Introduction:
In genetics, when crossing a purebred white flower with a purple flower, we might expect its offspring to be a blend of both colors. Instead, we see that its offspring is purple as well. This is led to be by one trait being dominant over another trait. Gregor Mendel came up with a theory that each member of a pair of homologous chromosomes separates independently of the members of other pairs so the results would be random. This law is known as the law of independent assortment. In this laboratory experiment we will count and score the phenotypes of Drosophila melanogaster from a F2 generation of a dihybrid cross involving loci on the two major autosomes, chromosome two and three. A dihybrid cross is a cross between two parents that differ by two pairs of alleles. An example for this would be a parent from the F1 offspring who is (AABB) and the other parent being (aabb).
Crossing those two parents to make offspring would be a dihybrid cross.
A phenotype is an organism’s observable characteristics or traits while a genotype is a specific allelic combination for a certain gene or set of genes. The reason why we will be using drosophila melanogaster or a “fruit fly” as an organism to study genetics and compare Medal’s laws to its offspring is because fruit flies can be produced in small vials and life cycles can be complete in about twelve days. It is inexpensive to maintain or breed these flies and take up very minimal space in a lab room. Also, it is easy and simple to observe our results and if needed, recreate the same experiment to check for inconsistencies. Using the chi-square formula we will calculate the goodness of fit to our expected ratio and our observed ratio when we determine and record our results. My null hypothesis for this lab experiment is that our ratio for the fruit flies will be at 9:3:3:1 ratio. Alternately, my alternate hypothesis will be that the fruit flies will be of an uneven ratio because random chance will be greater than probability.
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Methods & Materials:
In this laboratory experiment, we were given a vial sample with fruit flies in them and with the help of a microscope; we determined what their specific trait that was passed on to offspring was. These were a F2 generation offspring of a dihybrid cross involving loci on the two major autosomes, chromosome two and three. It was determined that for our vial, dumpy wings and sepia eyes were our traits. The flies were counted separately by traits and sex to gather the observed results and calculate the expected results along with the Mendelian expected ratio. Next we used the chi- square formula gathered from our data.
Using the chi-square formula we can calculate the goodness of fit to our expected ratio and our observed ratio with the following example.
Sex
Observed Values
Expected Values
Mendelian ratio
Chi-square Value (X2)
Male:Female
238:282
260:260
1:1
3.72
Sex Observed Values – Expected Values=(x)^2/Expected Result=sum of results Sex Observed Values: 238-260=22^2=484/260=1.86
282-260=22^2=484/260=1.86
1.86+1.86=3.72 (Chi square value)
Results:
Testing for Gregor Mendel’s law of independent assortment required us to examine the data that was collected for the phenotypes and sex of the fruit flies for the vial we recorded. With that information we were able to calculate chi-square values. For my results, we counted a vile of fruit flies with dumpy wings and sepia eyes for traits to be observed. The class data observed values were not so closely related to the expected values. The attached tables were chosen for dumpy wings and sepia eyes which was the vile we counted during the lab. Table one in the attached lab report shows that the chi-square calculations gave us a p-value of .0002 making it reject the null hypothesis. Table two in the attached lab report shows that the chi-square calculations gave us a p-value of 0.06 which made it failed to reject the null hypothesis. The chi-square calculation sample can be observed in the methods and materials section of this lab report.
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Discussion:
The results that were obtained for the fruit fly dihybrid cross were closely observed for a 9:3:3:1 ratio which was expected. The likely P phenotypes were wild type with wild type. The F1 generation was wild type with sepia eyes or wild type with dumpy wings. The F2 dihybrid cross phenotypes and genotypes were wild type with wild type, wild type with dumpy wings, wild type with sepia eyes, or dumpy wings with sepia eyes. The ratio expected for the P generation would be a one to one ratio. The F1 generation would expect a 3:1 ratio and the F2 generation we studied expected a 9:3:3:1 ratio which was also closely observed. We must also remember that an expected ratio we expect to occur might not occur because of chance.
Although we know what to expect, the results are not always as seen. The chi-square results allowed us to sum up the class data total and acquire a p-value which would reject the null hypothesis because it was less than .05. This could be because the observed values of data were not exactly on point with the expected values. This is also because these estimates run on probability but at times, even probability seems to surprise the results given. The chi-square p-value results for sex in class total failed to reject the null hypothesis. This is likely because the expected values were closely similar to the observed results. The significance of these results helped us observe and understand Gregor Mendel’s law of independent assortment because we observed and recorded how out of two parent fruit flies, there could be a combination of four different offspring.
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The Drosophila melanogaster is appropriate to use for studying genetics because we can get constant quick results with them. This allows us to check our data and determine if it is consistent. This also demonstrates and proves that Mendel’s law of independent assortment is true. Finally, each ratio that was acquired for the fruit flies was very comparable to each phenotype, because our observed values were closely similar to our expected values.
