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Lab 8. Evolution

Introduction

Evolution is often defined as change over time, usually in response to a change in the environment. What this means is that the gene pool of a population shifts as time passes.

What is a gene pool? It is simply the collection of all the available alleles of all the genes on all the chromosomes in the population. What’s a population? It is the group of individuals that have geographic and behavioral mating access to each other. Geographic access is obvious; the dandelions in a yard in Virginia are not in the same gene pool as those in a yard in Pennsylvania, even if they are the same species. They could mate if they were brought together, but dandelion pollen doesn’t blow that far, so they are geographically isolated. Behavioral mating access can be less obvious; one example would be a stag that keeps all other males out of his territory and mates with all the does. The other stags are not allowed to mate, so their genes are not in the pool.

A great deal of the time for most species, evolution is not occurring. The gene pool stays the same, because the environmental situation is not changing. In the early days of population genetics, people argued over whether dominant alleles could “take over” a gene pool without any selection. Two mathematicians, Hardy and Weinberg, showed that this would not happen.

The basis of their argument is that the gene pool will not change, and the frequency of the various alleles will stay the same if the following conditions are met:

· The population is large.

· The population is freely interbreeding at random (this excludes the stag and the does).

· No individuals are taking their alleles out of the population (emigrating) or adding their alleles to the population (immigrating), so the percentages of the alleles can’t change because of migration.

· There are no mutations, so no new alleles appear.

· None of the alleles has a selective advantage (in other words, there aren’t any combinations of alleles that give some individuals a better chance of surviving that anyone else).

Here is the mathematical basis of their argument:

Imagine a simple situation in which a gene has only two alleles, A and a, and A is dominant. Let the frequency of A, expressed as a decimal with a value less than one, be p, and let the frequency of a, expressed as a decimal with a value less than one, be q. Because there are only two alleles, every allele must be either A or a, so,

p + q = 1

By definition, p and q are also the frequencies of the alleles in the eggs and sperm produced by this species. These sperm and eggs can come together in four ways when random mating occurs.

1. The chance that a male p sperm will meet a female p egg is p x p, or p2. The children produced by this cross will be genetically AA and express the dominant allele; they will have the A phenotype.

2. The chance that a male p sperm will meet a female q egg is p x q, or pq. The children produced by this cross will be genetically Aa and express the dominant allele; they will also have the A phenotype.

3. The chance that a male q sperm will meet a female p egg is also p x q, or pq. The children produced by this cross will also be genetically Aa and express the dominant allele; they will also have the A phenotype.

4. The chance that a male q sperm will meet a female q egg is q x q, or q2. The children produced by this cross will be genetically aa and express the recessive allele; they will have the a phenotype.

These four situations are the only possibilities, so

p2 + pq + pq + q2 = 1 (1.0 represents 100% of all possible events in a mating)

When we combine the middle two terms, we get

p2 + 2pq + q2 = 1

These two formulas,

p + q = 1

p2 + 2pq + q2 = 1

summarize what is known as the Hardy-Weinberg Law.

However, usually we don’t know the frequency of the alleles in a population; in most cases, we can’t even see the gametes! If we want to know what the frequencies of the alleles are, we have to use these two formulas to figure it out.

The most important things to remember are the two formulas above. In these formulas,

· p = the frequency of the dominant allele

· q = the frequency of the recessive allele

· p2 = the frequency of individuals in the population who are homozygous dominant

· 2pq = the frequency of individuals in the population who are heterozygous

· q2 = the frequency of individuals in the population who are homozygous recessive

Materials

· Three colours of beans (chili, pinto and navy are good, but any three contrasting objects will do – M&Ms, coins, beads, etc).

· Two bowls

· A pocket calculator (MS Windows has one too)

Procedure

Two alleles which control hair texture are incompletely dominant to each other, and the phenotypic expression of hair texture is a function of which alleles are present. The genotypes and phenotypes are:

Genotype	Phenotype
C1C1	curly
C1C2	wavy
C2C2	straight

This is where Hardy-Weinberg comes in. Recall:

p2 + 2pq + q2 = 1.0

Remember:

p2 = the frequency of the C1C1s

2pq = the frequency of the C1C2s

q2 = the frequency of the C2C2s

These percentages will remain stable through all subsequent rounds of mating of this population.

Please refer to the following table for calculation references. Also, please adjust the calculations for the rest of the tables.

Please submit this Lab Report Sheet in Webtycho in the Assignments folder.

Data Sheet – Sample table and calculations:

Student answers to questions

1. In the absence of selection, what happens to gene frequencies in a population?

Type your answer here. This textbox expands as you type.

2. What have you learned about population genetics so far? i.e., what do these results tell you about how genes in a gene pool behave under tightly controlled (i.e., artificial/hypothetical) circumstances?

Type your answer here. This textbox expands as you type.

<more below>

You’ll use your three colours of beans (or any 3 objects of your choice), from this point on, to represent the individuals in your population.  The red (chili) beans represent the homozygous dominants (C1C1 – curlies), the mottled (pinto) beans the heterozygotes (C1C2 – wavies), and the white (navy) beans the homozygous recessives (C2C2 – straights). 

Pick the beans (objects) two at a time and record your results here. Use the example above (page 6) to help you in your calculations).

Experiment 1

In this first exercise, you are going to determine what happens when you allow your population of 80 to interbreed freely.

In a bowl, place the correct numbers of the three colours of beans to represent the population of 80 people.  For 20 C1C1s, 40 C1C2‘s, and 20 C2C2’s, you would choose 20 red beans, 40 pinto beans, and 20 white beans (or any three different objects you chose).  Mix them thoroughly and then, without peeking (i.e., at random), withdraw two beans (or two objects).  Record the genotypes represented by the two beans.

Example :  if you withdraw a white bean and a pinto bean the first time, then you will record one (1) C2C2 x C1C2; you have mated one pair.

Put your first two beans in the second bowl and continue to draw pairs of beans from the first bowl until you have withdrawn all 40 pairs.  Your records will now show a series of 80 random matings from this population.

There are six possible combinations:

1. C1C1 x C1C1 (two chili beans),

2. C1C1 x C1C2 (one chili, one pinto),

3. C1C1 x C2C2 (one chili, one navy),

4. C1C2 x C1C2 (two pintos),

5. C1C2 x C2C2 (one pinto, one navy), and

6. C2C2 x C2C2 (two navies).

Record the total number for each of the six matings.

Data sheet (fill in the BLUE and YELLOW areas).

(see page 6 for detailed calculation help).

			MATINGS	OFFSPRING = Matings x 4	C1S	C2S
C1C1	X	C1C1
C1C1	X	C1C2
C1C1	X	C2C2
C1C2	X	C1C2
C1C2	X	C2C2
C2C2	X	C2C2
							total

P=	Frequency of C1 =
Q=	Frequency of C2 =

3.    How do these compare with the parental generation?

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4.    What principle have you demonstrated with this exercise?

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Experiment 2

Put your beans back in the bowl.  This time, withdraw only 20 pairs (= 20 random matings), and record the results as you did in Task 1.

Next, calculate the offspring of this generation:  again, assume 4 offspring per mating.  Total the numbers of C1C1s, C1C2s and C2C2s. and then calculate the allele frequencies.  Finally, determine the genotypic frequencies.

 

Data sheet (fill in the BLUE and YELLOW areas).

(see page 6 for detailed calculation help).

			MATINGS	OFFSPRING = Matings x 4	C1S	C2S
C1C1	X	C1C1
C1C1	X	C1C2
C1C1	X	C2C2
C1C2	X	C1C2
C1C2	X	C2C2
C2C2	X	C2C2
							total

P=	Frequency of C1 =
Q=	Frequency of C2 =

5.    How do these last P and Q (frequencies) compare with the P (parental) generation (Experiment 1)?

Type your answer here. This textbox expands as you type.

6.    If you repeated this experiment (i.e., you selected another 20 pairs from the bowl) would you expect to get the same result?  Why or why not?

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7. What principle have you illustrated this time?

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Experiment 3

Now you are going to assume that your population has been invaded by an ET which is a human predator.  It particularly fancies people with curly hair – eats them preferentially – and when it moves on (looking for more of its favourite lunch), your population has been denuded of curlies (C1C1).

Set up your new population in the bowl, and go through the mating (bean picking/ withdrawal) procedure again, recording your results.  Again, assume that each mating produces four offspring.

(see page 6 for detailed calculation help).

			MATINGS	OFFSPRING = Matings x 4	C1S	C2S
C1C2	X	C1C2
C1C2	X	C2C2
C2C2	X	C2C2
							total

P=	Frequency of C1 =
Q=	Frequency of C2 =

8.    What is going on?

Type your answer here. This textbox expands as you type.

9.    How have the relative proportions of C1s and C2s changed?

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10.    What principle have you demonstrated here?

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<scroll down for more>

Experiment 4

Go back to your population in Experiment 3.  This time, assume that through some further natural disaster which has discriminated against people with wavy hair, half the wavies have also been lost.  Allow this population to breed at random and determine the outcome of the next generation.

(see page 6 for detailed calculation help).

			MATINGS	OFFSPRING = Matings x 4	C1S	C2S
C1C2	X	C1C2
C1C2	X	C2C2
C2C2	X	C2C2
							total

P=	Frequency of C1 =
Q=	Frequency of C2 =

13.    What is going on this time?

Type your answer here. This textbox expands as you type.

14.    What if this trend continues?

Type your answer here. This textbox expands as you type.

SUMMARY

15.   Summarize what you have learned from this lab about the principles of evolution.

Type your answer here. This textbox expands as you type.

Define (one short sentence each):

1) Evolution

2) Microevolution

3) Macroevolution

4) Genetic Drift

5) Natural selection

What did you learn about or in each of the following?

a) The Hardy-Weinberg Equilibrium

b) Experiment 1

c) Experiment 2

d) Experiment 3

e) Experiment 4

 

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Question 1 

In the Yearly Mortality Bill for 1632, consumption referred to:

 

dysentery

 

tuberculosis

 

smallpox

 

edema

Question 2 

Cyclic variations in the occurrence of pneumonia and influenza mortality may reflect:

 

seasonal variations in cases of influenza.

 

the fact that influenza is a disappearing disorder.

 

long-term changes in mortality trends.

 

both A and B

Question 3 

Which of the following is not usually an aim of epidemiology?

 

To describe
the health status of the population

 

To fund new public health programs

 

To explain
the etiology of disease

 

To predict
the occurrence of disease

 

To control
the distribution of disease

Question 4 

Indicate the level of prevention that is represented by screening for breast cancer

 

Primary Prevention Active

 

Primary Prevention Passive

 

Secondary Prevention

 

Tertiary Prevention

Question 5 

The difference between primary and secondary prevention of disease is:

 

primary prevention means control of causal factors, while   secondary prevention means control of symptoms.

 

primary prevention means control of acute disease, while   secondary prevention means control of chronic disease.

 

primary prevention means control of causal factors, while   secondary prevention means early detection and treatment of disease.

 

primary prevention means increasing resistance to disease,   while secondary prevention means decreasing exposure to disease.

Question 6 

Indicate the level of prevention that is represented by pasteurization of milk

 

Primary Prevention Active

 

Primary Prevention Passive

 

Secondary Prevention

 

Tertiary Prevention

Question 7 

Determining workload and planning the scope of facilities and manpower needs, particularly for chronic disease. Is this a use for incidence or prevalence data?

 

This is a use primarily for incidence data.

 

This is a use primarily for prevalence data.

 

This application could apply equally for both incidence and   prevalence data.

 

This is a use for neither incidence data nor prevalence data.

Question 8 

An epidemiologic survey of roller-skating injuries in Metroville, a city with a population of 100,000 (during the midpoint of the year), produced the following data for a particular year:
 

Number of skaters in   Metroville during any given month

12,000

 

Roller-skating   injuries in Metroville

600

 

Total number of   residents injured from roller-skating

1,800

 

Total number of   deaths from roller-skating

90

 

Total number of   deaths from all causes

900

 

The cause-specific mortality rate from roller-skating was:

 

90/600 × 100,000

 

90/100,000 × 100,000

 

90/1,800 × 100,000

 

90/900 × 100,000

Question 9 

To provide a direct estimate of the risk of developing a disease. Is this a use for incidence or prevalence data?

 

This is a use primarily for incidence data.

 

This is a use primarily for prevalence data.

 

This application could apply equally for both incidence and   prevalence data.

 

This is a use for neither incidence data nor prevalence data.

Question 10 

Which of the following statements most accurately expresses the breeder hypothesis for schizophrenia?

 

The conditions of life in lower-class society favor its   development.

 

The conditions of life in upper-class society favor its   development.

 

The illness leads to the clustering of psychosis in the impoverished   areas of a city.

 

The illness is associated with increases in creative talents,   which contribute to wealth-enhancing achievements.

Question 11 

Descriptive epidemiology has the following characteristics (Choose the incorrect
option):

 

provides the basis for planning and evaluation of health   services.

 

allows causal inference from descriptive data.

 

allows comparisons by age, sex, and race.

 

uses case reports, case series, and cross-sectional studies.

 

identifies problems to be studied by analytic methods.

Question 12 

A null hypothesis is most similar to which of the following?

 

Positive declaration

 

Negative declaration

 

Implicit question

 

Explicit question

Question 13 

Which of the following data sources is most likely to provide a representative sample of the general health status of a population?

 

hospital outpatient statistics

 

absenteeism data

 

data from public health clinics

 

a morbidity survey of the general population

Question 14 

Cautious use of information from death certificates is warranted because:

 

certificates are not available for everyone who dies

 

certificates are often erroneous for date of death and sex

 

cause of death information may not be correct

 

autopsy results are not included

Question 15 

Ecologic studies:

 

are expensive and require a great deal of time to conduct

 

are a good approach for generating hypotheses

 

provide accurate measurements of exposure

 

yield results that can be applied directly to individuals

Question 16 

A large medical center’s oncology program reported an increased number of cases of pancreatic cancer during a certain month. The hospital’s epidemiologist decided to research the problem. Tumor registry records were searched to identify all cases of pancreatic cancer during a five-year period; cancer patients were matched with patients treated for other diseases during the same five-year period. All subjects in the study were questioned about lifestyle factors including alcohol, tea, and coffee consumption. The resulting data are as follows:

 

DATA

 

Cancer Patients

Other Patients

 

Men

Women

Men

Women

 

LIFESTYLE VARIABLE

 

Alcohol

185

120

270

260

 

Tea Drinking

140

110

230

225

 

Coffee Drinking

190

140

270

240

 

Note:   Total number of male cancer patients = 200.
Total number of female cancer patients = 150.
Total number of male patients (other diseases) = 300.
Total number of female patients (other diseases) = 300.
Does this study have an exposure status variable?

 

No

 

Yes, lifestyle

 

Yes, disease type

 

Yes, sex of patient

Question 17 

In case-control studies, the odds ratio is used as an estimate of the relative risk. In order for this approximation to be reasonable, some conditions must be met. Which of the following conditions is not necessary in order to use the odds ratio to estimate the relative risk?

 

With respect to exposure, controls are representative of the   population to which you want to generalize your results.

 

The event (disease) under study is rare in the population.

 

The exposure in question is rare in the population.

 

Cases are representative of all cases.

Question 18 

As an epidemiologist you are going to investigate the effect of a drug suspected of causing malformations in newborn infants when the drug in question is taken by pregnant women during the course of their pregnancies. As your sample you will use the next 200 single births occurring in a given hospital. For each birth a medication history will be taken from the new mother and from her doctor; in addition, you will review medical records to verify use of the drug. [N.B.: These mothers are considered to have been followed prospectively during the entire course of their pregnancies, because a complete and accurate record of drug use was maintained during pregnancy.]
The resultant data are:
Forty mothers have taken the suspected drug during their pregnancies. Of these mothers, 35 have delivered malformed infants. In addition, 10 other infants are born with malfunctions.
The number of individuals who both did not take the drug and did not give birth to infants who were malformed was:

 

140

 

150

 

155

 

160

 

170

Question 20 

Which of the following individuals helped draw people’s attention to the method of cohort analysis?

 

Snow

 

Frost

 

Graunt

 

Hill

Question 21 

A new screening test for Lyme disease is developed for use in the general population. The sensitivity and specificity of the new test are 60% and 70%, respectively. Three hundred people are screened at a clinic during the first year the new test is implemented. Assume the true prevalence of Lyme disease among clinic attendees is 10%.
Calculate the following values:
The predictive value of a positive test is:

 

33.0%

 

18.2%

 

94.0%

 

22.2%

 

6.0%

Question 22 

Drs. Poke and Jab (2014) conducted an employee health program that used 5 screening tests at the same time to detect diseases among workers. Which type of program is this?

 

Selective screening

 

Mass screening

 

Ad hoc screening

 

Multiphasic screening

Question 23 

Sensitivity and specificity of a screening test refer to its:

 

reliability

 

validity

 

yield

 

repeatability

Question 24 

You have just finished administering a food/drink questionnaire to ill and non-ill participants in a Minnesota summer picnic party. The ill individuals developed moderate to severe diarrhea 16 to 46 hours after the picnic. Six persons experienced vomiting. The following data were collected:

 

ATE

DID NOT EAT

 

Number of people

Number of people

 

Food item

Ill

Not ill

Total

Ill

Not ill

Total

 

Hot dogs

40

30

70

10

20

30

 

Hamburgers

32

8

40

20

40

60

 

Potato salad

45

25

70

15

25

40

 

Ice cream

48

12

60

2

38

40

 

Lemonade

20

40

60

20

20

40

 

Which food item appears to be the most probable vehicle for the salmonella (agent) infection associated with the illness?

 

Hot dogs

 

Hamburgers

 

Potato salad

 

Ice cream

 

Lemonade

Question 25 

An outbreak of salmonellosis occurred after an epidemiology department luncheon, which was attended by 485 faculty and staff. Assume everyone ate the same food items. Sixty-five people had fever and diarrhea, five of these people were severely affected. Subsequent laboratory tests on everyone who attended the luncheon revealed an additional 72 cases.
Foods served at the luncheon included home-canned olives, chicken salad, homemade flavored drink mix, freshly baked rolls, and raw vegetables. Based on your understanding of foods that potentially are capable of transmitting salmonella, the most likely source of the outbreak was:

 

home-canned olives

 

chicken salad

 

drink mix

 

freshly baked rolls

 

raw vegetables

Question 26 

The site where a disease agent enters the body is the:

 

reservoir

 

portal of entry

 

vehicle

 

portal of exit

Question 27 

A situation in which the combined effect of several exposures is greater than the sum of the individual effects:

 

threshold

 

latency

 

synergism

 

square

Question 28 

It has been suggested that occupational exposure to benzene in the petroleum industry increases the risk of developing leukemia. The levels of benzene to which workers in this industry have been exposed were high from 1940 to 1970, but since 1970 have been significantly reduced. What kind of study design, using petroleum workers, would provide the most useful information on whether benzene affects incidence rates of leukemia in this industry? You may assume that records of individual worker assignments to jobs involving benzene exposure have been maintained by the industry.

 

Experimental

 

Retrospective cohort

 

Prospective cohort

 

Case-control

 

Cross-sectional

Question 29 

The type A behavior pattern is hypothesized to be a risk factor for:

 

chronic obstructive pulmonary disease

 

coronary heart disease

 

rheumatoid arthritis

 

retirement

Question 30 

Which of the following statements describes a stressful life event?

 

discrepancy between husband and wife in social and educational   status

 

goodness of fit between the characteristics of the person and   environment

 

an occurrence that might cause readjustments in people’s   activities

 

sleeping

 

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