B. OBJECTIVE
C. METHODOLOGY
Over a period of time of four week study three generations of flies starting with a parental (P) generation that has already separate the flies in each generation of each cross into males and females and separate the flies according to their mutations. Based on the observed numbers of each phenotype in each generation of each cross hypothesize the parental cross and use the chi-square (x2) test and if there is little difference between the observed results and the expected results, the chi-square will have a low value, thus, the hypothesis will be supported in this case.
D. MATERIALS
Drosophila melagasta (fruit flies) graduated cylinder
Drosophila vials and bottles medial food for flies
Microscope brush
Piece of paper Fly Nag
Fly Morgue Funnel
E. PROCEDURES
1. In each of the three crosses— A, B, and C lines— the flies must first be sorted into male and female flies. Male and female flies can be determined based on male and female body morphology in fruit flies. These first flies are the established F1 generation of fruit flies, and each group must remain separated between male and female flies. The table below demonstrates the different morphological and distinguishing features that allow the researcher to visually see the difference between male fruit flies in the Drosophila family and female fruit flies in the Drosophila insect family.
2. Once the sexing of the fruit flies is complete, the goal is to find the common trait of both the male and the female flies. The traits considered in this discussion are either the white eyes trait or the wild type trait.
3. Pick 10 of the females and 10 of the males of cross B to mate with each other. Repeat with cross C. Cross A should remain the standard, control group of flies in this case.
4. The vial for the flies is then prepared by including food in the vial. The pre-substance, 9 mL of water, and grains are added to the vial. Special caution was used when adding the grains to ensure that few were used to avoid killing the flies.
5. Once the food is dry, put the flies in each cross vial that will mate with each other and label each vial with the cross and family iteration.
6. Wait for 2-5 days for the fruit fly larvae to develop into fruit flies, and separate the resulting flies into males and females using the same features as before. These flies have mutations, and have become the F2 generation.
7. Analyze the resulting cross, hypothesize the parental cross, and do the appropriate chi-square analysis associated with the F2 generation. The initial F1 generation can be seen below.
F. CONCLUSIONS
CONCLUSIONS FOR CROSS A
The resulting information for the second generation of the Drosophila in cross A is contained in the table below. The phenotypes demonstrated by Cross A are wild type flies with white eyes; this means that the crosses that occurred in Cross A are monohybrid crosses. Since both males and females have white eyes in the resulting cross, it is likely that sex linkage of these genes is the main driver of inheritance.
In both the first and second generations, both males and females demonstrate the ability to express the wild type phenotype; however, in the next generation of flies, the F2 generation, the number of male and female flies demonstrating the ability to express the white eye gene are approximately the same. This suggests that the allele for white eyes is recessive— further supported by its low expression in the Cross A pool— and that this allele is sex-linked.
The Punnett square for the F2 generation where XwhXwh represents a white-eye phenotype on a female, and XWY represents a wild-type male has been established. The resulting Punnet Square suggests that the ratio of wild type females to white eye males should be 1:1. The Punnett Square for the second generation can be seen below.
Figure 1.
It can be seen, then, that with a total of 28 flies in cross A, the chi-square test can be analyzed as follows:
The resulting chi-squared analysis demonstrates a very low chi-squared value. This was the predicted outcome for the lab as a whole, and it demonstrates that the genetic and phenotypic representation is closely in line with the predicted phenotypic representation for this group. The outcome of this experiment demonstrated that the allele for white eyes is a sex-linked recessive trait, and the expression of this trait adhered strongly to the predicted mathematical outcomes for the population of flies as a whole.
CONCLUSIONS FOR CROSS B
A similar analysis to cross A was completed for cross B. The results of the cross are in the table below:
In this particular part of the experiment, black eyes and vestigial wings were both expressed in the second generation. This cross, then, will be analyzed as a dihybrid cross rather than a monohybrid cross. Dihybrid crosses are seen when two traits or the expression of two traits are manipulated in the population. The allele for black eyes is not sex-linked, but it is recessive; the allele for vestigial wings, likewise, is not sex-linked, but it is recessive. The Punnet square for these phenotypical traits is below.
B represents the dominant allele for wild type eyes
b is the recessive allele for black eyes
N is the dominant allele for normal wings
n is the recessive allele for vestigial wings.
Figure 2. Punnett Square for black eyes/vestigial wings
The Punnett Square has been color-coded to appropriately express the different traits expected: purple shows flies demonstrating the normal phenotype, blue shows those flies with wild type eyes and vestigial wings, pink shows flies expected with black eyes and normal wings, and green shows flies expected with black eyes and vestigial wings. In the following table, the chi-square analysis is completed for this Cross B. The results from this analysis can also be found in the table below:
As can be seen from this very large chi-squared analysis, the deviations in the dihybrid crosses is very different than that of the monohybrid and sex-linked crosses. This is because there is a much more significant number of variables that are associated with dihybrid crosses; dihybrid crosses represent a more complex form of genetic variation. This is a fascinating look into the complexities of genetic inheritance, and a good demonstration of how sex-linked characteristics are more easily and more clearly inheritable than non-sex linked characteristics.
THINGS I LEARNED FROM THIS LAB
Genetics are complex, and although this lab simplified the expression of certain types of genes, the reality is that percentages and averages associated with the expression of certain genes are largely based on luck of the genetic draw, so to speak. Fruit flies are an amazingly simple organism, as far as genetic expression is concerned, and even in this very simple organism, the expression of genes was complex to model after it happened and nearly impossible to predict before it happened. This is one of the things that makes genetics so interesting, as well— even with two parents who demonstrate a very strong genetic proclivity in one direction, nature and genetics can force the organism to display a completely different set of phenotypical or morphological features than the parents in an effort to keep the gene pool from becoming too small. This kind of self-regulating genetic mutation and moderation system is fascinating, and it is one of the most interesting features of learning about genetics. The more I learn, the more fascinated I become by the way that organisms express their genetic code, and it makes me more and more interested in the way my personal genetic code is expressed as well.
