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The fruit fly Drosophila melanogaster as a source of much of the chromosome theory of heredity.

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Introduction

The fruit fly Drosophila melanogaster as a source of much of the chromosome theory of heredity. ABSTRACT This investigation centres upon the fruit fly Drosophila melanogaster as a source of much of the chromosome theory of heredity. The Drosophila was chosen due to its ease of cultivation within the laboratory as well as the many previous studies performed on it from which information could be drawn. The hypotheses under investigation are: Ho: The inheritance of both wing length and eye colour are not sex-linked. Hi: The inheritance of both wing length and eye colour are sex linked. This was tested by initially crossing three vestigial winged, red-eyed males with four normal winged, white-eyed females. The F1 generation were then allowed to interbreed after their number and phenotypes had been recorded. The F1 generation interbreeding cross consisted of three normal winged, white-eyed males being crossed with four normal winged, red eyes females. The F2 generation produced were then counted and their number and phenotypes were recorded (i.e. how many normal wing, red eye females produced etc) this cross concentrated upon investigating the inheritance of wing length, while a reciprocal cross was set up to investigate the inheritance of eye colour. The reciprocal cross was three normal winged, white-eyed males and four vestigial winged, red-eyed females. ...read more.

Middle

This will show that wing length is sex-linked, while the reciprocal cross will be set up with the F1 generation being interbred in order to determine that eye colour is also sex linked due to the presence of another 3:1 ratio with the male again showing the recessive characteristic more frequently than the females. PLAN During his study of Drosophila, Thomas Hunt Morgan concluded that the genes for eye, colour and wing length have the allomorphs, red and white-eye and normal and vestigial wing length. He also concluded that red eye colour was dominant over white-eye and that the inheritance of eye colour in Drosophila was related to the sex of the parent flies. He concluded this by crossing a white- eyed male with a red-eyed female and producing equal numbers of F1 red eyed males and females. However, in breeding of these F1 flies produced red-eyed females, red-eyed males and white-eyed males but no white-eyed females. By the fact that the males showed the recessive characteristic more frequently than the females it suggested that the recessive white-eye allele was present on the X chromosome, hence being masked in the females yet expressed in the males due to sex linkage. [Biological Sciences 2, 1990] After reading through and being fascinated by Morgan's work, I wish to repeat his investigation into the inheritance of eye colour and confirm his results of eye colour being sex linked. ...read more.

Conclusion

Therefore, if 100 flies were produced in the F2 phenotypic generation, 75 flies would show the dominant characteristic and 25 would show the recessive characteristic in order to give a 3:1 ratio. Also, of the 25, the majority of the flies would be males in order to satisfactorily conclude that sex linkage has occurred. Cross showing expected results if characteristics are sex linked: Let: - N represent normal wing - dominant ?= female = XX n represent vestigial winged - recessive ? = male = XY R represent red eye - dominant r represent white eye - recessive (This cross concentrates upon investigating if inheritance of wing is sex linked) Parental phenotypes: White eye, normal wing X Red eye, vestigial wing Parental genotypes: XNr XNr XnRY Meiosis: Gametes: Random fertilisation: F1 Genotypes XNr XnR XNrY XNrXnR XNrY F1 Phenotypes Normal wing Normal wing Normal wing Normal wing Red eye white eye red eye white eye F1 generation cross: Normal wing, white eye X Normal wing, red eye F1 phenotypes: F1 genotypes: XNrY XNrXnR Meiosis: xxxxxxxxxxxx cross if the characterisitics are not linked parental phenotypes: vestigial wing, red eye X normal wing, white eye parental genotypes: RRnn rrNN meiosis: gametes: Rn rN random fertilization F1 genotypes RrNn F2 phenotypes all heterozygous normal winged, red eyed F1 generation cross: normal wing, red eye X normal wing, red eye F1 genotypes: RrNn RrNn Meiosis ...read more.

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