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Drosophila melanogaster-A Model Organism

by: Maryna Gregulich

Drosophila melanogaster-A Model Organism 3251

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Maryna Gregulich


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This report demonstrates an investigation of genetic modes of inheritance in which Drosophila melanogaster is used as a model organism to identify mechanisms of genetic transmission in eukaryotic o...
Genetics lab
Kathrine Sampuda
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This 12 page Bundle was uploaded by Maryna Gregulich on Sunday February 14, 2016. The Bundle belongs to 3251 at a university taught by Kathrine Sampuda in Fall 2015. Since its upload, it has received 15 views.


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Date Created: 02/14/16
Drosophila melanogaster-A Model Organism Name: Maryna Yevsyukov Genetics 3251/F-13:50 Instructor: Kathrine Sampuda Date: 10/15/14 Abstract This report demonstrates an investigation of genetic modes of inheritance in which Drosophila melanogaster is used as a model organism to identify mechanisms of genetic transmission in eukaryotic organisms. In this experiment, two separate crosses were made, one by crossing a wild type female with a mutant male and the other by crossing mutant female with wild type male. The two F2 generations were obtained by performing 2 simple F1*F1 crosses. The F2 generations were the subject of genetic testing of phenotypic inheritance pattern. Hence, the main purpose of these crosses was to determine whether the wild type and mutant genetic traits are autosomal or sex-linked. Introduction In the 19 century, Gregor Mendel discovered the principles of inheritance through series of experiments with his garden peas. In his work, he determined that physical characteristics are transmitted from generation to generation according to some general patterns. Mendel supported his idea through series of genetic laws that later became known as Mendelian Genetics. Mendel’s “Law of Segregation” states that during gamete production, the two copies of each genetic factor separate so that offspring obtain one factor from each parent (W. Bateson, 1996 ) The “law of Independent Assortment”, states that the inheritance pattern of one trait will not affect the inheritance pattern of another. While Mendel's experiments with crossing one trait always resulted in a 3:1 ratio between dominant and recessive phenotypes, his experiments with mixing two traits ,a dihybrid cross, showed 9:3:3:1 ratios. He concluded from those results that each of the two traits are independently inherited with a 3:1 ratio (W. Bateson, 1996). Therefore, the main goal of this experiment would be detecting the mode of inheritance of each trait by analyzing the phenotypic ratios of the offspring generations and determine whether our data corresponds with Mendel’s theory. Henceforward, the hypothesis for this experiment is that the wing shape trait would exhibit a 3:1 ratio between autosomal /dominant and recessive traits and 1:1 for the eye color trait. I. Background Drosophila melanogaster, commonly known as fruit fly, has been used as a model organism for various research purposes for more than a century. Thomas Hunt Morgan was the first biologist studying the fruit fly in the 1900’s and the first who discovered the sex linkage and genetic recombination, putting the small fly in the front of genetic research (T. Morgan, 1915). II. Logistics -why use Drosophila melanogaster in experiments? Model organism: The fruit fly, has been a very useful organism for the study of genetics due to its correspondence to several criteria:  A relatively short generation time (approximately 10 days at 25oC)  Due to its simple food requirements and easy handling in the laboratory, large and varied cultures of Drosophila can be maintained within minimal cost and effort.  Adult flies are ready to mate within 10-12 hours after they emerge and can live under laboratory conditions for about a month.  Yield a large amount of breeding data in a short period of time.  Similar genome to humans (M. Ashburner and J.N. Thompson, 1978). III. Life cycle There are four stages in fruit fly development process, and they are: egg, larva, pupa, and adult. Stage 1: Egg – Fruit flies can lay up to 500 eggs in their lifetime of less than two weeks, and they will only lay their eggs in the surface of moist, organic material. Stage 2: Larva – approximately 24 hours after an egg is laid, the fruit fly will emerge as a larva. This is where the fruit fly starts to take its shape. Stage 3: Pupa – This final formative stage gives the fruit flies their wings and color. Stage 4: Adult – After four days the metamorphosis is complete, the fly is fully formed, it emerges as an adult and can begin reproducing after about 12 hours (M. Ashburner and J.N. Thompson, 1978) V. Possible Inheritance mode Based on the genetic research conducted by Hunt in 1910, the experiment examines the 4 possible inheritance modes in the fruit fly.  Autosomal Dominant/Recessive: males and females are affected with equal frequency as a result of a defective gene on an autosome.  Sex-linked Dominant/recessive: alleles that are located on the sex chromosomes are inherited in predictable patterns. Females could be heterozygous for a particular trait. (M.T, Hunt. 1910) Materials and methods I. In order to fully understand the concepts learned in this experiment one must be able to comprehend the distinguishing characteristic; hence, the morphology and heredity traits of Drosophila melanogaster. Males vs. Females: • Body size- female is generally larger than male. • Abdominal shape- the female abdominal curve to a point and the male abdominal much shorter is round and • Sex comb- only in males there is a fine, hair like structure located on the very tip of the front leg (M. Ashburner and J.N. Thompson, 1978) Wild type vs. Mutant: For the purpose of this experiment, each phenotype will have its specific notation, red eyes and normal wings (+/+) or white eyes and apterous wings (W/AP) • Eye color Wild type- red eyes. (The most abundant in nature) Mutant- white eyes. •Wings shape Wild type- smooth edges, uniforms venations, extends throughout the body. Mutant- Apterous wings lack of fully functional wings. Before we begin the crossing experiment: 1. Anaesthetize the flies- Rods soaked in Fly Nap are inserted past the foam stopper into a vial containing the flies. The vial is then placed on its side, wand up, until all flies are unconscious. The unconscious flies can be poured out of the vial onto a white card with the aid of a paintbrush. 2. Identify sex and the sort flies in vials -dissecting microscope can be used to identifying sex, eye color and wing shape. 3. New crosses are made by transferring a virgin female and a male to a culture vial containing fresh food. Any fly not needed for further experimentation is placed in the morgue. Set up Cross I: Parental generation: (+/+) ♀ x (w/apt) ♂ F1: ♀ ♂ F2 ♀ ♂ +/ 209 183 + +/+ 32 33 +/a 0 0 3 2 pt +/a 16 11 w/ 0 0 pt + w/+ 8 49 w/a 0 0 w/a 0 5 pt pt Results: Test hypothesis w/ X^2 analysis (eye color): 1:1 Observe expected (sum)(o- e)^2 /e d Degrees of freedom: 1 red 68 744*(2/4)= (682- 2 372 372)^2/372=2 58.3 whit 62 744*(2/4)= (62- 372)^2/186=5 e 372 16.66 X^2= 774.99 Test hypothesis w/ X^2 analysis (Winshape): 3:1 Observed expected (sum)(o-e)^2 /e Apt. 32 744*(1/4)=186 (32- 186)^2/186=127.505 Normal 712 744*(3/4)=558 (712- 558)^2/558=40.0179 +/+ 101 150 X^2= 167.529 +/apt 18 13 w/+ 66 47 Set up Cross II: Parental: (w/apt) ♀ x (+/+) ♂ w/apt 16 17 F1: ♀ ♂ F2: ♀ ♂ +/+ 65 0 +/a 0 0 pt w/+ 0 58 w/a 0 0 pt Results Test hypothesis w/ X^2 analysis (Wing shape): 3:1 ratio O e (su m) bs x (o- e) er p ^2 ve e /e d ct e d (3 (3 N /4 64 o ) - r (4 32 m 2 1) 8) ^2 = /32 3 1= 2 5.7 1 60 A (1 (6 p /4 4- t ) 10 (4 7) 2 ^2 8) /10 = 7= 1 17. 0 28 7 0 X^2=23.04 0 Test hypothesis w/ X^2 analysis (eye color): 1:1 Obsereved Expected (o-e)^2 /e red (282- 214)^2/214=21.6 282 428*(2/4)=214 0 white 146 428*(2/4)=214 (146- 214)^2/214=21.6 0 X^2=43.2 Discussion: After performing the chai square analysis, to test the mode of inheritance of each trait, I have encountered a significant problem with the data. At this point, I would have to reject the hypothesis due to the very big chi-square values that significantly exceed the critical value of 3.841 for 1 degrees of freedom. So what went wrong? One source of error could be the lack of gentility towards the fruit flies which may have unintentionally killed some of them. Another error that could have developed, is improperly distinguishing between the genders of the flies or characteristics such as their eye color and wing shape, and the experimenter can easily make a mistake when manually counting the flies. The limit of the chi- square test analysis states that all values of 5 and under are often rendered not valid in the overall results, so that could be another reason for the high chi-square values. The experiment could have been improved is if larger sample of the model organism was available for experimentation. Also, more involvement of teaching assistant in certain parts of the lab, dealing with counting and distinguishing flies, could lead to more precise results, and avoid student error. Conclusion The purpose of this lab was to learn about the inheritance patterns when performing a dihybrid cross, and be able to predict whether the inheritance mode is autosomal or sex-linked. In my opinion, even though the class data did not worked to be any know and acceptable ratio, during this extensive course period we were able to grasp the idea of genetic inheritance. Therefore, I feel the goal of this experiment was achieved. Work cited 1). Ashburner M, Thompson JN (1978). "The laboratory culture of Drosophila". In Ashburner M, Wright TRF. The genetics and biology of Drosophila 2A. Academic Press. P.1– 81.Print. 2). Morgan, Thomas Hunt. The mechanism of Mendelian heredity. New York: Holt, 1915.p.164-166 Print. 3). Morgan T.Hunt. 1910. Sex-limited inheritance in drosophila. Science 32: p.120- 122.print


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