Population Genetics

Its importance

Its predictive value

Ch 23

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 relatedGene 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(heterozygotes)

F(homozygous recessives)

If the frequency of cystic fibrosis, a recessive allele, is 0.4 in Europe,

1. what is the frequency of the normal allele?

2. What is the frequency of people with the disease?

3.What is the frequency of carriers?

4.If the population size is 20,000, how many people have the disease?

Want to solve for pp=1-0.4= 0.6

Want to solve for q2 q2=0.42=0.16

Want to solve for 2pq2pq=2(0.4)(0.6)=0.48

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

neutral

negative

Ability to leap away from predators

Color of toenails

Inability to live in arid environment

Genetic change--> phenotypic change-->impacts fitness

Changes in a population’s gene pool--> changes in gene frequency in a population

Evolutionary ecologists take all these factors into account.

Focus on:

Microevolution

Hardy-Weinberg equilibrium1=p2+ 2pq +q2

1= p+ q

Assumes: no change over time; no evolution from one generationto 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?