The Drosophila Melanogaster has been studied extensively for over a century as a model organism for genetic investigations (Mackay, 2014). Regarding the pigment that gives the eye its color, the brown, white and scarlet genes of Drosophila Melanogaster encode proteins which are responsible for the transportation of tryptophan and belong to the ABC transporter superfamily. Current genetical models show that both the white and the brown genes products contribute in forming a guanine specific transporter, while the white and scarlet gene products form a tryptophan transporter. The Drosophila Melanogaster has a small genome size regarding base pairs with huge salivary gland chromosomes known as the polytene chromosomes (Benson et al., 2014). The presence of a low number of chromosomes in Drosophila Melanogaster’s body made it an attractive model organism in the early years of genetic studies (Dos Santos et al., 2014). However, the density of genes per chromosome is quite higher in the fruit fly than in human beings. About 13,600 genes encode the entire Drosophila genome as compared to 27,000 human genes. These 27,000 genes are located on twenty-three pairs of chromosomes, while the 13,600 genes are located on only four pairs of chromosomes (Mackay, 2014).
The structural composition of the Drosophila’s and the human beings’ genes have shown striking similarities in their entire genome sequencing (Mackay 2014). The Drosophila Melanogaster has a large genetic toolkit which is well curated and structured simply. Of the four Drosophila Melanogaster chromosome pairs, the first one is the sex chromosome which is highly compacted, dense in repeats and has transcriptionally silent DNA. These acrocentric X chromosomes and submetacentric Y chromosomes are gene poor and are almost completely made up of heterochromatin (Potier et al., 2014). The other two pairs are large metacentric autosomes, while the fourth one is a tinier one, described as the dot chromosome (Potier et al., 2014). Before the Drosophila Melanogaster genome was sequenced, mapping of DNA fragments on to a genomic region was done by in situ hybridization to polytene chromosomes, making Drosophila Melanogaster just the second multicellular genome to be sequenced (Schultz, Nilsson, and Westermark, 2011). Detailed molecular and phenotypic characteristics of the Drosophila Melanogaster have been discussed in this paper as well as the gene mutation and gene product characteristics.
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Phenotypic Characteristics of Mutation in the gene.
The Drosophila Melanogaster has a few classic phenotypes which are attributed to their wings, eyes and body type (McQuilton, 2012). My mutant fly, scarlet , has orange eye color. However, the only difference that scarlet had from the wild Drosophila Melanogaster was the orange eye color, which has dark red eye color. A variation in eye color is explained to be as a result of multiple allele inheritances (Mackay, 2014). My mutant fly is white eyed just like brown doubles because of its eye color. The fruit fly’s compound eye contains 760 ommatidia and is one among the most advanced insect eyes (Holoch, and Moazed, 2015). Each ommatidium is made up of 8 photoreceptor cells, a cornea, support cells and pigment cells (Attrill et al., 2015). The wild Drosophila Melanogaster has reddish pigment cells, whose purpose is to absorb excess blue light which would otherwise make the fly blind because of ambient light (Potier et al., 2014). Around two-thirds of the fruit fly’s entire brain are dedicated to vision (Potier et al., 2014).
If scarlet-eyed Drosophila Melanogaster is crossed with brown eyed Drosophila Melanogaster , then the F1 generation will all be wild type while the F2 generation will have wild-type, brown, scarlet and white eyes because it is an autosomal dihybrid cross (Potier et al., 2014). The eye color mutation that occurred had to be recessive because it involved two genes (St. Pierre et al., 2013). Therefore, wild-type, scarlet and brown eyes are autosomal while white eyes is a sex-linked trait. As for this sex-linked inheritance, the presence of the X and Y chromosomes bring forth the question of what happens with these genes which are located on either the X or the Y chromosome (Attrill et al., 2015). The white eye type in Drosophila Melanogaster only occurs in female flies, meaning that the white gene can be found on the Y chromosome. Very few genes are on the Y chromosome, leaving most of the sex-linked inheritance to be associated with the X chromosome (Holoch, and Moazed, 2015).
Two main pathways give rise to the phenotypical characteristics of mutation in the genes (Dos Santos et al., 2014). These characteristics are the wild-type red color and the white one. The location and variation of this white gene can give any color from red to white (orange and yellow). The simplest mutation that involves the white gene is one that disables its function entirely ( w- ), such as w1118 (Dos Santos et al., 2014) . Homozygous flies for w1118 have white eyes while heterozygous w1118 have the normal red eyes, like the wild ones. In as much as the white gene is located on the X chromosome, it does not necessarily lead to the production of eye color (Dos Santos et al., 2014). The white gene can, therefore, be used as a marker, to show the genome’s transposable elements (P-elements), which can hop around the genome, landing on any chromosome. This happens when a transposase enzyme is available. They occur naturally but are extensively exploited for genetic study, where an artificial P-element is created and inserted into a genome (Dos Santos et al., 2014). Some mutants have eye colors produced by the P-elements that they contain (Schultz, Nilsson, and Westermark, 2011).
Molecular Characteristics of the gene and gene Product
The kind of mutation that occurs on the genetic structure of Drosophila Melanogaster resembles the albino phenotypes in human beings where the dark color pigment has a complicated structure which is the product of a synthetic biochemical pathway (Benson et al., 2014). Each step in the pathway involves the conversion of one molecule into another catalyzed by a separate enzyme protein encoded by a specific gene (Benson et al., 2014). Most genetic mutations are silent, not affecting a species’ phenotype. Some mutations do not bring changes to the amino acid sequence simply because multiple codons usually encode the same amino acids (Benson et al., 2014). The mutational location of a DNA can be of different types. The simplest one is the nucleotide-pair substitution, which leads to amino acid substitution or premature stop codons (Benson et al., 2014). At the protein level, mutation turns the protein’s amino acid composition regarding shape and size. This kind of change results in no biological role, which would otherwise be the basis of a null allele (Benson et al., 2014).
There are hundreds of alleles of the white gene (Attrill et al., 2015). Each mutant allele is name white (to indicate what gene it is), followed by an allele designation (Attrill et al., 2015). The protein produced by the white genes build pigment granules. In case the amount of this protein is normal (50 – 150 percent of wild-type), then the eyes will accumulate a normal amount of pigment. No pigment will accumulate if the protein cannot function at all due to mutation (Attrill et al., 2015). However, the eyes can still accumulate a small amount of pigment if a mutation in the gene affects the protein to the extent that it does not function properly (Schultz, Nilsson, and Westermark, 2011).
Since the beginning of research and studies that were aimed at understanding the coding and manifestation of genes in living organisms, genetics has proved to be worth the resources that were invested in doing these studies. From the beginning of the twentieth century until today, the knowledge that has been amassed over the years has benefitted many health-related sectors of the world. Previous studies that have been conducted about the genetic characteristics of Drosophila Melanogaster have helped a lot in understanding the genetic characteristic of other species of living organisms too. Crops, human beings, guinea pigs and even rats have been understood better regarding their genetically inherited characteristics, courtesy of such studies, and more so the genetical study of Drosophila Melanogaster . Since the inception of genetical studies by T.H
Morgan, great advancements have been witnessed in technological and technical approach in this field. The level of sophistication of equipment that is used has tremendously increased, as the field broadens with time. This particular study of Drosophila Melanogaster is therefore crucial in the field of genetics simply because it contributes to the advancement of this field, with a potential of making a great positive impact in the future. For now, genetical engineering has enabled us to modify the genetical structures of plants and animals positively, and the study on Drosophila Melanogaster has contributed to drug discovery, treatment and a better understanding of human inheritance. Many researchers have projected the impact of genetics to be extraordinary in the near future, and seemingly this will come to happen.
References
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