Population Genetics: An introduction

Change in Populations & Communities:

Population Genetics

Major topics & questions in Unit 4:

Change in Populations & Communities:

Population Genetics

1. Populations can be described in terms of gene pools:

• What kinds of events can change a gene pool?

• Can we measures changes in a gene pool?

• What happens to native gene pools if we introduce an exotic species into an ecosystem?

2. Populations interact with each other:

• What kind of relationships exist in ecosystems

(Biology 20 review)

Major topics & questions in Unit 4:

Change in Populations & Communities:

Population Genetics

2. Populations grow in characteristic ways:

• What factors influence population growth?

• How does chaos theory affect population growth?

Change in Populations & Communities:

Population Genetics

Community

Population

Species

Gene

Mutation

Allele

Gene Pool

Random change in DNA

All genes in a specific population

Members of 1 species within a certain area & time

Segment of DNA that codes for a protein

All populations that interact with each other

Individuals that can mate & produce viable offspring

Alternative form of a gene

Pause for Math Review

Change in Populations & Communities:

Population Genetics

Convert the following percentages to decimals:

42% =

9% =

Hardy – Weinberg Principle

Change in Populations & Communities:

Population Genetics

• We can measure the allele frequency of genes in a given population using the Hardy-Weinberg principle.

• The principle predicts that if all factors remain constant, the gene pool will have the same composition of alleles generation after generation.

• This stability of the gene pool is called genetic equilibrium.

Hardy – Weinberg Principle

Change in Populations & Communities:

Population Genetics

• Notice that the equation applies to an idealized population that is not evolving.

• Conditions where no change (evolution) occurs:

1. Large populations ( to reduce effect of chance)

2. Random mating

3. No mutations

4. No immigration or emigration

5. No selection (all genotypes reproduce equally)

Change in Populations & Communities:

Population Genetics• If a population is not evolving, the frequency of

alleles will remain the same from generation to generation and can be predicted as follows:

• Where p = frequency of allele A

q = frequency of allele a

• Note: p + q = 1

p2 + 2pq + q2 = 1

Change in Populations & Communities:

Population Genetics• The formula shows the frequency of alleles as they appear in

the genotypes in an entire population.

• Example – What are genotype frequencies in a population for alleles A and a if the frequency of allele A is 0.7.

Step 1:

p2 + 2pq + q2 = 1

( )2 + ( ) + ( )2

= 1

( ) + ( ) + ( ) =

1

P = frequency of allele A = 0.7

1-p = q

1-0.7 = 0.3

q = frequency of allele a = 0.3

Step 2:

Change in Populations & Communities:

Population Genetics• The formula shows the frequency of alleles as they appear in the genotypes in an entire population.

p2 + 2pq + q2 = 1

A2 + 2Aa + a2 = 1

( )2+ ( ) + ( )2 = 1

( ) + ( ) + ( ) = 1

Frequency of allele A = 0.7

Frequency of allele a = 0.3

(p + q = 1; 0.7 + 0.3 = 1)

A (0.7) a (0.3)

A (0.7)

a (0.3)

Change in Populations & Communities:

Population GeneticsExample 1: A recessive genetic disorder occurs in 9% of the

population. What percentage of the population are carriers, but do not have the disorder?

Change in Populations & Communities:

Population GeneticsExample 2: For a moth population 60% of moths are white

(dominant) and 40% are black. Three years later 65% of moths are white and 35% are black. What is the change in allele frequencies for W and w.

Change in Populations & Communities:

Population Genetics Challenge: