Mary Jo Miller BIOL 171L TA: Sam B.
The Aldox Gene in Drosophila melanogaster
Abstract Drosophila melanogaster, well-known as the vinegar fly, is used in this experiment to decipher whether the aldox gene is: dominant or recessive, and autosomal or sex-linked. The presence of the aldox gene is determined by observing the presence or absence of the aldehyde oxidase enzyme (which the gene codes for) in two filial generations of Drosophila melanogaster. The first hypothesis, pertaining to the aldox gene as a dominant one, is supported. The second hypothesis is falsified; the aldox gene is not a sex-linked gene. The work of Thomas Hunt Morgan and Gregor Mendel is appreciated to this day, for the two are regarded as the forefathers of genetics.
Introduction The purpose of this experiment was to determine if the aldox gene in vinegar flies is autosomal or sex-linked, and whether the mutant allele is dominant or recessive. Almost the entire genetic content of the Drosophila is on three chromosomes referred to as: X(1), 2, and 3. These were the genes of focus in the experiment. The Aldox gene codes for an enzyme which catalyzes aldehyde oxidase (AO) activity in Drosophila. The presence of the aldox gene was studied by observing the presence of the AO enzyme in the F1 and F2 generations of the flies that were created. Though the importance of the AO activity is not yet known, most Drosophila flies possess it, so the absence of AO activity is considered the mutant condition.
Blue color indicated the presence of AO enzyme while absence of blue color indicated absence of the AO enzyme. (The symbols AO- and AO+ were used for no aldehyde oxidase activity and presence of aldehyde oxidase activity, respectively.) Thus,
the first hypothesis was: the aldox gene is dominant. Although a majority of one trait does not always mean it is “dominant” over the minority, it seemed appropriate for the hypothesis to consider dominance of the aldox gene since it has been expressed in most Drosophila flies.
The law of segregation (Mendel) proposes that alternative forms of a gene (alleles) do not blend, but pass intact from one generation to the next. During meiosis, two alleles for each gene segregate at random and the ratios of offspring expressing each allele can be predicted. A sex-linked gene is one that has an allele only on the X chromosome, so the second hypothesis tested was: the aldox gene is a sex-linked gene. If the males carried the mutant allele, then all the F1 generation would be phenotypically wild type (AO+) but if the females carried the mutant allele, then the F1 females would be wild type (AO+), and the males would be mutant (AO-).
Cross between wild type male and mutant female: XAO+ Y XAOXAOXAO-XAO+ XAO-XAO+
Cross between mutant male and heterozygous female: XAO-
Y XAO- Y XAO- Y XAO+ XAO-
XAO+ Y XAO- Y
This is a photograph of the resulting F1 and F2 generation of males (M) and females (F) that possessed the AO+ (blue) and AO- (yellow) genes. It should be noted that the parents of F1 were heterozygous. In the F1 generation, the ratio of AO+ to AOwas 15:3. One of nine males and two of nine females possessed the AO- gene. The F2 generation included a ratio of 18:6 (AO+ to AO-). In this generation of offspring, two of twelve males as well as four of twelve females had no aldehyde oxidase activity present.
Conclusion Given the results of this experiment, several inferences can be made. The first hypothesis was supported; the aldox gene was dominant. This is not necessarily because it occurred most often, but because it was expressed dominantly over the AO- gene that was masked in most of the offspring.
The second hypothesis was falsified. The aldox gene was not a sex-linked gene, but appeared to be an autosomal gene in both F1 and F2 generations. Both the AO- and AO+ gene were expressed in both males as well as females and seemed to have little to no relation to the genes specific of the sex chromosomes of the parents. That being said, the gene for aldehyde oxidase activity must be on chromosome 2 or 3 since we only focused on X(1), 2, and 3.
Even though there was no clear error in the experiment, outcomes might have been altered by a couple factors. For example, the determination of AO being present was based on the substrate benzaldehyde mixing with the color indicator to form benzoic acid and a blue color. If there happened to be a miscalculation in measuring the amount of substrate used or a reaction did not take place to wrongly oxidze (or not oxidize) the PMS, it could have given off an incorrect color. However, no error was apparent to the students.
As previously stated, the presence or absence of the aldox gene is not very significant because the importance of it is not known. However, this experiment served as a tool to amplify understanding and appreciation for geneticists such as Mendel and Thomas Hunt Morgan and their developmental work toward what we now know as the fundamental principles of genetics.