fruit fly experiment

Fruit fly experiment

Introduction
The purpose of our fruit fly experiment was to learn about genetics, which is the study of DNA, genes and traits. We also studied mutations, which are the changing of an organism's genes and DNA sequence which makes the organism different than that of its species, and the effects of it on our flies I tested the sex-linkage model of inheritance, and then predict the results using a Punnett square. I crossed female flies homozygous for the normal allele with a male that expresses the mutant trait. If this gene is on a sex chromosome I will see a different pattern than if the gene is on an autosome. ( Martin Brookes, March 2001)
Fruit fly Drosophila melanogaster is very suitable for experimental crosses in the classroom laboratory to demonstrate classical Mendelian laws and to study the inheritance of selected qualitative traits. It is an ideal organism for genetic studies because it has the following characteristic:
• has a short life cycle (10-14 days at 25°C)
• Possesses an abundance of genetic variability (body colour, eye colour, wing shape, wing length, etc)
• Is small and easy to handle and culture.
• Is convenient and inexpensive.
Drosophila melanogaster was among the first organisms used for genetic analysis, and today it is one of the most widely used and genetically best known of all eukaryotic organisms. Charles W. Woodworth is credited with being the first to breed Drosophila in quantity and for suggesting to W. E. Castle that they might be used for genetic research during his time at Harvard University. Thomas Hunt Morgan began using fruit flies in experimental studies of heredity at Columbia University in 1910. Morgan and his students eventually elucidated many basic principles of heredity, including sex-linked inheritance, epistasis, multiple alleles, and gene mapping. (Morgan, T. H., 1909)







Method
I used “Ether-ators” to put the flies to sleep. I placed the etherator sponge side down on the bench, removed the cone top, and tapped the fly vial gently to send the flies down to the blue media. I quickly placed the etherator on top of the fly vial. I inverted the two so that the fly vial is on the etherator. I tapped gently to send the flies down to the snoozing chamber. I quickly placed the cone lid back on the etherator and gave the flies three minutes to sleep.
Then, I poured the flies out onto the card for observations. I observed each of the stages: larva, pupa and adult under a dissecting microscope. I counted the flies by categorizing on their sexes. Males are smaller than the females with five abdominal segments. Males have dark hair like projections and solid colored tip on their abdomen. Females have stripes on their abdomen.
To calculate the chi square statistic x2, for each observed number in the table subtract the corresponding expected number (O — E), square the difference [(O —E)2 ], divide the squares obtained for each cell by the expected number for that cell [ (O - E)2 / E ]. Finally, sum all the values for (O - E) 2 / E.
To find the degrees of freedom, I subtracted one from the number of classes. There were four classes,therefor 4-1=3. Then I compared calculated value to "critical" value in a table at appropriate degrees of freedom and significance level. This indicates whether or not there is a significant difference between the actual and predicted values. Look up the chi-square value on a Chi-square distribution table; find the degree of freedom and the probability of 0.05; move down the columns and across rows to find the Chi-square value. If your value is lower than 7.815, then accept the null hypothesis of no difference. (Alemadi, 2011)
Results
The hypothesis of the experiment is traits could in fact be passed on in the same manner predicted by the inheritance of sex chromosomes. The inheritance of the sex determining chromosomes in Drosophila seemed to follow closely with the inheritance of the white-eye phenotype. The data I collected in vial #1, 2 and 3 are accepted but the data I collected on vial #4 is rejected. According to the statistical results Vial #1(0.34), vial#2(0.364) and vial#3(1.8) X^2 values were less than 7.218 which support our predictions and support the null hypothesis. Via #3 did not support our hypothesis. The computed results are less than 7.218 because our actual result is reasonably close to the expected result with minor variations. Vial #2 is so important. If a gene is on sex chromosome we will see different pattern than if it is on autosome. We check this phenomenon by comparing vial #2 between the sex chromosome prediction and the autosome prediction.



Actual and expected counting results of flies
Females males
vial actual expected actual expected
red white red white red white red white
1 325 0 334 0 342 1 334 0
2 274 4 274 0 6 264 274 0
3 135 7 137 0 125 7 137 0
4 4 394 0 356 6 308 0 356

Results of Chi-square Analysis
vial Chi-square calculated Chi-square from table Accept/reject null hypothesis
1 0.34 7.815 accept
2 0.364 7.815 accept
3 1.8 7.815 accept
4 10.53 7.815 reject

Discussion
The tested null hypotheses were accepted. As explained in class the genes are linked. The mutant gene for eye color appears on chromosome number. According to results, the chi-square calculated numbers indicate reason to accept the hypotheses because the observed data had only small difference from the expected. The actual results of an experiment are unlikely to match the expected results precisely. But how great a variance is significant. As such “implies” the difference can be due only to chance sampling variation. Had the statistic been greater than the critical value the null hypothesis would have been rejected? Vial #4 has significantly much variation than the expected value. Reasons for this outcome range from numerous potential errors. It is possible I did not count and or observe all the flies in the vials. Flies often “escape” despite best efforts to contain them. It is very likely they flew away. Some flies were still in their pupae stage and thus yet to enclose. If the count was done two days later maybe different ratios would have been observed supporting the null hypotheses. Other error could have been the results of poor cultivating on my part. Flies could have “hid” themselves under shed exoskeleton, the media, etc. thus it is possible all the adult flies were not removed from the vials after each count. In general the experimentation is recommended to determine the null hypotheses truly should be accepted. The only problem we had was that we had contamination in vial two which probably messed up some of our data. If we were to repeat this we would probably double check all of our work. In conclusion the experiment helped us to learn that genes are related to chromosomes. (Molly Runner, 2000)


References
Benson, K. R. T. H 2001
Morgan, T. H., 1909
Martin Brookes, March 2001
Molly Runner, 2000