Introduction
During prehistoric times, human beings recognized that specific physical characteristics of animals, plants, and people could be passed from one generation to the next. This basic knowledge of genetics was of great importance in agriculture and also in the domestication of animals such as cattle and dogs. However, significant understanding of genetics, inheritance and variation came with the work of an Austrian monk- Gregor Mendel. In 1860, Mendel studied heritable traits of pea plants. He found out that these traits were passed form the parents to the offspring. His experiments with the garden pea plants led to the discovery of several fundamental principles of genetics (Essentials of Genetics 132). Mendel, however, did not know the role of the nucleus, chromosomes and DNA in inheritance (Mendel, Corcos, and Monaghan 193). These were discovered later. Mendel attributed the passing of traits from parents to offspring to heritable factors, which are now called genes.
Objectives
The experiment was conducted to determine the ratio of a monohybrid cross, dihybrid cross, and sex linkage cross of Drosophila melanogaster based on Hardy- Weinberg equation. In this experiment, the progeny from the cross between wild type and vestigial (monohybrid cross), wild type and vestigial, sepia eye (dihybrid cross) and wild type and white eyes (sex linkage cross) will be determined. This experiment is helpful in introducing the use of the Chi-square statistic to test hypotheses concerning expected and observed ratios and also in discussing the life cycle of Drosophila melanogaster.
Hypothesis
It is expected that allele frequency will not change from one generation to the next because of natural selection.
It is expected that the frequency of sepia flies in the population will decline as they were not allowed to reproduce. Also, the inheritable allele producing sepia flies is expected to decrease in frequency.
Materials
Male and female common fruit flies, filter paper, substrate, fly nap, falcon tube, filter paper, petri dish cover, Petri dish, and a soft brush.
Procedure
We dropped Fly nap (ether) on cotton wool below the etherizer cap and corked the bottle until ether gas filled the bottle. Then, we lightly hit the base of the bottle to enable the fruit flies to drop. This was followed by removal of the bottle cap, which was immediately replaced with etherizer’s mouth. The bottles were then inverted over ether gas and shaken so that the drosophila get to the etherizer. Thereafter, the bottle was separated from the cover. This was followed by subjection of drosophila to ether until they become immobile. After this, the etherized drosophila were transferred to a filter paper. Using a dissecting microscope, we observed the drosophila. To move the flies on the stage of a microscope, a soft brush was used. At the end of the experiment, the flies were disposed in soup water with the exception of those that were going to be used for further genetic crosses. This, we placed on a dry surface.
Red-eyed drosophila (male) were mated with scarlet-eyed drosophila (female) in monohybrid cross. 10 males and 10 females were transferred to a bottle containing a substrate and the bottle corked with cotton. Other fruitflies were killed and traits observed. After three days, the fruit flies mated and laid eggs which later hatched. We then removed the F1 generation and placed them in the jars provided. We then used ether to kill F2 Drosophila and transferred to the filter paper. After this, we counted the number of F2 phenotypes and found to be780 wild type, and 222 sepia eyed. From this cross, two phenotypes were investigated and thus their distribution was tested using X2.
On the other hand, dihybrid crosses were carried by mating wild type fruit fly (male) and sepia eyes (female) fruit fly. 10 males and 10 females were transferred to a bottle containing a substrate and the bottle corked. The remaining was killed, and characteristics examined. After three days, the fruit flies mated and laid eggs which then hatched. Thereafter, we discarded all the parental Drosophila so as not to mix up with F2 generation. The same experiment was conducted using a male with sepia eyes and wild type (female). Using ether, the F2 fruit flies were killed and placed on the filter paper. After counting, F2 phenotypes were found to be 1130 wild type, and 271 sepia eyed. The table below shows the results of the experiment.
Results
We expected the number of red-eyed flies to be752 and the number of sepia-eyed flies to be 250 (see table 1). At the end of the experiment, our observed number for red-eyed and sepia-eyed flies were 780 and 222 respectively. The Chi-square value obtained was 4.1. Given that the p-value is 0.56, no significant differences was found between our expected and observed values. It can be concluded that F2 generation ratio is 3:1.
As shown in Table 1 (F3 generation), we expected the number of red-eyed flies to be 1051 and the number of sepia-eyed flies to be 350. At the end of the experiment, our observed number for red-eyed and sepia-eyed flies were 1130 and 271 respectively. The Chi-square value obtained was 23.7. This is significantly high compared to the theoretical value. Hence, there was a significant difference in our data.
DISCUSSION
Works cited
Essentials of Genetics. I. K. International Pvt Ltd, 2010. Print.
Mendel, Gregor, Alain F. Corcos, and Floyd V. Monaghan. Gregor Mendel’s Experiments on Plant Hybrids: A Guided Study. Rutgers University Press, 1993. Print.
