In the 1800s, an Austrian monk named Gregor Mendel used pea plants to study the way heritable traits were passed down across the generations. Eventually, he was able to quantify the passage of these traits and attributed them to heritable traits, which were eventually to be named genes. In the early twentieth century, 1910, Thomas Hunt Morgan used these same heritable traits to study the Drosophila melanogaster
, commonly known as the fruit fly, to carry out extensive studies on the genes and chromosomes. This he did by carrying out experiments that were centered on the functions of the genes and chromosomes. The choice of these fruit flies for these experiments was very convenient in a number of ways. This is because they possess certain characteristics that enable easier studying of the genes and chromosomes. These important characteristics include:The life cycle of fruit fly consist of four complete stages namely egg, larva, pupa and adult.From the time the egg is laid by a mature female to the hatching and the end of the larval stage, a total of about eight days will have lapsed. The pupa stage of the fruit fly may last for about six days and the adult fruit fly may live for up to several weeks.The sexing of these fruit flies, that is the separation of the males from the females is easily done. This is because the genitalia and sex combs are easily visible to the eye. The male has sex combs on their forelegs while the females do not, and the male flies have a darker, divided anal plate, penes, and scleritized genital arch which are absent in the females. For these experiments, virgin females must be used in order to get pure breed characteristics. This is ensured by removing all living males from the vial under experiment. Males however, do not have to be virgins. For dihybrid crosses, alleles that code for two separate traits that are found on different chromosome are used. An example would be a case where vestigial wings and wild type eyes which are mutant recessive alleles found on Chromosome 2 and Chromosome 3.The crossing over effect of these alleles will lead to the production of different phenotypes which may be either dominant or recessive (Cunnings, 1996).
The null hypothesis of this study is that there is no significance difference between the observed and the expected numbers in the experiment. The alternative hypothesis on the other hand is that there is a significant difference between the observed and the expected numbers in the experiment.In this experiment, the variables that were being studied include the eye type and wing types. The eye types were either wild type (red) or sepia (brown) while the wing types were either normal or dumpy. The body type was either normal or black body. The constant variables were the body type.
These include fruit flies, incubator, counter, culture vials freshly prepared, yeast, water, FlyNap, anesthetic wand, white sorting card, brush, dissecting microscopeStatistical Analysis
The Chi-square analysis (X2
), also known as the goodness-of-fit test was used to compare the expected and observed values from the dihybrid crossing of the fruit flies with the four traits under study. X2
/e =61.48 X= 7.8415 From the Chi-square tables, the C value of X is found to be between 0.05 and 0.01.Discussion and conclusion
The expected ratios of the dihybrid crosses where the two traits are linked should give 9:3:3:1 where 9 represents the dominant trait and 3 represents a combination of the two dominant and recessive traits while 1 represents the combination of both recessive trait. In this experiment, the total number of fruit flies observed totaled 702. Therefore, a quick calculation into the ratios gives the expected numbers of the phenotypes of the fruit flies. This gives the numbers of the red-eyed normal fruit flies to be 394; the red-eyed dumpy fruit flies to be 132, the same number for the sepia normal fruit flies and finally the sepia dumpy were expected to be 44, giving 702 fruit flies. A point to note is that decimal numbers from the calculations of the expected values were rounded off to the nearest ten, as there are no fraction fruit flies!From the tables it was found that the p-value lies between 0.05 and 0.01. This value is much lower than the expected value of 0.05 and hence we cannot reject the null hypothesis. However, the data I recorded did not support my null hypothesis, which stated that there was no significant difference between the observed and expected numbers, when indeed the observed differences were much greater than the expected.Some of the factors that might have contributed to the discrepancy of the numbers obtained may be that the F1 generation fruit flies were mixed with the F2 generation hence the crossing was irregular giving significantly different numbers from the expected.