
does it have if any?