Inheritance of the Vestigial Wing in Drosophila Melanogaster

Abstract 

The purpose of this experiment was to test inheritance patterns of an assigned fly mutation; the mutation studied was an autosomal recessive vestigial wing mutation. In the parental cross, virgin truebreeding female wild-type flies were crossed with vestigial truebreeding mutant flies to produce the F1 generation. The F1 flies were then crossed to produce the F2 generation. The expected phenotypic ratio of the F2 generation was 3 normal wing :1 vestigial wing because the vestigial mutation is autosomal, however, the observed phenotypic ratio was 2.7 normal wing :1 vestigial wing. Since the ratios were different, a Chi-square analysis was necessary to test if there was any statistical significance in the difference of the observed and expected phenotypic ratios. Consequently, the Chi-square analysis failed to reject the null hypothesis of whether the observed and expected phenotypic ratios were statistically different with a p-value range of 0.20 to 0.05. 

Results 

Again, this experiment's purpose was to test the vestigial mutation's inheritance pattern. The vestigial mutation is an autosomal recessive mutation and results in loss of function in the wings of Drosophila melanogaster (Williams, et al. 1991). The first cross that was made was between virgin truebreeding wild-type female flies and vestigial mutant truebreeding male flies to make what was considered the parental cross. The F1 flies all had the normal wing phenotype; in other words, they all had the wild-type phenotype. When the F1 flies were crossed they produced the F2 generation, in which the expected phenotypic ratio of the normal wings to vestigial wings was 3:1.  Out of 163 F2 flies counted, 115 flies had normal wings, and 48 flies had vestigial wings. The expected ratio of 3:1 normal wing to vestigial wing was calculated to yield 122.25 flies with normal wings and 40.75 flies with vestigial wings. Since the numbers were different, the observed ratio was calculated to be 2.7 normal wing:1 vestigial wing. Because the expected ratio and observed ratio were different, a Chi-square analysis was performed to determine if the difference was statistically significant, or if the difference was simply random, and the overall Chi-square value was found to be 1.72 (Table 1). 

Table 1: Chi-Square Table by Wing Phenotype of Drosophila melanogaster 

		Chi-Square Values
Phenotype	Observed Value	Expected Value	O - E	((O – E) ^2)/E
Normal Wings	115	122.25	-7.25	0.43
Vestigial Wings	48	40.75	7.25	1.29
Total	163	163	N/A	1.72

Discussion 

The experiment tested the inheritance patterns of the vestigial mutation in Drosophila melanogaster on chromosome two. Truebreeding mutant vestigial male flies were crossed with virgin truebreeding wild-type female flies, and their offspring were crossed to create the F2 generation. The calculated Chi-square value was 1.72 with 1 degree of freedom. The critical alpha value was 0.05, and the critical Chi-square value was 3.841.  Since the critical Chi-square value was greater than the calculated Chi-square value, the p value for the calculated Chi-square ranged from 0.20 to 0.05.  Because the p range was not at or below 0.05, there was not enough evidence to conclude that the null hypothesis could be rejected. There is not sufficient evidence that there is a difference between the expected phenotypic ratio of 3:1 wild-type wings to vestigial wings and the observed phenotypic ratio of 2.7:1 wild-type wings to vestigial wings.  

The vestigial gene is on the second chromosome in Drosophila melanogaster. Since chromosome one contains the sex chromosomes, the trait for vestigial wings is not sex-linked. If mutant truebreeding vestigial female flies were crossed with wild-type truebreeding male flies, the conclusion would be the same because the gene studied was not on the sex chromosome. This gene is simple in the sense that the wild-type function of the vestigial gene is normal, fully functioning wings (Williams, et al. 1991). The gene itself encodes for a hypothesized modular protein, and the hypothesis that homopolymeric repeats in the vestigial gene are not required for the protein to be functional is supported by the presence of poor sequence conservation in the glycine-rich region of the gene (Williams, et al., 1991).  If this gene were to be mutated to lose function, phenotypic evidence would include a loss of wing structure and shape, as well as other defects such as erect postscuttellar bristles (Williams, et al., 1991).   When the vestigial gene in a Drosophila melanogaster specimen consists of both mutant alleles, the wing function is dead. However, four different combinations of mutant vestigial alleles exist (Williams and Bell, 1988). These four classes are recessive viable alleles, recessive lethal alleles, complex alleles, and finally, dominant alleles. In particular, the recessive viable alleles have varying levels of strength. The recessive alleles that are weaker exhibit a slight crumpling of the wing (Williams and Bell, 1988), whereas the stronger recessive alleles show death of the full wing. Additionally, a combination of strong and weak vestigial alleles produce an intermediate wing phenotype where the wing has a medium amount of dysfunction (Williams and Bell, 1988). Using this knowledge, another experiment could be done to reflect the disparity in these phenotypes. If there are multiple different phenotypic ways for the mutant vestigial allele to present in Drosophila melanogaster, the experiment could attempt to track these separate groups and calculate their frequencies within the species. However, a hypothesis mentioned that many of the vestigial alleles in the recessive viable categories are mutations within the introns of the gene. These mutations cause the mutated phenotype through reduction of the expression of the vestigial gene, but they do not change the gene product itself (Williams and Bell, 1988). Given that the vestigial mutation follows an autosomal recessive pattern of inheritance, it is plausible that many of the cited genotypes investigated by Williams and Bell fall into the “recessive viable” category. They also found that a few of the genotypes they investigated were either completely dominant or complex mutants. Since one copy of the wild-type vestigial allele is sufficient to produce a wild-type phenotype, the dominant and complex mutants may be able to mask the wild-type phenotype (Williams and Bell, 1988). Therefore, one experiment would be to test Drosophila melanogaster with both the wild-type phenotype and the recessive vestigial wing phenotype and genotype them. This could be done by PCR amplification of the flies in the F2 generation; the F2 generation is where the process of genotyping is non-lethal (Carvalho, et al.). After doing the PCR and identifying the genotypes, a Chi-square analysis could be performed to analyze the statistical differences, if any, between the hypothesized ratio of each class of genotype.  

Literature Cited 

Carvalho, Gil B., et al. “Non-Lethal Genotyping of Single Drosophila.” BioTechniques, vol. 46, no. 4, 25 Apr. 2018, pp. 312–314, www.ncbi.nlm.nih.gov/pmc/articles/PMC2793178/, https://doi.org/10.2144/000113088. Accessed 19 Nov. 2024. 

Fisher and Yates, 1963. Statistical Tables for Biological, Agricultural and Medical Research, 13th ed. Oliver and Boyd Ltd., Edinburgh. 

Williams, J A, et al. “Control of Drosophila Wing and Haltere Development by the Nuclear Vestigial Gene Product.” Genes & Development, vol. 5, no. 12b, 1 Dec. 1991, pp. 2481–2495, https://doi.org/10.1101/gad.5.12b.2481. 

Williams, J. A., and J. B. Bell. “Molecular Organization of the Vestigial Region in Drosophila Melanogaster.” The EMBO Journal, vol. 7, no. 5, May 1988, pp. 1355–1363, https://doi.org/10.1002/j.1460-2075.1988.tb02951.x. Accessed 19 Nov. 2024.