AP Biology Lab 7 :Genetics of Organisms

Introduction

Drosophila Melanogaster, the fruit fly, is a great organism for genetic use because it has simple food requirements, occupies little space, is hardy, completes its life cycle in 12 days, makes a large number of offspring, can be knocked out easily, and it has many types of hereditary variations that can be seen with low power microscopes. Drosophila has a small number of chromosomes, four pairs. They are easily located in the large salivary glands. The Drosophila can be obtained from many places. Research of Drosophilae has led to a lot of knowledge about many of its genes.

Many factors combine to affect the length of the Drosophila life cycle. Temperature affects the life cycle the most. At room temperature the average life cycle of the Drosophila is about 12 days. Eggs of the Drosophila are small, oval shaped, and have two filaments at one end. They are usually laid on the surface of the culture medium, and with practice, can be seen with the naked eye. After one day the eggs hatch into the larva.

The larval stage of the Drosophila eats all the time. Larvae tunnel into the culture medium when they eat. The larva will shed its skin as it increases in size. In the last of the three larval stages, the cells of the salivary glands contain giant chromosomes that can be seen under low power in a microscope.

The pupal stage. Before a larva becomes a pupa it climbs the side of the container. The last larval covering then becomes harder and darker, forming the pupal case. Through this case the later stages of metamorphosis to an adult fly can be seen. In particular, the eyes, the wings, and the legs become visible.

The adult stage. When metamorphosis is over, the adult fly emerges form the pupal case. They are fragile and light in color and their wings are not fully expanded. They get darker in about an hour. They live about a month and then die. A female refrains from mating for about 12 days after she emerges from the pupal case. After she mates her receptacles contain large amounts of sperm and she lays her eggs. Make sure that the first flies you use are virgins.

Methods

1.  Separate fruit flies from original vial with eggs and medium into new vial with adult flies only

2. Sedate the fruit flies with anesthetic 3. Put fruit flies onto blank paper4. Put blank paper under microscope5. Observe fruit flies for gender-specific differences such as sex combs in males and

larger body shape in females 6. Separate fruit flies into corresponding male vial or female vial

Results 

Red Male × Sepia FemaleR = Dominant red-eye trait (wild type)

r = Recessive sepia-eye trait (mutant)

P Generation:[RR] × [rr]

F1 generation

F2 generation

R R

r Rr Rr

r Rr Rr

R r

R RR Rr

r Rr rr

F1 F2

Expected 1 Red Male1 Red Female

3 Red: 1 Sepia1 Male: 1 Female

Actual 42 Red23 Female

37 Red Male34 Red Female10 Sepia Male

16 Sepia Female

Chi-Square Analysis

Introduction

Statistics can be used to determine if differences among groups are significant, or simply the result of predictable error. The statistical test most frequently used to determine whether data obtained experimentally provide a good fit, or approximation to the expected or theoretical data is the Chi-square test. This test can be used to determine if deviations from the expected values are due to chance alone or to comeother circumstance.

To determine if the observed data fall with in acceptable limits, a Chi-Square analysis is performed to test the validity of a null hypothesis; that there is no statistically significant difference between the observed and expected data. If the Chi-Square analysis indicates that the data vary too much from the expected 3: 1 an alternative hypothesis is accepted.

Methods

The formula for Chi-square is:

X2=E(o-e) 2             E

O= observed number of individuals

e= expected number of individuals

E= the sum of the values

The (df) are determined by taking the number of possible phenotypes and subtracting one from it. If the Chi- Square answer is greater than the number from the critical values chart then the null hypothesis is incorrect. The results are said to be significant at .05. This means that only 5 % of the time you would expect to see similar data if the null hypothesis were correct. The probability can also be rejected at .001. This time it means that less than 1 % of the time would you expect to see similar data.

Results

Critical Values Chart

Degrees of Freedom (df)

1 2 3 4 5

.05 3.84 5.99 7.82 9.49 11.1

.01 6.64 9.21 11.3 13.2 15.1

.001 10.8 13.8 16.3 18.5 20.5

Observed Phenotypes

Expected (e)

(o-e) (o-e)2 (o-e)2/ e

LL 333 -103 10609 31.86

Ll 666 -156 24336 36.54

ll 333 -73 5329 16.00

84.4

Error Analysis

Results from this lab could have been affected by many things. The constant knocking out of flies could have caused some of the larvae to not hatch therefore affecting our numbers. Also, incorrectly identifying the characteristics of the flies could have also greatly affected the results received. Improper calculation of numbers could have also caused inaccurate results. Finally, some flies could have gotten stuck in the medium and could have been identified.

Conclusion

From the results of the experiment I can conclude that I received results that were close to a 1:1 ratio. The Chi- Square worked from my data was accepted at a possibility greater than .05. The null hypothesis in this case can be accepted.