Observable Phenotypes in Crossbreeding

Drosophila melanogaster
Drosophila melanogaster have been very useful in determining genetic patterns  across generations of these organisms. This study was conducted to determine the inheritance patterns and whether there is predictable results in the independent assortment of alleles. Over the course of seven weeks, crossbreeding between flies was observed and the results from the F2 generation were used to determine genetic patterns. The results of the data derived from the laboratory assignment were consistent with the ratios derived from the Punnett Square.

Genes are units of heredity first discovered by Gregor Mendel in the simple traits he saw in crossbreeding his pea pods. The simplistic traits had only two alleles to distinguish between genotypes. These traits were easily observable phenotypes round peas versus wrinkled peas and yellow peas versus green peas. Though his work didnt take the complexity of multiple-allele traits and other intricacies into account, Mendels work served as a foundation for all genetic research in the future.

Drosophila melanogaster have prominent features that are easily distinguished as well. The fly has red eyes, a brown body, antennas, wings, and any variations in these prominent features are easily observable under the a microscope. Despite the fact that the phenotype and genotypes of flies were easily observable, the biochemical mechanisms at work on a molecular level were observed more clearly in unicellular organisms such as bacteria or simple eukaryotic organisms such as fungi.

In 1910, Thomas Hunt Morgan  identified the fruit flies as a good organism for the study of genetics due to their short lifespan between generations. Morgan was able to identify the dominant red alleles he found in the eyes of the Drosophila melanogaster by crossbreeding homozygous white-eyed flies, which were found to be recessive. Anytime a white homozygous individual was crossbred with a red homozygous or heterozygous individual, the resulting offspring were red-eyed.

Drosophila melanogaster have about 5000 genes. Most of these genes are necessary for survival such as the translation of various proteins. Other genes are used for specific, observable traits, or phenotypes. Mutants in any of these genes are obvious in the malformations or abnormalities of the physical characteristic.

Wild type Drosophila melanogaster are defined as having absolutely no mutations in their genomic make up. Genetic recombination factors should provide a mutant gene that provides for the genetic variations seen between generations of drosophila. Generally F2 generations will show the cross-linkages that are a natural occurrence during mitosis. When the arms of two chromosomes are close together, there is a strong possibility of recombination. At this proximity, genes can transfer to the other arm and alter the original chromosomes.


The whole process for obsecrating the drosophila is around seven weeks.
Week 1
A vial was placed in a clear space with the foam stopper in easy reach. Then, the vial with the flies was placed in the flies in the ice and was waited no more than 1 minute. The vial was removed from the ice and was quickly transferred into beside the newly prepared vial.

By now the flies should be immobile. The vials were tapped gently on the bench to position flies at the bottom of the vial. The foam stoppers were removed from the vials and was put the mouths of the two vials together so no flies could escape. The immobile flies were tapped down into the desired vial and were replaced foam stopper. It should be careful not to disturb the food in this process as it would engulf the flies and kill them.

Your tutor or Patrick would be called if the flies were still highly mobile after no more than 1 minute in the ice.

With the help of your tutor and Patrick, the male flies were decanted into the female fly vial (or vice versa).

The drosophila was observed for a few moments as they got used to each other.

The vial with the male and female flies in it was labelled as described above.

What happens next
The flies that had just introduced to each other were constituted the parent generation or P.

Their offspring would be constituted the F1 generation.

The parent female would lay eggs,  and it was necessary to decant the P flies before the F1 flies emerge from their pupa. This was to ensure that there is no breeding between the parents and their offspring. This would be done  by Patrick.

Week 2
After 2 weeks the F1 Flies would had hatched, mated and laid eggs, which would be the F2 generation. At that stage the F1 flies could be decanted, killed, identified and counted ( and were not needed any more).

Week 5
The vial was put into ice for 5 minutes or until all the flies in the cial were immobile. Then the flies was decanted into a vial with no food, Ethanol was added to kill the F1 FILES. The dead flies was observed under the microscope to confirm their gender and traits. Data was recorded in the Experiment journal.

Week 7
F2 offspring was observed for each cross.

The F1 generation of flies were kept consistently paired with one another and the results of the F2 generation are shown in the following charts.  The ratio of genotypes follows a consistent pattern with the Mendelian  model of anticipated offspring.

X4t X4T
X4t X4tX4tX4TX4tYX4t YX4T Y

In crossbreeding the generations, the patterns in the eye color of the Drosophila melanogaster is consistent with the ratios predicted by Punnett Squares. In the dominant traits expressed here, the heterozygous dominant alleles produced the dominant characteristic. Homozygous dominant produced the dominant traits and homozygous recessive were the only alleles that produced a recessive phenotype. All of the results were consistent with the predictive model of the Punnett Square.


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