Inheritance Patterns of Drosophila melanogaster, the Fruit Fly

Transcription

1 InheritancePatternsofDrosophilamelanogaster,theFruitFly KevinReynolds SchoolofNaturalSciencesatFerrumCollege Abstract Thislabwasperformedtofurtherour understanding of the basis of Mendelian genetics.wedidthisbycrossingfruitflieswe found in our laboratory. We performed four crosses:femalewild typetomaleebonybody, female white eyes to male wild type, female wild typetomalesepiaeyes/ebonybody,and femaleebonybodytomalevestigalwings.we found that each cross showed an inheritance pattern of either a standard monohybrid cross, sex linked monohybrid, dihybrid cross of unlinked genes, and a dihybrid cross of linkedgenes. Introduction Thepurposeofthislabwastofurther ourunderstandingofinheritancepatternsand to get hands on experience with Mendelian genetics. To do this we are looking at the inheritance patterns of the mutant fruit flies wediscoveredinourlaboratory.wewantto look at the phenotypic characteristics of our specimens and determine the patterns for howeachtraitisinherited. Many organisms are used to study Mandeliangeneticssuchasfruitflies,yeast,E. coli, and mice. We have chosen fruit flies to focusforafewkeyreasons;becausefruitflies are small, cheap, easy to keep in large numbers,andhaveashortlifecycle,fromegg to adult in about 10 days at room temperature, they are idea for a classroom study. In this procedure, we are looking for four distinct inheritance patterns, standard monohybrid cross, sex linked monohybrid, dihybrid cross of unlinked genes, and a dihybrid cross of linked genes. A standard monohybrid cross is a cross, of a single trait, between a homozygous dominant and a homozygous recessive to produce a heterozygous F1 generation, which when is selfed produces an expression rate of three dominant to one recessive. A sex linked monohybridcrossisacrosswherethetraitis linked to the X chromosome, since in fruit fliesamaleisdenotedbylackingasecondxchromosome. In both make and female you would see roughly equal numbers of both traitsexpressed.thethirdtypeofinheritance pattern we could have is a dihybrid cross of unlinked genes. This is a cross between two parents, one being homozygous dominant for both genes and the other being homozygous recessive for both genes. When we examine ourf1generationwewouldseethatwehave aninetothreetothreetooneratio.thenine offspring would look like the parent whom was dominant for both genes and the one offspring would look like the parent whom was homozygous recessive for both genes. The final possible inheritance pattern we could observe is a dihybrid cross of linked genes.thisiswheretwogenesaregenerally inherited together. However, due to recombinationyouwillfindbothrecombinant offspring and parental offspring. How much recombinationdependsontherecombination frequency, how often crossing over occurs between genes, which is how far apart the genes are on the chromosome. In the F2 generation we would see a deviation from a ninetothreetothreetooneratio,whereboth parental phenotypes and recombinant phenotypesarepresent,butjusthowmuchof a deviation depends on the recombination frequency. Methods ExaminationofFlies: To look at the flies we anesthetized them using flynap, which is composed of ethanolandtriethylamine.wedippedawand intotheflynapandputthatintothetubes,for aboutthreetofiveminutes,offliesuntilthey wereanesthetized,beingcarefulnottoletthe fliesescapeorfallintothefoodatthebottom ofthetubes.wethenputthefliesonanindex card to look at them under a dissecting microscope at 10X to 25X magnification. To 1

2 move the flies around we used a small paintbrush.welookedtoseeiftheflywasa maleorfemale.thiswasachievedbylooking at the size(females are usually larger), shape(the males abdomen is narrow where the female is spherical), color(the male has a largeblackdotonhisabdomen),andexternal genitalia(maleshaveadarklycoloredexternal genitaliaontheventralsideoftheabdomen). We also looked at distinguishable characteristics so we could separate the differentfliesthatwereneededforeachcross. MatingSetup: Tosetupthecrosses,weputascoop of dry food and a scoop of water into each tube;onetubewasusedforeachparentcross, and four parent crosses were done. We placed about three virgin females into each tube for every one male. We ended up with roughly ten females and three males in each tubepercross.thetubeswerethenputinan incubatorat25 C. CrossesPerformed: Weperformedfourcrosses.Ourfirst crosswasamonohybridcrossoffemalewildtype to male ebony body. Our second cross was another monohybrid cross of female whiteeyestomalewild type.thethirdcross was a dihybrid cross of female wild type to male ebony body/sepia eyes. The final cross weperformedwasfemaleebonybodytomale vestigalwings. AnalysisofF1andF2: It takes approximately ten days for flies to develop from an egg to an adult at 25 C, so after about two weeks we look at what the parental cross had produced, which arethef1generation.welookedatthemand transferred them to a separate tube for each cross. We then let the F1 generation from eachtubeselfcross.afteranothertwoweeks, when the F1 offspring had matured, we counted and looked at our F2 generation. When we looked at the flies, we examined to see what phenotype they were and to see if theyweremaleorfemale.forthef1andf2 generationwecountedonehundredflies.we countedjustaswehaddonewiththeparents. We a anesthetized them with flynap and examined them under a dissecting microscope. Results Four flies of the five we crossed had differentanddistinctcharacteristicsfromthe others. Wild type flies have a light brown body, red eyes, and long oval wings. White eyedflieshavealightbrownbody,whiteeyes, andlongovalwings.ebonybodyflieshavean ebony body, red eyes, and long oval wings. The vestigal flies have a light brown body, with re eyes, and tiny wings. The final fly type, ebony/sepia, have black bodies, brown eyes,andlongovalwings. F1PhenotypesandCounts: Table 1.1 shows the offspring from athecrossbetweenfemalewild typeandmale ebonybody(cross1). Table1.1 Phenotype #offemales #ofmales Wild type Table 1.2 shows the offspring from teh cross between female white eyes to male wild type(cross2). Table1.2 Phenotype #offemales #ofmales Whiteeyes 0 47 Wild type 53 0 Table 1.3 shows the offspring from thecrossbetweenfemalewild typeandmale ebonybody/sepiaeyes(cross3). Table1.3 Phenotype #offemales #ofmales Wild type Table 1.4 shows the offspring from the cross between female ebony body and malevestigalwings(cross4). Table1.4 Phenotype #offemales #ofmales Wild type F2PhenotypesandCounts: Table 2.1 shows the offspring when the F1 generation, from the first cross, was allowedtoself matewitheachother. 2

3 Table2.1 Phenotypes #offemales #ofmales Wild Type EbonyBody 15 8 Table 2.2 shows the offspring when thef1generation,fromthesecondcross,was allowedtoself matewitheachother. Table2.2 Phenotypes #offemales 3ofMales Whiteeye Wild type Table 2.3 shows the offspring when the F1 generation, from the third cross, was allowedtoself matewitheachother. Table2.3 Phenotype #offemales #ofmales Wild type Ebonybody 7 10 Sepiaeyes 14 1 Ebony/Sepia 5 4 Table 2.4 shows the offspring when thef1generation,fromthefourthcross,was allowedtoself matewitheachother. Table2.4 Phenotype #offemales #ofmales Wild type Ebonybody Vestigalwings 7 1 Ebony/Vestigal 0 2 Conclusions AnalysisofF1GenerationData: Because all of our parent crosses were either a homozygous monohybrid or a homozygous dihybrid, one parent being dominant and the other recessive, we can determine which traits were dominant and whichtraitswererecessive.inthefirstcross between female wild type and male ebony body all of the F1 generation were wild type, thereforewild typeisdominant. Thecrossbetweenfemalewhiteeyes andmalewild typeproducedallfemalewildtype and all white eye males. Since the parental male wild type gives his only X chromosome to his daughters, and all of his daughters express wild type, we can say that wild type is dominant and white eyes is recessive. In the cross between female wildtypeandebonybody/sepiaeyes,allofthef1 generation was wild type. Therefore, we can say that wild type is dominant to ebony body/sepiaeyes. Finally, since the cross between femaleebonybodyandvestigalwingsyielded all wild type F1 generations, we can determine wild type is dominant to both vestigal wings and ebony body, and that vestigal wings and ebony body are linked genes. AnalysisofF2GenerationData: With a first look at the data it is my hypothesis that the cross between female wild type and male ebony body will be a standardmonohybridcross.ialsohypothesis thatthefemalewhiteeyesandmalewild type isasex linkedmonohybridcross.thefemale wild type crossed with male ebony body/ sepiaeyeswillmostlikelybeadihybridcross of unlinked genes. Therefore, the cross betweenfemaleebonybodyandmalevestigal wings must be a dihybrid cross of linked genes.however,wecannotbecertainuntila Chi SquaredTestisperformed. Chi SquaredTest: Table3.1showstheChi Squaredtest on the cross of female wild type and male ebonybody. Table3.1 Traits Observed Expected (O E) 2 /E Wildtype Ebony body df=1 Σ 2= Prange= AfterperformingtheChi Squaredtest we determined or P range to be between and 0.500, therefore we can say that any discrepancies in our data is caused by random chance. So the hypothesis holds correct that this cross is a standard monohybridcross. 3

4 Prange=0.100to0.050 Prange=0.500to0.250 Here the hypothesis also holds up, showingthatthecrossisastandarddihybrid cross. Table3.4showstheChi Squaredtest on the cross of female ebony body and male vestigalwing. Table3.2showstheChi Squaredtest on the cross of female white eyes and male wild type. Table3.2 Traits Observed Expected (O E) 2 /E Wildtype White eyes df=1 Σ 2 =3.24 Again we see that because our P range is between and 0.050, our hypothesis is proven valid, and the data statistically matches the inheritance pattern ofasex linkedmonohybridcross. Table3.3showstheChi Squaredtest on the cross of female wild type and male ebonybody/sepiaeyes. Table3.3 Traits Observed Expected (O E) 2 /E Wildtype Ebony body Sepia eyes Ebony/ Sepia df=3 Σ 2 =2.714 Table3.4 Traits Observed Expected (O E) 2 /E Wildtype Ebony body Vestigal wings Ebony/ Vestigal df=3 Σ 2 =8.473 Prange=greaterthan0.050 Sincewedidnotperformattestcross withthef1generation,wecannotprovethat this is a dihybrid cross of linked genes. However, we can prove that it is not a standard dihybrid cross of unlinked genes. Since our P range is grater than 0.050, the cross fails the Chi Squared test thereby proving that the cross cannot be a standard dihybrid cross. If a test cross had been performed it is probable that parental types wouldhavebeenthemostabundantinthef2 Generationandtherewouldhavebeenasmall percentage of recombinant phenotypes presentintheresultsfromthetestcross. References 1. Hartwell, Leland H. Hood, Leroy. Goldberg,MichaelL.Reynolds,AnnE. Silver, Lee M. Verus, Ruth C. GENETICS: From Genes to Genomes Third Edition. New York: The McGrawHillCompanies,Inc., Mertens and Hammersmith. BIO 305 Introduction to Genetics LaboratoryManual.FerrumCollege FLYNAP July 13. < herresources/msds/flynap.pdf> Acknowledgments 4

5 IwouldliketothankJoshLiptak,John Harper, Ross Beckner, and Jessica Foley for theirassistancewiththislab.iwouldalsolike to thank Dr. Gazdik for her guidance throughouttheentirelab. 5