Drosophila melanogaster dihybrid cross

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Drosophila melanogaster DIHYBRID CROSS

Drosophila melanogaster dihybrid cross

The studies ofgenes of living organisms have been aided by experimental animals andother microorganisms. Drosophila melanogaster is the most commonorganism used in genetics. Drosophila’s natural habitat ischaracterized by availability of moisture and hence has a worldwidedistribution. It belongs to the order Diptera and undergoes completemetamorphosis as it develops from eggs to adult. Drosophila has threebody parts and is covered by a chitinous exoskeleton (MetzW. C, Moses M. S. &amp Mason E. D. 1923). Drosophila has beenpreferred in scientific studies due to various reasons. First, theorganism is cheap and relatively easy to maintain in the laboratory.These flies have a short life cycle hence favoring time limit of mostscientific studies. They also produce many offspring from whichscientific observations can be made. Various genetic mutations can bestudied using this fly. The mutations can result to altered wingstructure, abnormal body color, odd colored eyes and strangely formedheads or a mixture of the mutations (Appel, S., &amp Birmingham, L.1998).

Mendel isreferred to as the father of genetics and he developed the Law ofSegregation and the Law of Independent Assortment. He described agene as a small part of DNA of at specific locus that organismsinherit from their parents. Powell, J. R. (1997) argues that alleleis the alternate form of a gene and every individual has two allelesof a given gene. Genotype is defined as the genes that an individualinherits form his/her mother and father. The gene for a particulartrait that an individual inherits from the mother does have to besame with that he/she inherits from the father. Each parentcontributes one allele (haploid state) which fuses with the otherduring fertilization to form a diploid organism. The genes that aperson has determine the person’s physical appearance also known asphenotype. Mendel used monohybrid and dihybrid crosses during hisstudies. Monohybrid cross involves one pair of allele where only onetrait of an organism is studied. When two heterozygous organisms arecrossed, the resulting phenotypic ratio of the offspring is 3:1.Dihybrid cross involves two pairs of alleles and the resultingphenotypic ratio of the offspring is when two individual, AaBb, arecrossed is 9:3:3:1 (Johnstone, A., 2001).

This experimentis performed to determine the genotype and phenotype of an unknown F1heterozygous generation using its F2 generation. Punnett square wasused to analyze the phenotypic characteristics of the F2 generation.The physical appearance is always used to describe individuals. Thegenotype that an individual have is what determines his/her phenotypeand hence the physical appearance can be used to study genes of anorganism.

Hypothesis: Thered colored eye and winged drosophila was produced by a dihybridcross of unknown F1 heterozygous generation resulting in atheoretical ratio of 9:3:3:1

Materials andmethods

The experiment began with culture tube setup. About one spoon of drymedia was placed in the vial. Distilled water was added to the mediauntil it became completely moistened after which the vial was left tostand for a few minutes. Additional water was added as necessary toensure complete hydration of the media shiny appearance of surfaceof the media was an indication of hydration. Two grains of yeast wasadded to each tube and then closed. The plugged tube was given fiveminutes to on the bench while confirming solidification of the mediaby tipping the tube sideways. The F1 adults were observed and sexed(Biology 101 Laboratory Manual, Chap 7).

Results

Table 1: Punnettsquare

RW

Rw

rW

rw

RW

RRWW

RRWw

RrWW

RrWw

Rw

RRWw

RRww

RrWw

Rrww

rW

RrWW

RrWw

rrWW

rrWw

rw

RrWw

Rrww

rrWw

rrww

Red eye withwing: RRWW, 2 RRWw, 2RrWW, 2 RrWW, 2 RrWw =9

Red eye withoutwing: RRww, Rrww, Rrww =3

Black eye withwing: rrWW, rrWw, rrWw =3

Black eye withoutwing: rrww =1

Ratio- 9:3:3:1

Table 2:Chi-square

Phenotype

Red eye with wing

Red eye without wing

Black eye with wing

Black eye without wing

Raw total

O

71

21

15

7

114

E

64.125

21.375

21.375

7.125

114

O-E

6.875

-0.375

-6.375

-0.125

NA

(O-E)2

47.266

0.141

40.641

0.016

NA

(O-E)2/E

0.74

0.01

1.90

0.00

2.65

(Data tableretrieved from Biology 101 Laboratory Manual, Chap 7)

Discussion

Mutation isresponsible for changes in the body layout and behavior of drosophilamelanogaster. In this experiment, the product of mutations of thegenes of drosophila responsible for eye color and presence of wingwas studied. The gene mutation was studied using dihybrid crosses.Mendel used dihybrid crosses to study the law of independentassortment. During meiotic cell division in the reproductive cells,segregation of genes responsible for different traits of anindividual takes places. However, segregation of one pair of allelesoccurs independently of the other pair of alleles. Therefore, all thepossible combination of alleles took place in the gametes of thedrosophila melanogaster. During meiosis in each sex of the fly, everypair of homologous chromosome separated and assorted independently.Alleles also segregate and assort independent (Starr, C., Evers, C.A., &amp Starr, L. 2011).

Duringfertilization, two alleles responsible for a particular trait pairresulting to the genotype of the offspring. Within the allelic pairthere is one gene that expresses itself over the other. In thisexperiment, it is evident that the gene responsible for the redcolored eye is dominant over the gene that codes for black coloredeye and that the gene that codes of presence of wings is dominantover the gene coding for lack of wings. The dominant gene red coloredeye expressed itself in both homozygous and heterozygous state i.e.RR and Rr. The gene for black colored eye is recessive and expresseditself only in homozygous recessive state i.e. rr (Callihan, L. A.,Hart, S., &amp Research and Education Association. 2008).

The hypothesis isjustified since the p value is &lt0.5. The F1 generation used forthe dihybrid cross done in this experiment was (RrWw). Each pair ofthe alleles (Rr and Ww) segregated during meiotic division and thenpaired up after fertilization. Red colored eyed drosophila with wingfrom the F2 generation consisted of homozygous and heterozygousstates RRWW, RrWW, RrWw and RRWw. Experiments are characterized withmanipulation of variables in order to obtain a desired type ofresults. The experiment is performed without natural parameters inplay and thus will have an effect on the outcome. For this reasoncontrol are used in scientific studies. They are used to judge theeffect of the manipulations by comparing it with the experimentalgroups that had manipulation of variables. For example, in thisexperiment there was manipulation of environmental parameters and acontrol was necessary to help in comparing the outcome of the entirestudy (Patrick J. C., Sharon L. &amp Emily J. H. 2007).

References

Appel, S., &amp Birmingham, L. (1998).&nbspArthropods.Portland, ME: J. Weston Walch.

Biology 101 Laboratory Manual, Chap 7.

Johnstone, A. (2001).&nbspBiology: Facts &amp practice for alevel : [written for the 2000 specification]. Oxford: OxfordUniversity Press.

Lindsley, D. L., &amp Zimm, G. G. (1992).&nbspThe Genome ofDrosophila melanogaster. Oxford: Elsevier Science.

Metz W. C, Moses M. S. &amp Mason E. D. (1923),Genetic Studies on Drosophila Virilis:WithConsiderations on the Genetics of Other Species of Drosophila,The Carnegie Institution of Washington, Washington, DC.

Patrick J. C., Sharon L. &amp Emily J. H. (2007), The BioinformaticEnhancement of Exercises in Drosophila Genetics, The American BiologyTeacher, Volume 69, Issue 8, page 482-487

Powell, J. R. (1997).&nbspProgress and prospects in evolutionarybiology: The Drosophila model. New York: Oxford University Press.