Generally, Drosophila melanogaster is a diploid organism which has 8 chromosomes where 3 pairs are autosomes and the remaining 2 are sex chromosomes which can be X or Y The life cycle of Drosophila may last from 10 – 14 days. Moreover these organisms are also sensitive to high temperature. The optimum temperature where Drosophila could grow is at 25 degrees Celsius. (Socolich, 2003)
Through the years Drosophila melanogaster has been used in genetics lab for it is easy to study the basic principles of Mendelian Inheritance. The high rate of reproduction and the ease of culture manipulation of this organism only imply that it is the most understood organism in the field of genetics (Microsoft Encarta, 2008). In the Mendelian principle of inheritance it is said that there is a complete dominance between the homozygous crosses. That is the dominant gene in the cross product masked the expression of recessive alleles.
This is easily observed in the species Drosophila melanogaster whether the mode of inheritance is autosomal or sex-linked. However results of reciprocal crosses are same only for autosomal traits. (Klug, 2000) In this study the researchers will determine the mode of inheritance of Drosophila melanogaster with respect to mutant traits. Methodology To determine the mode of inheritance of the mutant trait of Drosophila melanogaster, a test cross was prepared from the stock culture of a male wild-typed Drosophila and a female white-eyed, having miniature wings, and forked body Drosophila.
Wild type Drosophila species are red-eyed and they are marked as dominant. While the white-eyed ones which also have miniature wings and fork body are marked as recessive. Virgin female cultures were placed in the test cross to ensure a pure culture. Moreover, a reciprocal cross was made as a reference to check the dominance or recessiveness and the sex-linkage of the traits of the test cross. As soon as the offspring of the parental cross has reached the pupa stage, the flies were released. The pupae in the stock or the expected progeny were marked as F1 or the first filial generation.
From the first progeny of the parental cross (i. e. the mutant female and normal male), six virgin wild-typed females that has emerged were isolated and crossed to 6 mutant males for F2. The rest of the progeny has emerged were counted and segregated based sex and phenotype. All F2 progeny were counted and segregated based on the phenotype of each individual organism. Results Table 1. 1 Chi Square Test of Sex Ratio of Progeny of Drosophila melanogaster Ho: Sex ratio in F1 progeny is 1:1 F1 (Wild) F1 (Mutant) male female Male Female Observed no. of flies (O) 0 49 45 0
Expected no. of flies (E) 0 47 47 0 Deviation (D) 0 2 -2 0 d2 0 4 4 0 d2/E 0. 000 0. 85 0. 85 0. 000 X2 0. 007 0. 007 n=2 Degrees of freedom (n-1) = 1 ? = 0. 05 X2table= 3. 84 X2 < X2table Decision: accept null hypothesis that sex ratio in F1 is 1:1 for both Wild-typed and mutant. Table 1. 2 Chi Square Test of Sex-linked traits of Progeny of F1 Drosophila melanogaster Ho: P1 Wild-typed Male x Mutant Female therefore F1 1 Wild-typed Female: 1 Mutant Male F1 (Male) F1(Female) Wild White-eyed Wild White-eyed Observed no. of flies (O) 0 45 49 0 Expected no. of flies (E) 0 47 47 0 Deviation (D) 0 -2 2 0 d2 0 4 4 0 d2/E 0 0. 85 0. 85 0 X2 0. 007 0. 007 n=2 Degrees of freedom (n-1) = 1 ? = 0. 05 X2table= 3. 84 X2 < X2table Decision: accept null hypothesis that sex-linked trait ratio in F1 is 1:1 in male and female. F1 Cross: 6 Wild Female x 6 Mutant Male Wild-typed (Normal) (+) Normal trait has F, M, and W gene F= normal body f = forked body M = normal wings m = miniature wings W = red w = white Table 1. 3 F2 Progeny # of flies Sex of Flies Phenotype shown 25 female + 19 male + 2 Male F 4 Male F 7 female W 9 Male W 1 Female F,w
1 Male F,w 2 female M 62 Male M 2 female F,m 7 Male F,m 5 female W,m 3 Male W,m 16 female W,m,f 13 Male W,m,f Discussion Based from the results that were obtained in the study the mode of inheritance of the mutant characteristic of Drosophila melanogaster is X-linked for the eye color. This X-linked mode of inheritance means that a pattern of inheritance is formed that results from genes located on the X chromosome. Moreover male parents can only transmit their X chromosome to a daughter while female parents can randomly pass the X chromosomes to all progenies.
In addition, X-linked inheritance could either be dominant or recessive. (Klug, 2000) Table 1. 1 shows that the sex ratio of the crosses in F1 is 1:1 regardless of the phenotypic expression in both autosomal and sex-linked mode of inheritance. Because the trait regarding eye color is X-linked there is a 50 % probability that the progeny would acquire the recessive trait. This is shown in Table 1. 2. Reciprocal crosses involving sex-linked traits differ from those involving autosomal traits in such a way that sex affects the level of gene expression.
A male parent can only transmit their X chromosome to a daughter while female parents can randomly pass the X chromosomes she has to all progenies. Crossing autosomal traits do not affect whether the progeny is male or female because the traits may be express to both sexes (Klug, 2000). Such autosomal traits in this study involve traits like forked body type and miniature wings which is manifested in the progeny of F1 cross. Errors regarding to the experiment may be accounted due to the mishandling of the crosses. Moreover, some statistical tests were not performed because some aspects of data were failed to be observed.
“Fruit Fly. ” (2007). Microsoft® Student 2008 [DVD]. Redmond, WA: Microsoft Corporation. “Introduction to Drosophila Genetics”. Date Accessed: 23 April 2008 <http://www. hhmi. swmed. edu/Labs/rr/world/Methods/Drosophila%20Genetics%20Primer%20May%202003. pdf> Klug, W. S. (2001). “Genetics. ” Singapore: Prentice-Hall Inc. Socolich, M. (2003). “Drosophila Genetics. ” Date accessed: 23 April 2008 <http://www. mun. ca/biology/dinnes/B2250/DrosophilaGenetics. PDF>