10. Errors and Redesign.
Throughout this experiment a number of random and procedural errors were apparent; these errors could have affected the results of the experiment in a number of ways. One experimental error that occurred during the experiment was that some flies became stuck in the food source and died. The main cause of this was the fact that the fly vials were stood up (vertically) before the flies had fully recovered from the anaesthetic. This could be overcome in future experiments by ensuring that the vials are kept horizontal until all of the flies fully recover from the anaesthetic.
One possible error that may have occurred was that some of the adult flies may have accidentally been left in the vials with their offspring, which would have affected the results due to the fact that these flies could have bred with their offspring. This could be overcome in further experiments by ensuring that all adult flies were either removed from the vial or pushed into the food source inside the vial.
It is also possible that some of the maggots and pupa in the vials were killed when the adult flies were anaesthetised. This would have reduced the total number of offspring from each generation ultimately lowering the accuracy of the experiment. This could be prevented in further experiments by anaesthetising and removing the flies faster to lower the amount of time the offspring were exposed to the CO2 gas or by using a less harmful anaesthetising agent.
It is possible that there were mathematical or calculation errors made during the experiment (for example when the fly totals were being tallied). Such errors could be overcome by being more thorough when counting flies and doing calculations, and by double checking calculations.
The sample size of this experiment was quite small; this may have affected the accuracy of the experiment, preventing the hypothesis from being tested properly. This could be overcome in future experiments by breeding more flies and performing multiple trials.
The virginity of the flies used in this experiment was guaranteed. However, it is possible, that the female flies were not virgins and had mated with other flies previously. Female Drosophila flies have seminal receptacles that collect sperm, which is used to fertilise all of the eggs that they have in their lifetime. This means that all of a female fly’s offspring could be from a single male making all offspring from a non-virgin female potential “errors” (This is important as a single female fly can lay hundreds of eggs in her lifetime). This would affect the results obtained by both the F1 and F2 generations. It is important to note that no flies with phenotypes other than those predicted were observed. This could be prevented in future experiments by checking the female fly vial thoroughly for eggs, larva and pupa before placing the male flies with them, as this would help ascertain wether the females were truly virgins.
Inaccurate identification of flies may have cause the validity of the experiment to be diminished. One of the main reasons why identification errors may have taken place is that flies with lozenge eyes were hard to identify and required the use of a stereo microscope. When the microscope was not properly in focus gridded flies could easily be mistaken for lozenge flies. Another major source of identification error may have been due to immature males and females as these could easily be confused. In this case the use of a stereo microscope was needed in order to look for the distinct sex combs on the male flies. As this experiment was performed in a group in which a number of individuals sorted the flies the results may have varied from what a single individual would get if they had sorted and counted all of the flies themselves. In future the flies could be double checked by a specific group member in order to standardise the results.
Drosophila melanogaster rely heavily on their sense of sight, (2/3 of their brain capacity is dedicated to image analysis.) because of this the white lozenge males may have been at a general disadvantage and may have been more prone to getting stuck in the food supply. This may have been the cause of the low numbers of white lozenge in the F2 generation of flies. However, the cause of white eyes is a defective red pigment gene and should not affect the vision of the flies, whereas the lozenge gene should have a greater affect due to it causing the malformation of the fly’s eyes. Therefore the lozenge flies should have also been in lower than expected numbers, but it was found that they were actually in higher than expected numbers making the validity of this argument questionable.
It is possible that the temperature at which the flies were kept dropped significantly below 20 degrees Celsius; this could have caused the death of some of the files or slowed down their growth and reproduction rate. This would result in there being fewer flies. This problem could be overcome in further experiments by using a larger heating device with a more responsive thermostat to keep the Fly house within the recommended temperature range for D. melanogaster.
It is also possible that a number of other random experimental errors (not mentioned above) affected the results of this experiment (for example the possible death of flies and their offspring due to mites and mould). These random errors could also be overcome by doing a large number of trials using a larger number of flies than used in this experiment.
The purpose of this experiment was to study the inheritance of the sex-linked genes for both lozenge and white eyes in Drosophila melanogaster flies from a sample of pure bred white lozenge eyed males, and to examine the affect of linkage distance on the assortment of alleles during meiosis, in three generations of Drosophila melanogaster flies. This was done by comparing the predicted and actual values for the inheritance of these alleles.
Drosophila melanogaster is used extensively in genetic research. Some of the reasons that Drosophila melanogaster are so popular for genetic research are that they are quite small and are easily reared in the laboratory. They have a short life cycle, which allows for a new generation of flies to be produced every two weeks. Female fruit flies can lay hundreds of fertilised eggs during their short life span, which allows for larger populations that
give easy and reliable statistical analysis. D. melanogaster’s embryos grow outside the body allowing for every stage of development to be studied. Two other benefits of this organism are that their genome is relatively small (less than one tenth of that of humans and mice) and that there is already a large amount of research available to scientists.
The inheritance of the white and lozenge alleles was greatly affected by the crossing over that occurred on the X chromosome. Crossing over is a process in genetics by a homologous pair of chromosomes exchange equal segments of DNA with each other. Crossing over occurs in the first division of meiosis and results in the recombination of genes found on the same chromosome, called linked genes that would otherwise always be transmitted together. Because the frequency of crossing over between any two linked genes is proportional to the chromosomal distance between them, crossing over frequencies are used to construct genetic, or linkage maps of genes on chromosomes. Mutations, temperature changes, and radiation all affect crossing over frequency. Crossing over can occur in a single pair of chromosomes up to four times during meiosis. This assists evolution and reduces the genetic linkage between genes on the same chromosome.
The major reason for a fly to have a certain phenotype in this experiment was sex linkage. Sex linkage is where a recessive allele is carried on the X chromosome, because it is on the X chromosome males will show the phenotype resulting from that allele at a much higher frequency than a female will. This is because a female (XX) requires two X chromosomes with the recessive allele to show its phenotype, whereas a male (XY) only requires one X chromosome with that allele.
In this experiment both of the wild alleles (red and gridded) were dominant over the recessive mutant genes (white and lozenge). A dominant gene is one that overrides the effect of another. This means that whenever a fly was heterozygous to both the wild and mutant genes its phenotype would be that of a wild fly. This can be seen in the offspring from the first cross.
The genes for lozenge and white eyes are linked as they are on the same chromosome. This means that there is a chance of crossing over occurring resulting in these genes being separated. The total distance between these two genes was 26.2 mapping units meaning that there was a 26.2% chance of the genes not being together on the same chromosome after meiosis. This percentage was high enough for crossing over to be evident in the results from the small sample of flies used in this experiment.
The parent cross of true breeding white lozenge males and wild females resulted in a F1 generation ratio of 1 wild male: 1 wild female: 0 mutant males: 0 mutant females. This ratio was identical to the predicted ratio for this generation. The chi-square analysis of these results showed that the predicted cross was highly supported by the data (P was greater than 0.99). The reason why there were no mutant flies in this generation was that both the males and females obtained the wild alleles (which are dominant) from their mothers, which prevented the mutant genes from being expressed.
The results of the F1 fly cross were predicted to result in a F2 phenotype ratio of 50% wild females: 18.45% wild males: 6.55% white males: 6.55% lozenge males: 18.45% white lozenge males. The reason for there being 50% wild females was that the F1 males gave either the Y chromosome or a X chromosome containing wild genes to their offspring, meaning that any cross with these males would result in 50% wild females and 50% male (whose phenotypes are dependant on the X chromosome given by the female F1 fly) flies.
The above predicted percentage ratio shows that when 166 flies are bred (using F1 males and females) the ratio will be 83 wild females: 31 wild males: 11white males: 11 lozenge males: 31 white lozenge males. The actual results of the F1 fly cross was 94 wild females: 20 wild males: 20 white males: 12 lozenge males: 20 white lozenge males. These results strayed from the predicted results, although the results did show a general pattern that seemed to support the predicted ratios. The reasons for the results straying from the predicted results could be explained by the errors stated in the Errors and Redesign section of this report.
The results from the F1 cross did not support the hypothesis (predicted crosses) of this experiment as the P value was less than 0.01. The reasons for this have been stated in the Errors and Redesign section of this experiment. It is unlikely that this hypothesis is inaccurate as the linkage distances used in this experiment came from professional scientists who would have used a greater number of flies and trials than used in the experiment. Despite this fact further experimentation could be done to either prove or disprove the results of these researchers.
This experiment could be used to find the linkage distance between certain genes in Drosophila melanogaster as well as other organisms; however a larger number of trials involving a larger number of flies would have to be performed in order to get a precise linkage distance. These linkage distances could be added to linkage maps in order to gain a better understanding of where specific genes are within certain chromosomes.
There are a number of other practical applications of this experiment and genetics in general. One of these is creating biological “machines” for specific purposes. These organisms behave in predictable ways, use interchangeable genetic components and have special genetic code that allows them to do things that no other organism can. These organisms could be used to repair humans, detect certain chemicals, create biological computers, create energy or turn wastes into useable items.
These organisms could significantly change the way in which we live. Another practical application of this specific experiment could be the use of fast breeding mammals that have a similar genetic structure to humans in helping to find the exact location of sex linked genes responsible for diseases such as: Red and green colour blindness, Haemophilia and Duchenne muscular dystrophy. This could aid in producing a cure for these diseases or help to produce a test to ascertain whether a baby was going to have these diseases even before it is born.
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Manning, G. (2005). Introduction to Drosophila. [Online] Available:
http://ceolas.org/VL/fly/intro.html [August 20, 2005]
Manning, G. (2005). About Drosophila. [Online] Available:
http://www.ceolas.org/fly/ [August 20, 2005]
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http://www.cafamily.org.uk/inherita.html [August 20, 2005]
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