AP Biology

AP Biology Lab

Population Genetics

{Hardy Weinberg}

http://www.phschool.com/science/biology_

place/labbench/lab8/intro.html

AP Biology

A Large Breeding Population Helps to ensure that chance alone

does not disrupt genetic equilibrium.

AP Biology

Random Mating Population at equlibrium:

mating must be random.

Assortative mating: individuals

tend to choose mates similar to

themselves

does not alter allelic frequencies, but results in fewer heterozygous individuals than you would expect in a

population where mating is random.

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No Change in Allelic Frequency Due to Mutation

Any mutation in a particular gene would

change the balance of alleles in the gene pool

Mutations may

remain hidden in

large populations

for a number of

generations, but

may show more

quickly in a small

population.

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No Immigration or Emigration Both immigration and emigration can

alter allelic frequency.

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No Natural Selection If selection occurs, those alleles that are

selected for will become more common.

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Lab 8: Population Genetics

Description

simulations were used to study effects

of different parameters on frequency of

alleles in a population

selection

heterozygous advantage

genetic drift

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Lab 8: Population Genetics Concepts

Hardy-Weinberg equilibrium p + q = 1

p2 + 2pq + q2 = 1

required conditions

large population

random mating

no mutations

no natural selection

no migration

gene pool

heterozygous advantage

genetic drift founder effect

bottleneck

AP Biology

Lab 8: Population Genetics

Conclusions

recessive alleles remain hidden

in the pool of heterozygotes

even lethal recessive alleles are not

completely removed from population

know how to solve H-W problems!

to calculate allele frequencies, use p + q = 1

to calculate genotype frequencies or how

many individuals, use p2 + 2pq + q2 = 1

AP Biology

Estimating Allelic Frequency If a trait is controlled by two alternate

alleles, how can we calculate the

frequency of each allele?

For example, let us look at a sample

population of pigs.

The allele for black coat is

recessive to the allele for

white coat. Can you count the

number of recessive alleles in

this population?

Can you count the number of

dominant alleles?

AP Biology

The Hardy-Weinberg Equation Used to estimate the frequency of alleles in a population,

According to this equation:

p = the frequency of the dominant allele (represented here by A)

q = the frequency of the recessive allele (represented here by a)

For a population in genetic equilibrium:

p + q = 1.0 (The sum of the frequencies of both alleles is 100%.)

(p + q)2 = 1

So p2 + 2pq + q2 = 1

The 3 terms of this binomial expansion indicate the frequencies of

the 3 genotypes:

p2 = frequency of AA (homozygous dominant)

2pq = frequency of Aa (heterozygous)

q2 = frequency of aa (homozygous recessive)

This page contains all the information you need to calculate allelic

frequencies when there are two different alleles.

We start with some sample problems that will give you practice in

using the Hardy-Weinberg equation.

AP Biology

Sample Problem 1 The allele for black coat is recessive.

We can use the Hardy-Weinberg

equation to determine the percent of the

pig population that is heterozygous for

white coat.

1. Calculate q2

2. Find q.

3. Find p.

4. Find 2pq.

AP Biology

Sample Problem 1 The allele for black coat is recessive.

We can use the Hardy-Weinberg

equation to determine the percent of the

pig population that is heterozygous for

white coat.

1. Calculate q2

2. Find q.

3. Find p.

4. Find 2pq.

Answer:

Four of the sixteen individuals show the

recessive phenotype, so the correct

answer is 25% or 0.25.

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What % of the pig population is heterozygous for white/pink coat color?

2. Find q. Take the square root of q2 to obtain q, the

frequency of the recessive allele.

3. Find p. The sum of the frequencies of both alleles = 100%,

p + q = l. You know q, so what is p, the frequency of the

dominant allele?

1. Calculate q2

Count the homozygous recessive individuals in the illustration.

Calculate the percent of the total population they represent.

This is q2.

4. Find 2pq.

The frequency of the heterozygotes is

represented by 2pq. This gives you

the percent of the population that is

heterozygous for white coat:

AP Biology

What % of the pig population is heterozygous for white/pink coat color?

2. Find q. Take the square root of q2 to obtain q, the

frequency of the recessive allele.

3. Find p. The sum of the frequencies of both alleles = 100%,

p + q = l. You know q, so what is p, the frequency of the

dominant allele?

1. Calculate q2

Count the homozygous recessive individuals in the illustration.

Calculate the percent of the total population they represent.

This is q2.

4. Find 2pq.

The frequency of the heterozygotes is

represented by 2pq. This gives you

the percent of the population that is

heterozygous for white coat:

Answers:

Four of the 16 individuals show the

recessive phenotype,

so the correct answer is 25% or 0.25.

2. q = 0.5

3. p = 1 - q, so p = 0.5

4. 2pq = 2(0.5) (0.5) = 0.5 ,

so 50% of the population is

heterozygous.

AP Biology

How many individuals would you expect to be homozygous for red eye color?

In a certain population of 1000 fruit

flies, 640 have red eyes while the

remainder have sepia eyes. The sepia

eye trait is recessive to red eyes.

Hint: The first step is always to calculate q2! Start by determining the

number of fruit flies that are homozygous recessive. If you need help

doing the calculation, look back at the Hardy-Weinberg equation.

AP Biology

How many individuals would you expect to be homozygous for red eye color?

In a certain population of 1000 fruit

flies, 640 have red eyes while the

remainder have sepia eyes. The sepia

eye trait is recessive to red eyes.

Answer:

You should expect 160 to be homozygous dominant.

Calculations:

q2 for this population is 360/1000 = 0.36

q = = 0.6

p = 1 - q = 1 - 0.6 = 0.4

The homozygous dominant frequency = p2 = (0.4)(0.4) = 0.16.

Therefore, you can expect 16% of 1000, or 160 individuals, to be homozygous dominant.

AP Biology

FRQ Evolution is one of the unifying themes of biology. Evolution involves

change in the frequencies of alleles in a population. For a particular

genetic locus in a population, the frequency of the recessive allele (a) is

0.4 and the frequency of the dominant allele (A) is 0.6.

(a) What is the frequency of each genotype (AA, Aa, aa) in this

population? What is the frequency of the dominant phenotype?

(b) How can the Hardy-Weinberg principle of genetic equilibrium be

used to determine whether this population is evolving?

(c) Identify a particular environmental change and describe how it might

alter allelic frequencies in this population.

AP Biology

Lab 8: Population Genetics ESSAY 1989

Do the following with reference to the Hardy-Weinberg model.

a. Indicate the conditions under which allele frequencies (p and q)

remain constant from one generation to the next.

b. Calculate, showing all work, the frequencies of the alleles and

frequencies of the genotypes in a population of 100,000 rabbits of

which 25,000 are white and 75,000 are agouti.

(In rabbits the white color is due to a recessive allele, w, and agouti

is due to a dominant allele, W.)

c. If the homozygous dominant condition were to become lethal, what

would happen to the allelic and genotypic frequencies in the rabbit

population after two generations?

FRQ

AP Biology

Application The Hardy-Weinberg equation is useful for

predicting the percent of a human population that

may be heterozygous carriers of recessive alleles

for certain genetic diseases. Phenylketonuria

(PKU) is a human metabolic disorder that results

in mental retardation if it is untreated in infancy.

In the United States, one out of approximately

10,000 babies is born with the disorder.

Approximately what percent of the population are

heterozygous carriers of the recessive PKU

allele?

AP Biology

Design of the Experiment The experiment in this laboratory is a test of

Hardy-Weinberg equilibrium in a "mating

population," represented by you and your

classmates. Your instructor will assign each

class member a genotype (AA, Aa, or aa). The

initial allelic frequency of the population will be

0.5 A and 0.5 a. Your population will simulate

several different conditions that might alter allelic

frequency, and you will determine the allelic

frequencies over several generations.

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Analysis of Results Your analyses should have included

Hardy Weinberg and Chi square.

http://www.phschool.com/science/biology_place/labbench/lab8/analysis.html

AP Biology

Any Questions??