Genetics of Organisms

Introduction

Drosophilia melanogaster, the fruit fly, is an excellent organism for genetics studies because it has simple food requirements, occupies little space, is hardy, completes its life cycle in about 12 days at room temperature, produces large numbers of offspring, can be immobilized readily for examination and sorting, and has many types of heredity variations that can be observed with low power magnification. Drosophila has a small number of chromosomes ( four pairs). These chromosomes are easily located in the large salivary gland cells.

The Life Cycle of Drosophila

 

Figure 7.1 The Life Cycle of Drosophila melanogaster

 

 

Design of the Exercise

This genetics experiment will be carried on for several weeks. Drosophila with well-defined mutant traits will be assigned to you by your teacher. You are responsible for making observations and keeping records concerning what happens as mutant traits are passed from generation to the next.

You will be assigned to study a certain mode of inheritance using particular genetic crosses of flies having one or two mutations. The modes of inheritance most commonly used are:

Procedure

1. Obtain a vial of wild- type flies. Practice immobilizing and sexing these flies. Examine these flies and note the characteristics of their eyes, wings, bristles, and antennae.

2. To make handling easier, immobilize the flies with fly-nap, or by twirling the vial in ice for several minutes. Place the immobilized flies on a piece of filter paper inside a petri dish. Place this under a dissecting microscope to view the flies.

3. Distinguish male flies from female flies by looking for the following characteristics:

a). Males are usually smaller than females.

b). Males have dark, blunt abdomens, and females have lighter, pointed abdomens.

c). Only males have sex combs. which are groups of black bristles on the upper most joint of the forelegs.

 

Figure 7.2: Female and Male Drosophilia

 

Female

 

Male

 

Male with vestigial wings

 

4. Obtain a vial containing pairs of experimental flies. Record the cross number of the vial. This number will serve as a record as to which cross you obtained. These flies are the parental generation(P1) and have already mated. The female should have already laid eggs on the surface of the culture medium. The eggs represent the first filial, F1 generation and will be emerging from their pupal cases in about a week.

5. First week (today): Immobilize and remove the adult flies. Observe them carefully under the dissecting microscope. Separate the males from the females and look for the mutation(s). Note whether the mutation(s) is/are associated with the males or females. Identify the mutation(s) and give them a made up name and symbol. Record the phenotype and symbol in Table 7.1. The findings should be confirmed by your teacher.

6. Place the parents in the morgue(jar containing alcohol). Label the vial containing the eggs or larvae with the symbols for the mating. Also label the vial with your name and date. Place the vial in a warm location.

7. Second week: Begin by observing the F1 flies. Immobilize and examine all the flies. Record their sex and characteristics. Consider the conclusion s that can be drawn from these data. Place five or six pairs of F1 flies in a fresh culture bottle and the rest of the flies in the morgue. For this cross the females need not be virgins. Label the vial with the symbols , name, and date.

8. Third week: Remove the F1 flies from the vials and place them into the morgue. The F2 generation are the eggs and /or larvae in the vial. Place the vial in a warm place.

9. Fourth week: Begin removing the F2 flies. Record their sex and the presence or absence of mutation(s). The more F2 flies collected, the more reliable the data will be. You may have to collect flies over a three-or four day period. Try to collect at least 200 flies.

10. To analyze your data, you will need to learn how to use the Chi-Square Test. Go to the Statistical Analysis Section to review the technique.

 

Table 7.1: F1 Generation Data

Date _________________

Phenotype and Symbol
Females
Males
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eeeeejjjjjjjjjjje
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eeeeeebbbbbbbbbbb
eeeeejjjjjjjjjjjjjjje
eeeeee
eeeeeebbbbbbbbbb
eeeeejjjjjjjjjjjjjjjjjje
eeeeee
eeeebbbbbbbbbee
eeeeejjjjjjjjjjjjjjje
eeeeee
eeeebbbbbbbbbbbbee
eeeeeejjjjjjjjjjjjjjjj

 

Table 7.1: F2Generation Data

 

Date _________________

Phenotype and Symbol
Females
Males
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eeebbbbbbbbbbbeee
eeeeeejjjjjjjjjjj
eeeeee
eeeeeebbbbbbbbbbbb
eeeeejjjjjjjjjjje
eeeeee
eeeeeebbbbbbbbbbb
eeeeejjjjjjjjjjjjjjje
eeeeee
eeeeeebbbbbbbbbb
eeeeejjjjjjjjjjjjjjjjjje
eeeeee
eeeebbbbbbbbbee
eeeeejjjjjjjjjjjjjjje
eeeeee
eeeebbbbbbbbbbbbee
eeeeeejjjjjjjjjjjjjjjj

Analysis of Results

 

1. Describe and name the observed mutation(s).

___________________________________________________________________________________

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2. Write a hypothesis which describes the mode of inheritance of the trait(s) you studied. This is your null hypothesis ( as described in the statistical analysis section).

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3. Refer to a textbook and review Punnett squares. In the space below, construct two Punnett squares to predict the expected results of both the parental and F1 crosses from your null hypothesis.

Parental Cross
F1 Cross
f
r

4. Refer to the Punnett squares above. Record the expected ratios for the genotypes and phenotypes of the F1 and F2 in the experiment below.

 

Expected Genotypic Ratiollll
Expected Phenotypic Ratiolll
F1
dddddddddd
dddddddddd
F2
dddddddd
ddddddddddd

5. Do the actual results deviate from what was expected? If so explain how.

________________________________________________________________________

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6. From the results describe your cross: is it

Sex-linked or autosomal?fffffffffffffffffffffffffffffffffffffffffffffffffffff_________________________

A dominant mutation or a recessive mutation? fffffffffffffffffffffffffffff_________________________

Monohybrid or dihybrid? ffffffffffffffffffffffffffffffffffffffffffffffffffff _________________________

 

7. Are the deviations for the phenotypic ratio of the F2 generation within the limits expected by chance? To answer this question, statistically analyze the data using the Chi-square analysis. Calculate the Chi-square statistic for the F2 generation in the chart below.

 

Observed Phenotypes (o)
Expected (e)
(o-e)
(o-e)2
(o-e)2

e

rrrrrr
rrrrrr
rrrrrr
rrrrrr
rrrrrr
rrrrrr

rrrrrr

rrrrrr

rrrrrr

rrrrrr

rrrrrr

rrrrrr

rrrrrr
rrrrrr
rrrrrr
rrrrrr

rrrrrr

rrrrrr

rrrrrr

rrrrrr

X2 =

a). Calculate the Chi-square value for these data.

sssss1. How many degrees of freedom are there? ___________________

sssss2. Chi-square (X2) = __________________

sssss3. Reffering to the critical values chart, what is the probability value for these data? _____________

 

b). According to the probability value, can you accept or reject your null hypothesis? Explain why.

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Topics for Discussion

1. Why was it necessary for the females of the parental generation to be virgin?

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2. Why was it not necessary to isolate virgin females for the F1 cross?

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3. Why were the adult flies removed from the vials at weeks 2 and 4?

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